Document / Report Control Form - Redland City · 2016-09-02 · Draft 21/04/05 S. Holt T. Rannard, J. Westerman Final 1/06/05 S. Holt J. Westerman Final Rev 1 16/09/05 S. Holt J

33529 Redland Landslide Hazard Assessment: Final Report: September 2005 1

Document / Report Control Form

Project Name: Redland Landslide Hazard Assessment

Project No: 33529

Report For: Redland Shire Council

PREPARATION, REVIEW AND AUTHORISATION

Revision # Date Prepared by Reviewed by Approved for Issue by

Draft 21/04/05 S. Holt T. Rannard, J. Westerman

Final 1/06/05 S. Holt J. Westerman

Final Rev 1 16/09/05 S. Holt J. Westerman

ISSUE REGISTER

Distribution List Date Issued Number of Copies

Redland Shire Council 3 copies + 1 electronic

SMEC staff:

Associates:

Brisbane Office Library: 1 copy

Report/EOI/Project File: 1 copy

SMEC Australia Pty Ltd Level 2, 60 Leichhardt Street SPRING HILL 4000 PO Box 940, SPRING HILL QLD 4004

Tel: (07) 3831 8988 Fax: (07) 3831 8977 Email: [email protected]

www.smec.com.au

The information within this document is and shall remain the property of SMEC Australia Pty Ltd.

33529 Redland Landslide Hazard Assessment: Final Report: September 2005 i

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Table of Contents

1 INTRODUCTION.......................................................................................... 1 1.1 Purpose of the Study................................................................................................ 1 1.2 Landslide Hazard Overview ..................................................................................... 1 1.3 Methodology............................................................................................................. 2

2 DEFINITIONS............................................................................................... 3

3 CHARACTERISTICS OF A LANDSLIDE .................................................... 5 3.1 What is a Landslide?................................................................................................ 5 3.2 What Causes a Landslide? ...................................................................................... 5 3.3 What Types of Landslides Occur? ........................................................................... 6

3.3.1 What Surface Features Indicate Landslides?..................................................... 7

4 HAZARD IDENTIFICATION SYSTEM......................................................... 8 4.1 Basis of Hazard Identification System...................................................................... 9

4.1.1 Evaluation of Previous Landslides ................................................................... 10 4.1.2 Assessment of Site Features and Relative Frequency Analysis ...................... 10

4.2 Risk Assessment, Evaluation and Treatment......................................................... 11 4.2.1 Likelihood ......................................................................................................... 11 4.2.2 Consequence of Slope Failure ......................................................................... 12 4.2.3 Risk Assessment and Implications................................................................... 12

5 RESULTS OF STUDY................................................................................ 14 5.1 Study Area.............................................................................................................. 14 5.2 Data Available ........................................................................................................ 14 5.3 Desktop Study........................................................................................................ 15

5.3.1 Identification of Previous Landslides within the Region.................................... 15 5.3.2 Assessment of Topography, Geomorphology and Geology Information.......... 16

5.4 Site Inspection........................................................................................................ 16 5.4.1 Redland Shire Mainland ................................................................................... 16 5.4.2 North Stradbroke Island ................................................................................... 17

5.5 Guidelines for development of the Hazard Map ..................................................... 17

6 HAZARD AND OVERLAY MAPS.............................................................. 20 6.1 Hazard Map............................................................................................................ 20 6.2 Overlay Map........................................................................................................... 20 6.3 Limitations of the Maps .......................................................................................... 20

7 GUIDELINES FOR USE OF THE OVERLAY CODE................................. 22 7.1 What Guidelines Apply to Development Applications? .......................................... 22 7.2 What Guidelines Apply to Road Design Over Sloping Ground? ............................ 25 7.3 What Guidelines Apply to Existing Developments in Hazard Management Areas?25 7.4 Guidelines for Assessment in Hazard Rating Categories ...................................... 26

7.4.1 Very High.......................................................................................................... 27 7.4.2 High .................................................................................................................. 27 7.4.3 Moderate .......................................................................................................... 28 7.4.4 Low................................................................................................................... 28 7.4.5 Very Low .......................................................................................................... 28

8 CONCLUSIONS......................................................................................... 29

9 REFERENCES........................................................................................... 31

33529 Redland Landslide Hazard Assessment: Final Report: September 2005 ii

APPENDIX 1 : LIST OF PREVIOUS LANDSLIDES FROM NATIONAL DATABASES

APPENDIX 2 : LIST OF PREVIOUS LANDSLIDES FROM COUNCIL RECORDS

APPENDIX 3 : SITE REPORTS

APPENDIX 4 : HAZARD AND OVERLAY MAPS

APPENDIX 5 : OVERLAY CODE

List of Figures Figure 1. Common Types of Landslides

Figure 2. Flowchart for Landslide Risk Management

Figure 3 Hazard Classification Matrix for Shallow Soil Landslides on the Mainland and Southern Moreton Bay Islands

Figure 4 Hazard Classification Matrix for Landslides on North Stradbroke Island

Figure 5. Examples of Good and Poor Hillside Construction

List of Tables Table 1. Implications of Hazard Classification

Table 2. Correlation between Relative Frequency and Hazard Rating

Table 3. Qualitative Measures of Likelihood

Table 4. Qualitative Measures of Consequences to Property

Table 5. Qualitative Risk Analysis Matrix – Level of Risk to Property

Table 6. Risk Level Implications

Table 7. Guidelines for Hillside Construction


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1 INTRODUCTION

1.1 Purpose of the Study SMEC Australia Pty Ltd was commissioned by Redland Shire Council to carry out a regional study, to assess the potential for landslide instability and produce an overlay map and associated overlay code consistent with the Draft Redland Planning Scheme and State Planning Policies.

In accordance with the State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and landslide”, risks to the community for proposed developments in identified Natural Hazard Management Areas should be adequately considered during design and assessment of a proposed development. Where the risks are unacceptable they should be suitably minimised and/or controlled.

This report details the results of a landslide hazard assessment study for the Redland Shire Council area, some 528km2 comprising:

Redland Shire Mainland (235km2); Coochiemudlo Island (approx. 1km2); Southern Moreton Bay Islands (approx. 22km2); and North Stradbroke Island (approx. 270km2).

This report provides details of the Redland area and its topography, geomorphology and geology and evaluates the landslide hazard using an adopted classification system.

Natural Hazard Management Areas have been mapped on a GIS Overlay Map (Appendix 4) and this has formed the basis of the Overlay Code. Additional information such as Geotechnical guidelines and good practices have been included to assist council assess future development applications and determine the appropriate type and form of development that best reflects the capability of the land.

1.2 Landslide Hazard Overview Historical records on national landslide databases indicate little evidence of landsliding within the Redland Shire Council area. It is likely that several landslides may not have been reported/recorded on the databases, however review of topography, geology, geomorphology and hydrological conditions suggest that given the generally low lying terrain the occurrence of landslides is relatively minor and mainly confined to:

the steeper slopes in the Naranleigh-Fernvale geological formation, overlain by colluvium, around the Mount Cotton range;

the steeper slopes within the basalt flows that occupy the coastal areas near Wellington Point, Cleveland, Victoria Point and Redland Bay; and

the steeper slopes of North Stradbroke Island, in particular where there is little vegetation cover.

Historical information suggests that almost half the landslides recorded in Australia were the result of human activity, and occur during intense storm events. Planners should be aware of the potential impact associated with future development on sloping sites, particularly those which comprise considerable cut/fill operations, where there is a material change and increased water concentration into the slope. These have the potential to increase the likelihood of a landslide occurring.


1.3 Methodology The methodology undertaken by SMEC Australia in assessing potential Landslide Hazard Management Areas was based on:

a review of historical landslide information from national databases (Geoscience Australia), plus other known landslide areas as provided by Redland Shire Council;

aerial photographic interpretation to identify areas apparently affected by slope instability; a review of regional features including topography, geology and geomorphology; identification of potential hazard zones based on topography, geology and geomorphology; visits to selected sites, site reporting and assessment of relative frequency to determine

hazard rating; and transfer onto overlay map that was used to develop a planning scheme code.

This study has identified hazard ratings for the Redland Shire using a classification system consistent with the procedures detailed in the paper entitled “A Method of Zoning Landslide Hazards”, prepared by McGregor and Taylor. This method has been adopted on a wide range of projects and has proven to be robust.

The hazard ratings (based on a five level hazard rating system consistent with national standards) are based on a grid system of 25m squares for the mainland, and 5m squares for the Islands. Each square is defined by a point at its centroid, and data related to the average slope instability rating for that square is attached (Appendix 4). A Hazard Classification Table has been developed in section 4.1 that ranks the hazard areas in very low to very high categories to reflect those areas identified on the Hazard Map. The Overlay Map was established from this Hazard Map to produce Landslide Hazard Management Areas consistent with the Moderate to Very High Hazard ratings.

The present study is regional and assessment has been restricted to a qualitative evaluation of hazard rating. Due to unknowns associated with development applications and their influence on the surrounding environment, only natural slope instability has been considered here.


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2 DEFINITIONS

The terminology adopted in this report has been designed to be consistent, as far as practicable, with national standards including the Australian New Zealand Standard AS/NZ 4360-1999 “Risk Management”, and the paper entitled “Landslide Risk Management Concepts and Guidelines” prepared by the Australian Geomechanics Society Sub-Committee on landslide risk management and published in the Australian Geomechanics Journal Volume 37 No. 2 May 2002.

The following definitions have been adopted for the study:

Acceptable Risk: A risk for which, for the purposes of life or work, we are prepared to accept as it is with no regard to its management. Society does not generally consider expenditure in further reducing such risk justifiable.

Consequences: The outcomes or potential outcomes arising from the occurrence of a landslide expressed qualitatively or quantitatively, in terms of loss, disadvantage or gain, damage or loss of life.

Elements at Risk: Meaning the population, buildings and engineering works, economic activities, public services utilities and infrastructure in the area potentially affected by landslides.

Frequency: A measure of likelihood expressed as the number of occurrences of an event in a given time.

Hazard: A condition with the potential for causing an undesirable consequence. Descriptions of a landslide hazard, particularly for zoning purposes, should include the volumes or areas of the landslides and the probability of their occurrence. There may also be value in describing the velocities and differential velocities of the landslide.

Individual Risk: The risk of fatality and/or injury to any identifiable (named) individual who lives within the zone exposed to the landslide, or who follows a particular pattern of life that might subject him or her to consequences of the landslide.

Likelihood: Used as a qualitative description of probability or frequency.

Probability: The likelihood of a specific outcome, measured by the ratio of specific outcomes to the total number of possible outcomes. Probability is expressed as a number between 0 and 1, with 0 indicating an impossible outcome, and 1 indicating that an outcome is certain.

Risk: A measure of the probability and severity of an adverse effect to health, property or the environment. Risk is often estimated by the product of probability and vulnerability. However, a more general interpretation of risk involves a comparison of probability and vulnerability in a non-product form.

Risk Analysis: The use of available information to estimate the risk to individuals or populations, property, or the environment, from hazards. Risk analyses generally contain the following steps: scope definition, hazard identification and risk estimation.

Risk Assessment: The process of risk analysis and risk evaluation.

Risk Control or Risk Treatment: The process of decision making for managing risk, and the implementation, or enforcement of risk mitigation measures and the re-evaluation of its effectiveness from time to time, using the results of risk assessment as one input.

Risk Estimation: The process used to produce a measure of the level of health, property or environmental risk being analysed. Risk estimation contains the following steps: frequency analysis, consequence analysis and their integration.

Risk Evaluation: The stage at which values and judgments enter the decision process, explicitly, by including consideration of the importance of the estimated risks and the associated social, environmental, and economic consequences, in order to identify a range of alternatives for managing the risks.

Risk Management: The complete process of risk assessment and risk control.


Societal Risk – The risk of multiple injuries or death to society as a whole; one where society would have to carry the burden of a landslide accident causing a number of deaths, injuries, financial, environmental and other losses.

Tolerable Risk: A risk that society is willing to live with so as to secure certain net benefits in the confidence that it is being properly controlled, kept under review and further reduced as and when possible. In some situations risk may be tolerated because individuals at risk cannot afford to reduce risk even though they recognise that it is not properly controlled.

Vulnerability: The degree of loss to a given element or set of elements at risk within the area affected by the landslide(s); i.e. a normalised consequence of the hazard occurring. It is expressed on a scale of 0 (no damage) to 1 (total loss). For property, the loss will be the value of the property; for persons, it will be the probability that a particular life (the element at risk) will be lost, given that the person(s) is affected by the landslide.

Additional definitions are given below:

Landslide Hazard Management Area: An area that has been defined for the management of naturally occurring landslide, but may not reflect the full extent of the area affected by the hazard.

Basic Frequency: The estimated frequency (annual return period) of a landslide event for regional area.

Relative Frequency: The relationship between the frequency of a landslide event at any particular site (or small area) within the regional area and the basic frequency based on analysis of the site conditions.

Site Frequency: The combination of the Basic Frequency of the regional area with the Relative Frequency of the site within the area to provide of estimate of the likelihood of a landslide affecting the site.


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3 CHARACTERISTICS OF A LANDSLIDE

Landslide hazard identification requires an understanding of the processes and influencing factors that lead to a landslide occurrence. This section briefly describes the characteristics of a landslide and gives an appreciation into the factors that contribute to their occurrence.

3.1 What is a Landslide? A landslide is the movement of a mass of rock, debris or earth down a slope. They are the result of shear failure of the soil and/or rock materials that make up the hill slope and they are driven by gravity.

3.2 What Causes a Landslide? The stability of sloping ground is controlled by three main factors:

the angle of the ground surface; the strength of the materials below the ground surface; and the level of water within the slope.

Landslides can be triggered by both natural causes or by human activity.

Natural causes may include: saturation of slope material from rainfall or seepage; undercutting of cliffs and banks by erosion; prying loose of rock masses from vegetating growth within joints; or vibrations caused by earthquakes.

Human activities may include: the modification of slopes by cut and fill activities associated with construction; interference with or changes to natural drainage; leaking pipes (water, sewer); changes to materials; the removal of vegetation; mining activities; or vibrations from heavy traffic, blasting or excavation.

In comparison to many other countries, much of Australia is subject to minimal landslide activity. Generally we receive little rainfall and the landscape has minimal influence from the processes of uplift. Intense rainfall is by far the most common trigger of landslides in Australia.

Areas that are affected by landslides commonly have cliffs, steep colluvial deposits, or gentler slopes of unstable geology subjected to prolonged or intense rainfall events. Landslide prone areas commonly comprise:

coastal cliffs; existing or old landslides; areas at or on the base of slopes; within or at the base of minor drainage hollows; at the base or top of cut and fill slopes; and any sloping ground in an area known to have a landslide problem.


3.3 What Types of Landslides Occur? Once a landslide is triggered, the material is transported in three main forms:

by sliding along a failure surface; by falling down a steep slope; or by flowing as a suspended mass, usually in water for example a mudslide or debris flow.

The rate of landslide movement varies from extremely slow (millimetres to centimetres per year) to a sudden and extremely rapid (metres per second) as with rock fall or debris flow. Sudden and rapid events are the most dangerous because of the lack of warning, and the speed at which they can travel down the slope and the force of impact.

Landslides may be classified into the flowing main types:

Translational Slides: where failure occurs on a planar surface or surfaces, usually natural defects in the material such as fissures, joints or bedding. Material within the slide can remain relatively undisturbed.

Creep Slides: where failure occurs as a gradual downslope progression (often extremely slow rates) of slope material. The slide area may appear relatively undisturbed and identification of the slide is often reliant on surface features.

Rotational Slides: where failure occurs through the material substance commonly on a concave surface. Material within the slide is considerably disturbed.

Topple: where failure occurs from the end over end motion of rocks down a slope. Often resulting from closely spaced sub-vertical jointed rock outcrops.

Falls: where movement is by free-falling or rolling of fragments on steep slopes with outcrops of closely jointed rock.

Flows: where, after failure along a planar or concave surface, the material is transformed into a viscous fluid consisting of soil and rock particles suspended in water.

Complex: where there is a combination of one or more of the above mechanisms.

Figure 1: Common Types of Landslides

TRANSLATIONAL

CREEP ROTATIONAL

TOPPLE

FALL

FLOW

The above figure is courtesy of the Geosciences Australia Web-site. (www.ga.gov.au)


3.3.1 What Surface Features Indicate Landslides? In the natural environment the progressive development of hill slopes by weathering and erosion involves a gradual incision of the stream beds into higher ground and results in the formation of slope surfaces that are essentially uniform, convex or planar.

The occurrence of natural landslides on these slopes produces an irregular profile, often concave, accompanied by features reflecting the disturbance that has taken place. In the case of recent landslides these features are usually sharp and distinct.

With time, the effects of weathering and erosion modify these features which become indistinct but usually can be recognised by close observation. Individually the features may not be related to landsliding but the presence of several features at one location indicates that some mass movement of material may have occurred.

Features that indicate existing natural slope instability include: irregular surfaces: areas of hummocky ground and depressions indicating disturbed material; benches: anomalous flat areas in uniform sloping areas; scars: areas where vegetation has been stripped during slope movement; scarps: linear features showing the location of vertical displacement of the ground surface; cracks: linear features showing lateral displacement of the ground surface; debris mounds: deposits of loose soil and rock on or at the base of slopes; disturbed vegetation: tilted trees; and seepage: presence of springs and dense vegetation regrowth.

Features that indicate that some lateral mass movement of material may have occurred in areas that have been developed include:

cracking or tilting of walls and retaining structures; cracking or slumping of embankment slopes; cracking and fall of material from excavated slopes; broken/fractured water pipes and underground facilities; tilted powerlines, retaining walls and fences (or offset); and sunken or cracked road surfaces.


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4 HAZARD IDENTIFICATION SYSTEM

This section describes the hazard identification system that has been adopted in carrying out the assessment of landslide hazard management areas throughout the Redland Shire.

In accordance with the State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and Landslide” (SPP 1/03) the identification of areas prone to natural hazards in a planning scheme can be used to guide the location and form of developments so that potential risks to the community associated with the development can be suitably minimised and/or controlled.

The general processes in assessment and management of such risk with regards to landslides is given in detail in the paper entitled “Landslide Risk Management Concepts and Guidelines” prepared by the Australian Geomechanics Society Sub-Committee on Landslide Risk Management and published in the Australian Geomechanics Journal Volume 37 No. 2 May 2002. Figure 2 is an extract from the paper showing the typical processes in flow chart form.

Figure 2: Flowchart for Landslide Risk Management

Extract from “Landslide Risk Management Concepts and Guidelines”, Australian Geomechanics Society Journal, Volume 37 No. 2, May 2002, p6.


As shown in Figure 2, hazard identification is an initial step in this process, and is the basis for this study. Due to unknowns associated with development applications and their influence on the surrounding environment, this study has been restricted to a qualitative evaluation of hazard rating, with respect to natural slope instability. Landslide hazard management areas were then derived from these hazard ratings.

The effects of earthquake on slope stability have not been included in this study.

For developments within identified Landslide Hazard Management Areas, there will be some element of risk associated with the development. The level of risk varies depending on three factors: the level of hazard, elements at risk (such as people, houses and so on) and the consequence associated with the occurrence of the hazard. Assessment of the risk in these areas should be adequately considered and where unacceptable these risks should be suitably minimised and/or controlled. The reduction of any of these factors would consequently reduce the risk. Assessment of risk should be carried out on development specific basis and is beyond the scope of this study however risk assessment, evaluation and treatment are briefly introduced in section 4.2 below for completeness.

For proposed developments in identified landslide hazard management areas the implementation of the Overlay Code (Appendix 5) will impose the required mechanisms to achieve this outcome, and allow for suitable restrictions or control of the risk to be imposed as Council deem appropriate.

For existing developments in identified landslide hazard management, there is little that can be done to reduce the elements at risk and consequence. So attention should be directed at reducing the hazard component to reduce the risk to acceptable levels. Some guidance with respect to this is given in section 7.3

4.1 Basis of Hazard Identification System The hazard identification system adopted for this study is consistent with the procedures detailed in the paper entitled “A Method of Zoning Landslide Hazards” prepared by McGregor and Taylor and published in the Australian Geomechanics Journal Volume 36 No. 3 Sept 2001. This is a commonly used qualitative assessment and such assumptions should be confirmed by a detailed analysis as part of the development assessment process.

This study provides an evaluation of the level of site hazards in relation to landsliding. The hazard rating is based on a five level system and classified into Very Low, Low, Moderate, High and Very High categories. The implications of this hazard rating classification are indicated in Table 1.

Table 1: – Implications of Hazard Classification Hazard Rating

Description Implications

VH (Very High)

The event is expected to occur

Extensive investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels.

H (High)

The event will probably occur under adverse conditions

Detailed investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels.

M (Moderate)

The event could occur under adverse conditions

May be acceptable provided treatment plan is implemented to maintain or reduce risk level.

L (Low)

The event might occur under very adverse conditions

Can be accepted. Treatment to maintain or reduce risk level should be defined.

VL (Very Low)

The event is conceivable but only under exceptional circumstances

Accepted. Managed by routine procedures.


4.1.1 Evaluation of Previous Landslides Several factors combine to define the complex relationship between the physical environment and land instability, however two basic rules can be concluded. Firstly, it is likely that landslides will occur in areas where they have occurred in the past, and secondly they are likely to occur in areas exhibiting similar conditions to these areas. Thus, the first activity of any landslide study is to identify previous landslide occurrences. This will give an appreciation of the areas which are more prone to landsliding.

With time, features identifying previous landslides become indistinct from the effects of weathering, erosion and vegetation growth and may not be discovered in this assessment. In addition to this, development on such areas may remove or cover the features indicating previous landslides. Thus, identification of the majority of landslides is usually rather difficult, especially where there is no, or limited, records of previous landslide.

Methods used as part of this study to assess previous landslides comprised: gathering information of existing landslides from national databases; review of aerial photographs for identification of landslides not recorded in the above step;

and site inspection for identification of evidence of instability.

In undertaking this task an appreciation of the historical occurrence of landslides within the Redland Shire was obtained, and similar areas could then be identified based on review and assessment of topography, geomorphology, geology and hydrology and climate information.

4.1.2 Assessment of Site Features and Relative Frequency Analysis From the potential landslide hazard areas identified in the above task, site inspection was undertaken to confirm the geological and geomorphology mapping and where possible refine the zone boundaries and assess the likely hazard rating based on site features which contribute to the initiation of a landslide.

Site inspection was undertaken on a number of selected sites (Appendix 3), representative of similar conditions within the project area. For each site a site report and frequency analysis was undertaken. The site report is a standard form that identifies the site features that contribute to the occurrence of a landslide. The major site features relevant to this assessment for natural slopes were:

Slope Angle: In general the steeper the average surface slope angle the higher the risk of slope instability. Slope angle can be obtained from contour plans or directly measured on site;

Slope Shape and Features: The shape of the slope provides an indication of the method of slope development and the materials below the surface. Concave shapes often indicate past movements;

Engineering Properties of Sub-surface Materials: The engineering properties of the materials forming the subsurface profile contribute to the risk of slope instability. Most slope failures occur in soil strength material and deeper soils increases the likelihood of slope failure;

Concentration of Surface Water: The most important factor in the occurrence of landslides is water. The majority of slope failures occur during or following rainfall events when there is a combination of surface erosion and saturation of subsurface materials. The potential for this to occur can be evaluated by consideration of how much surface water can be concentrated depending on the slope situation;

Concentration of Groundwater: The presence of a high groundwater table can provide a similar long-term situation to a rainfall event and contribute to slope failure without a major rainfall event; and

Evidence of Instability: In many cases slope failures occur due to the reactivation of previous landslides. The presence of features that indicate past slope instability provide evidence which could initiate future slope movement.


In many cases most of the above information on site conditions can be obtained during a walk-over survey by a suitably experienced geotechnical professional. It also may be necessary to supplement the site observations by subsurface investigations such as boreholes or test pits.

Evaluation of these features in a Landslide Frequency Assessment Form provided an indication of their relative importance to slope failure. The assessment of the features included the allocation of a weighted factor for each feature based on judgment and experience. This allows a Relative Frequency to be calculated by multiplying the selected factors together for each of the above site conditions.

The objective of this geotechnical assessment is to establish an understanding – a geotechnical model – of the site for qualitative analysis of the level of slope instability and to provide a basis for planning, design and remedial works. At the design stage, slope stability analyses to confirm an acceptable factor of safety, will be required.

From the assessment of results from this study and other studies in similar terrain a correlation between Relative Frequency and Hazard Rating has been established as shown in Table 2.

Table 2 : Correlation between Relative Frequency and Hazard Rating Relative Frequency Hazard Rating

< 0.2 Very Low 0.2 – 0.6 Low 0.6 – 2.0 Moderate 2.0 – 6.0 High

>6.0 Very High

To ensure consistency certain rules were adopted. For example if there is evidence of active slope instability the Hazard Rating must be at least High.

The present study has determined the Hazard Rating which has been used as the basis of the G.I.S. overlay map for identified natural hazard management areas. The determination of risk during an individual site assessment includes an evaluation of the consequences of failure and is explained in section 4.2.

4.2 Risk Assessment, Evaluation and Treatment As shown in Figure 2, hazard identification is the first step in any landslide risk assessment. During the development application processes the risk of landsliding needs to be assessed where the development is located within an identified natural hazard management area. This assessment should be undertaken to assess the risks to the community associated with the proposed development and measures required to manage the risks should be incorporated into the design and maintenance of the development.

The following sections give a brief explanation of such processes to give guidance into assessment and control of risk associated with the proposed development. For further information this is given in detail in the paper entitled “Landslide Risk Management Concepts and Guidelines” prepared by the Australian Geomechanics Society Sub-Committee on Landslide Risk Management and published in the Australian Geomechanics Journal Volume 37 No. 2 May 2002.

4.2.1 Likelihood In the case of natural slopes the likelihood (or Site Frequency) of a landslide occurring is the combination of the Relative Frequency of slope failure (indicated by the assessment of site features) and the Basic Frequency (annual return period) of failure for the region as indicated by records of significant events, for example past landslide events, major storms or earthquakes.

Likelihood can be grouped as: Almost certain (very high): Expected in most circumstances. Likely (high): Probably occur under adverse conditions. Moderate: Could occur under adverse conditions.


Unlikely (low): might occur under very adverse conditions. Rare (very low): Conceivable but only under exceptional circumstances.

An indication of probable range of frequency of events per annum is given in Table 3.

Table 3: Qualitative Measures of Likelihood Indicative Frequency per Annum Descriptor

10-6- 10-5 10-5- 10-4 10-4- 10-3 10-3- 10-2 10-2- 10-1 10-1- 1 Very High ------- ======= --------- High ------- ======= ------- Moderate ------- ======= ------- Low ------- ======= ------- Very Low ======= -------

4.2.2 Consequence of Slope Failure In the evaluation of risk, likelihood has to be associated with an assessment of the consequence of slope failure. The consequences may not be limited to property damage or loss of life, and may include such factors as public dissatisfaction and loss of business. Many of these consequences may not be readily quantifiable and will require judgement in their assessment.

The consequence can be grouped as shown in Table 4.

Table 4 : Qualitative Measures of Consequences to Property Descriptor Description

Catastrophic Structure completely destroyed; major engineering works required for stabilisation

Major Extensive damage to structure; significant stabilisation works required to whole of site

Moderate

Moderate damage to structure; large stabilisation works required to some of site

Minor

Limited damage to part of structure; some stabilisation works required

Insignificant

Little damage

4.2.3 Risk Assessment and Implications The evaluation of the level of risk is based on a combination of the assessed level of likelihood and consequences. The categories involved are obtained from the risk analysis matrix given in Table 5 with the implications on development given in Table 6.

Table 5 : Qualitative Risk Analysis Matrix – Level of Risk to Property Consequences to Property Likelihood

1:Catastrophic 2: Major 3: Medium 4:Minor 5:Insignificant Very High VH VH H H M

High VH H H M M Moderate H H M M L

Low H M M L VL Very Low M M L VL VL


Table 6: Risk Level Implications Risk Level Implications

VH (Very High Risk) Extensive investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels

H (High Risk) Detailed investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels

M (Moderate Risk) May be acceptable provided treatment plan is implemented to maintain or reduce risk levels.

L (Low Risk) Can be accepted. Treatment to maintain or reduce risk levels should be defined

VL (Very Low Risk)

Accepted. Manage by routine procedures.

The main objective of risk evaluation is to decide whether the levels of risk are acceptable, and involves making judgements about the significance and acceptability of the estimated risk. At the end of the evaluation process, the decision will be made to either to accept the risk, remove the risk, treat (and possibly monitor) the risk, or not accept the risk.


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5 RESULTS OF STUDY

5.1 Study Area The landslide hazard assessment study was carried out for the Redland Shire Council area comprising:

Redland Shire Mainland (235km2); Coochiemudlo Island (approx. 1km2); Southern Moreton Bay Islands (approx. 22km2); and North Stradbroke Island (approx. 270km2).

The study comprised a qualitative evaluation of hazard rating on a regional level and only natural slope instability has been considered.

5.2 Data Available The following GIS (Arcview GIS 8.3) files were made available by Redland Shire Council for the purpose of this study:

Topographic: 0.5m Contour (Mainland); 0.5m Contour (Southern Moreton Bay Islands); and 5.0m Contour (North Stradbroke Island).

Geological Data

Soils Data

Slope Data: 25m grid Slope Data (Mainland, Southern Moreton Bay Islands & North Stradbroke Island);

and 25m grid Aspect Data (Mainland, Southern Moreton Bay Islands & North Stradbroke

Island).

Orthorectified imagery (Mainland, Southern Moreton Bay Islands)

Satellite Imagery (2004 Quickbird data 0.6m pixel) In addition to this, the following hard copies of aerial photography were made available for review:

1973 Approx. 1:8000 Photo Run; Mainland, Macleay Island and Russell Island 1985 Approx. 1:10000 Photo Run;

Mainland, Macleay Island, Russell Island and part of North Stradbroke Island 1998 Approx. 1:8000 Photo Run;

Mainland 2002 Approx. 1:8000 Photo Run;

Mainland


5.3 Desktop Study

5.3.1 Identification of Previous Landslides within the Region A search of the Geoscience Australia landslides database (www.ga.gov.au) was undertaken to identify any recorded landslides within the Redland Shire Area. The landslides database contains information on landslides that have been recorded in Australia since 1842. This data is used in risk assessments and to raise awareness about landslides and their impacts.

Results of this search found two records of previous landslides within the study area, both occurring on North Stradbroke Island. More landslides have occurred within the study area, however these have not been reported to Geoscience Australia and hence have not been included in their database.

A broader search of surrounding regions (extending from Redcliff to Southport) was undertaken to get an appreciation of landslide occurrences in areas of similar geology. The results of this search are given in detail in Appendix 1, together with a plan showing the approximate location of the landslides. From review of these records the following comments can be made:

The majority of the landslides occurred along or adjacent to riverbanks typically in alluvial soils. The most common trigger of landsliding was intense rainfall and flood drawdown. Where records show, slopes were typically between 55 to 100%. On several of these locations, the natural slopes were steepened or surcharged by fill, which may contribute to lower slope angles.

The next major occurrence of landslides resulted in saturated fill slopes during intense rainfall events. Where records were available, it indicated these slides occurred on slopes typical between 55 to 100%. Some of these landslides were due to the failure of retaining walls from pore pressure build up associated with inadequate drainage behind the wall.

Review of the records indicates that there have been some five landslides recorded within the nearby areas underlain by the Naranleigh-Fernvale Formation. The majority of these failures occurred in cuttings with batter slopes of some 85 to 175%, with the landslides triggered by heavy rain. Two other landslides were recorded in a filled gully and colluvium slope where high water levels in the slope would be expected.

Several landslides resulted from the over steepening of riverbanks through erosion and/or undercutting of the toe of the slope, often associated with a high groundwater table during or immediately following intense storm events.

Review of the 1973 aerial photography (approx. 1:8000 photo run) of the Mainland, Macleay Island and Russell Island, indicated little evidence of previous landslides was found on the lower lying areas of Wellington Point, Birkdale, Capalaba, Thornlands Victoria Point and Redland Bay areas. These areas typically exhibit gentle natural slope angles, typically 0 to 5% with some areas exhibiting slopes up to some 12%. At the approx. 1:8000 scale the detail of the aerial photos made it hard to distinguish between landslide scars and erosion of the riverbanks. Given the over steepening of riverbanks through erosion and/or undercutting of the toe of the slope, commonly associated with alluvial soils, it is anticipated that some minor landsliding would have resulted in some of these areas following intense storm events which was not detectable in the aerial photographs. In areas around Mount Cotton and Redland Bay where areas of higher elevation and steeper slope angles are experienced the vegetation cover was relatively thick and in such areas the identification of instability was dependant on vegetation disruption, and little signs of previous instability could be identified from the photographs. Evidence of former landslides was present in some of the gullies which had been cleared for rural residential purposes around the Mount Cotton Range where steeper slope angles were experienced. This was later confirmed during the site inspections, which found signs of previous landslides, hummocky ground or minor irregularities in the majority of the heads of the gullies which were cleared.

Further to this Redland Shire Council compiled a list of known landslides based on their knowledge in the area and available council records. This information is included in Appendix 2.


5.3.2 Assessment of Topography, Geomorphology and Geology Information

The topography of the Redland Shire Mainland is composed of gentle rolling hills with natural slope angles, typically 0 to 5% with some areas exhibiting slopes in the order of 12%. The elevation gradually increases in a westerly direction with steeper slopes experienced around the Mount Cotton Area, and to a lesser extent the south western portion of Redland Bay.

The general landforms comprise rounded convex slopes with local incisions from natural drainage paths and river valleys cut from erosion into the landforms over time. Erosion gullies, in particular around the Mount Cotton Area, frequently have concave extensions at the gully head due to a combination of erosion, landsliding or minor irregularities.

There are several geological formations that make up the Redland Shire Mainland, comprising: metasediments of the Naranleigh-Fernvale formation; sedimentary rocks of the Woogaroo Sub-group; basalt flows; and estuarine and alluvial deposits.

The topography of Coochiemudlo Island and the Southern Moreton Bay Islands typically comprise low lying gentle sloping land with typical slope angle less than 5%. Maximum elevations for the islands are in the order of 30m. The geological formations that make up the Islands comprise:

estuarine deposits; sedimentary rocks of the Woogaroo Sub-group; and metasediments of the Bunya Phyllite formation.

The topography of North Stradbroke Island comprised reasonable steep sand dunes with slopes angles up to some 100% experienced. Towards the coastal areas these slopes flatten out. The majority of the slopes are densely vegetated, and in areas where there was little to no vegetation angles of sand dune near their angle of repose could be measured.

The geological formations that make up North Stradbroke Island comprise: dune sand; and estuarine deposits

5.4 Site Inspection Site inspection was undertaken on a number of selected sites, representative of similar conditions within the project area, to confirm the geological, geomorphology mapping and assess the likely hazard rating based on site features which contribute to the initiation of a landslide. Particular concentration was given to areas where previous landslides were indicated.

For each selected site a site report and frequency analysis was undertaken. The site report is a standard form that identifies the site features that contribute to the occurrence of a landslide. Evaluation of these features in a Landslide Frequency Assessment Form provided an indication of their relative importance to slope failure. A copy of the site reports and frequency analyses, together with a map showing the approximate location of the selected sites is given in Appendix 3.

5.4.1 Redland Shire Mainland For the Redland Shire Mainland much of the site inspections were focused around the Mount Cotton area where signs of previous landslides were encountered in the heads of gullies underlain by Metasediments of the Naranleigh-Fernvale formation.

These metasediments are found in the major parts of the mainland including the steeper areas of Mount Cotton and Redland Bay and comprise interbedded Graywache (recrystallised sandstone), argillite (recrystallised mudstone), quartzite (recrystallised chert) and greenstone. The rock structure is well foliated and has been sheared and folded by several tectonic episodes.

The general soil cover (comprising clays with varying amounts of silt, gravel and cobbles) is thin, often less than 1m towards the upper slopes and increasing in depth towards the lower slopes and


within the gullies. In the absence of closer identification of these soils it is reasonable to assume that colluvium deposits, comprising soil and rock fragments that have been transported and deposited downslope under the influence of gravity, could be found on the steeper slopes around the Mount Cotton area and within the gullies. Commonly the colluvial material has a lower shear strength to that of residual material which has weathered in place.

Shallow landslides and surface irregularities, indicating potential previous instability, was observed on some of the steeper slopes estimated to be in excess of some 55% and in the heads of the gullies in this material. These locations typically demonstrated areas with a high potential for water concentration influenced by the shape of the land. Reference to the AGSO-Geoscience Australia “Natural Hazards and the risk they pose to South East Queensland” indicates that slumps were observed on lower slope angles as low as 30%, and noted that this may be as low as 20% on greenstone derived or red colluvial soils in the Naranleigh-Fernvale beds in the mount cotton area.

This was not encountered on the gentler slopes of the lower Mount Cotton area or on similar slopes around the Redland Bay area where residual soils were encountered.

Few signs of instability were encountered throughout other representative sites. However, along some of the steeper slopes within the basalt flows occupy the coastal areas near Wellington Point, Cleveland, Victoria Point and Redland Bay it was found that even despite only moderate slopes being encountered, some of the residential properties located on the edge of such plateaus comprise the levelling off of the building site from cut and fill operations (often associated with some types of retaining structures) resulting in steepening of the natural slope and alteration of natural surface drainage. This can further contribute to the potential for landslide hazard if a thorough assessment is not undertaken.

Although little evidence was observed it is reasonable to assume that some small landslips on the edges of rivers has resulted following intense storm events as a result of erosion and over steepening of the riverbanks.

5.4.2 North Stradbroke Island The slopes of North Stradbroke Island represent wind blown accumulations of drifted sand with dense vegetation on the majority of the slopes away from the coast line. The formation of these dunes results from the movement of sand particles by wind action until they are impeded by physical obstructions, such as vegetation. The sand is then trapped and small mounds occur. Wind passing over these mounds causes disposition of sand on the leeward side. Sand accumulates on these slopes until it reaches its angle of repose, at which point the sand slips away. It is by this means that the dunes advance. Factors affecting the angle of repose of granular slopes include moisture content, particle size, shape, grading and density.

Given the progressive movement and formation of these dunes, the typical features that identify landslides are often not identifiable. It is with this in mind that site inspection was more concentrated on observation and measurement of some of the steeper slopes which exhibited characteristics near the angle of repose.

Inspection indicated that non vegetated slopes exhibiting characteristics near the angle of repose occurred on slopes in the order of 70 to 100%. This was evident in a number of batters cut for construction of roads. Surface sample were taken at some of the sites from which the angle of repose of these samples in a very loose state was measured. The results indicated angles of repose in the order of 65 to 78% slope.

On densely vegetated slopes, higher slope angles were observed and measured with slope angles up to some 100% experienced. This is likely a result of the vegetation and tree roots contributing to slope stability with the roots contributing to the soil strength and to a minor extent reinforcing the slope.

5.5 Guidelines for development of the Hazard Map In the absence of any landslide hazard assessment, such as this study, the SPP1/03 nominates all land of 15% slope, or greater, and other land known or suspected by the local government as being geologically unstable as a Natural Hazard Management Area with respect to landslides. This figure


appears to coincide with lower bound slope angles where landslides have been experiences in some of the more unstable areas throughout Queensland.

Historical records and observations undertaken during the field investigation indicate that the Redland Shire Area exhibits favourable geomorphology, geology and topography characteristics with respect to natural slope stability, and stable slopes (no signs of previous instability) in excess of 15% can readily be observed.

To reflect this, hazard ratings were assigned in accordance with the descriptions detailed in Table 1, with a hazard rating of High corresponding to areas where previous landslides have occurred or areas exhibiting similar geomorphology, geology and topography conditions. The results indicate that these high hazard areas were mainly confined to:

The steeper slopes (typically 55% slope or greater) of the Naranleigh-Fernvale Formation, overlain by colluvium soils. Most of these areas are around the Mount Cotton range. The heads of gullies where greater soil depths and a higher potential for water concentration into the slope are typically the least stable areas.

The steeper slopes (typically on slopes in the order of 55%, but experience in similar material shows this could be as low as 25% particularly in areas prone to water concentration) within the basalt flows that occupy the coastal areas near Wellington Point, Cleveland, Victoria Point and Redland Bay. These areas have deep residual soils, and a high potential for water seepage into the slope (due development close to the top of the slope)

The steeper slopes of North Stradbroke Island, in particular where there is little vegetation cover.

Although no single set of characteristics can define the complex relationship between the environment and the occurrence of landslides the above observations, together with soil mechanic principals, were used to establish some parameters that contribute to slope instability within the area, based on:

The angle of the slope, The strength/depth of the subsurface materials, The level of water within the slope.

These parameters as depicted in Figures 3 and 4 comprise the core structure of the hazard map, with certain areas then being further refined to reflect a more rigorous analysis carried out on similar areas during the site inspections.

Figure 3: –Hazard Classification Matrix for Shallow Soil Landslides on the Mainland and Southern Moreton Bay Islands

Geomorphology & Hydrology

Alluvial / Estuarial + High WaterAlluvial / EstuarialColluvial + High WaterColluvialResidual + High Water (1) Residual

9050 60 70 8010 20 30 40 100Slope Angles (%)

0

Notes: (1) The residual basalt soils that occupy the coastal areas near Wellington Point, Cleveland,

Victoria Point and Redland Bay with high water concentration potential exhibit moderate hazard characteristics at slope angles as low as 25%.

Legend: Very Low Hazard

Low Hazard

Moderate Hazard

High Hazard

Very High Hazard


Figure 4: –Hazard Classification Matrix for Landslides on North Stradbroke Island

Geomorphology & Hydrology

Alluvial / Estuarial + High WaterDune Sand

10 20 30 40 50 60 70 80 90 100Slope Angles (%)

0

Legend:

Very Low Hazard

Low Hazard

Moderate Hazard

High Hazard

Very High Hazard


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6 HAZARD AND OVERLAY MAPS

From the above study, an appreciation of the factors that combine to define the complex relationship between the physical environment and land instability can be determined. Resulting from this a GIS Hazard Map of potential hazard ratings, based on a five level hazard rating system, was established. This was then used to generate the Overlay Map by adopting hazard areas of moderate or higher as per the request of Redland Shire Council. A copy of both the Hazard Map and Overlay Map is provided in Appendix 4 as Arcview GIS “.shp” files for incorporation into Redland Shire Councils GIS system. This section briefly describes how these maps were generated and the limitations of the maps. The following sections give explanation to the planning scheme, and associated geotechnical guidelines, that will assist Council with the assessment of future development applications, and can be used to determine appropriate type and form of development that best reflects the capability of the land.

6.1 Hazard Map In developing the hazard map the Arcview GIS information supplied by Redlands Shire Council was broken up into a grid system of 25m squares for the mainland and 5m squares for the Southern Moreton Bay Islands and North Stradbroke Island. Each square is defined by a point at its centroid, and data related to the average hazard rating for that square has been attached. To coincide with SMEC’s GIS mapping software, the overlay map was developed using Mapinfo GIS files which could later be transferred into Arcview GIS files for incorporation into Redland Shire Councils GIS system.

Given the differing topographic, geomorphology, geological conditions and frequency of landsliding with respects to such, the methodology used to create the hazard rating classifications for the varying geological units was different, as explained in section 5.5. This was based on previous landslide occurrences, information in similar conditions and identification of the likely conditions which will contribute to the initiation of slope failure.

Preliminary slope hazard categories were then prepared. Modification and confirmation of these areas was carried out to reflect a more rigorous analysis from the results of the aerial photographic interpretation, site inspection and landslide frequency assessment forms produced at selected sites.

A copy of the Hazard Map is provided in Appendix 4 as Arcview GIS “.shp” files for incorporation into Redland Shire Councils GIS system.

6.2 Overlay Map The overlay map was generated from the Hazard Map by adopting hazard areas of moderate or higher as per the request of Redland Shire Council. The planning and design of future development on these areas will be treated as self assessable or code assessable under the Overlay Code with varying levels of detail required for the various hazard ratings.

A copy of the Overlay Map is provided in Appendix 4 as Arcview GIS “.shp” files for incorporation into Redland Shire Councils GIS system. Details of Overlay Code are given in Appendix 5.

6.3 Limitations of the Maps The following limitations are considered relevant with respect to development of the Hazard and Overlay Maps:

The accuracy to the maps are subject to the quality and reliability of the input data. For example tree height corrections to topographic contours may not be accurate resulting in different contoured slopes.

Identification of previous landslides throughout steeper sloping areas to the south and southwest of the mainland was restricted as a result of thick vegetation obscuring identification from aerial photographic interpretation.


The use of the 25m square grids allows ease in calculating the slope angles for that individual square. In doing so the average slope angle for that square was assessed, with an accuracy of plus or minus 12.5m either side of the point. Thus small sharp changes in the landscape and/or individual site features may be underestimated when the average for that square is assessed.

For any particular development site, there may be one or more squares which cover the site. Should development be proposed on areas where this occurs the higher of the collective hazard ratings should be adopted.

Changes to natural slopes, drainage paths and material properties as a result of human activities can not be accounted and could significantly affect the hazard rating for that site. Such consideration should be addressed as part of the development application process based on sound engineering principals and the appropriate level of professional input with reference to the proposed development.

In areas such as landfill and quarries, where significant cut and fill operations have taken place, the geological conditions may be adversely altered. For such areas these should be blanketed with at minimum a moderate hazard rating and consideration of the likely hazards should be controlled as part of the development application.


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7 GUIDELINES FOR USE OF THE OVERLAY CODE

Every year in Australia landslides damage many houses and cause millions of dollars damage to the natural and built environment including buildings, roads, railways and pipelines. It is important for planners, engineers and geologists to assess slope stability and landslides with respect to appropriate development and if required implement effective and timely remedial measures.

The activities once landslide hazard have been identified can be directed in three areas to reduce the likelihood of a landslide occurrence:

The reduction of slope angle An increase in material strength A lowering of the water table and improvement of drainage

The nature and extent of the activity depends on the most appropriate, practical cost-effective solution.

7.1 What Guidelines Apply to Development Applications? The planning and design of future development comprising building or other works, should consider the relevance of the hazard rating to the type of development proposed. Specific geotechnical requirements will depend on the hazard rating category as discussed in section 7.4.

Any application should incorporate the following within their design.

As a general rule: Development should be practicable in areas with a High or Very High Landslide

Management Area provided comprehensive sighting supplemented by rigorous analysis and design measures are applied. Good hillside practices must be adopted. For the majority of situations a risk assessment with respect to landsliding will be required. Remedial measures may be required to reduce/control the risk of slope instability to acceptable levels.

Development should be practicable in areas with a Moderate hazard rating of natural landslides provided appropriate sighting and design measures are applied. Good hillside practices must be adopted. A risk assessment with respect to landsliding would be prudent.

Development is practicable in areas with a Low or Very Low hazard rating of natural landslides subject to the adoption of good hillside practices.

Development planning should: Avoid major modification of sloping ground. Avoid development in sloping areas with significant depths of colluvial soils. Restrictions

should apply where soil is between 1 m and 3 m deep and development on slopes where colluvium is greater than 3 m deep should be avoided.

Avoid areas of potential water concentration. These include drainage depressions, gullies, narrow valley floors and areas with surface seepage of groundwater. Areas with some moistness can be developed with specific drainage provisions. Generally wet areas should be avoided.

Avoid areas with evidence of active, recent or ancient landslide events. Development roads should be located in areas that require the minimum modification of the

natural surface. Roads located on ridge crests or in wide valley floor are preferred. Details of cut and fill road design requirements are described in section 7.2 below.

Guidelines for residential hillside construction are given in Reference 2. These guidelines provide a description of Good Engineering Practice which applies to residential design and construction in areas of landslide hazard. For convenience these have been reproduced below in Table 7 and Figure 5.


Table 7 : Guidelines for Hillside Construction

Extract from “Landslide Risk Management Concepts and Guidelines”, Australian Geomechanics Society Journal,

Volume 37 No. 2, May 2002, p43.


Figure 5: Examples of Good and Poor Hillside Construction



7.2 What Guidelines Apply to Road Design Over Sloping Ground?

Roads on side slopes usually are formed by a combination of cut and fill operations. The design must incorporate effective drainage, and should incorporate the following good practices:

The road cut slope design should incorporate:

The adoption of batter slopes appropriate to the engineering properties of the different materials exposed in the cut face. As a general rule batters in soil should be 2H:1V, in poor rock 1H:1V and in good rock 0.5H:1V.

Where cuttings in rock are proposed, road alignments should be planned as not to coincide with major jointing orientations of the rock.

The higher cut faces should include the provision of benches not less than 3 m wide at vertical intervals of not greater than 10 m. These benches are required to catch fallen material, to control drainage and to provide access for maintenance of the cut face.

The provision of formed drains at the top of the cut slope, on the benches and at the toe of the cut slope.

The provision of slope protection, slope treatment or slope support in areas of potential concern. Slope protection against erosion may utilise a cover of topsoil and grass. On steeper slopes treatment of erodible and closely joined rock is commonly by a cover mesh and shotcrete with rock bolts providing treatment of areas with adversely oriented jointing. In areas of greater concern slope support can be provided by an engineered retaining wall. The design of the wall depends on the site conditions and cut dimensions but could include gabion crib, masonry and reinforced concrete wall designs.

The road fill embankment design should incorporate: The removal of all unsuitable material including trees, vegetation and topsoil from the

embankment foundation The preparation of the embankment foundation by the formation of terraces across the slope.

These terraces should be at least 2 m wide with a maximum height of 0.6 m. The installation of drainage, if required, in the foundation. This drainage may involve

trench drains in areas of local seepage or a drainage blanket in an area that is generally wet. The embankment fill should be placed in an engineered manner. Placement of earth fill

should be in layers – each not thicker than 300 mm and compacted by roller to not less than 95 % relative to Standard Compaction.

The design of compacted earth fill slopes in soil should be no steeper than 1.5H:1V, and may often be lower subject to retained height and soil strength. Surface protection should be by grass or rock.

The provision of drainage at the crest and toe of the embankment as formed drains leading to an identified disposal area.

7.3 What Guidelines Apply to Existing Developments in Hazard Management Areas?

Historical information suggests that almost half the landslides recorded in Australia were the result of human activity, and occur during intense storm events. Unfortunately, for existing developments in Landslide Hazard Management Areas, there is little that can be done to reduce the elements at risk and consequence. So attention should be directed at reducing the hazard component to reduce the risk to acceptable levels.

Slope remedial measures to reduce the hazard component are typically associated with high economic costs. However regular maintenance can be implemented to reduce the level of exposure to factors that contribute to slope stability, and thus reduce the risk. This should be associated with


continual monitor and where signs of distress emerge, implement the most appropriate and cost-effective treatment in a timely manner. Such regular maintenance may comprise: check that all drains are effective and carry out regular cleaning of the drains to remove

material that may divert or restrict run-off; check that all retention structures have suitable drainage, and the drainage does not silt up or

become blocked with time; sewage/water tanks and pools should be water tight, suitably founded and where applicable

sewage effluent should be connected to sewer mains, or regularly pumped out; landscaped areas should be planned such that they minimise cut/fill areas (in particular

surcharging at the top of slopes and excavation at the toe of slopes), retain vegetation and where possible avoid ponding of water and water concentration into slopes;

repair broken water pipes and joints ASAP, as soon as leakage becomes evident; check that surface protection and slope support is effective including the husbandry of the

grassed surfaces, repair of eroded areas and regular observation of the condition of the retaining structures;

regularly inspect the house for signs of distress, such as cracking from differential settlement and/or creep movement and monitor concerning cracks by using by either tape measure or installation of tell tales;

where actual landslide movement is suspected, the displacements can be monitored by the regular survey of location and elevation of a series of marks installed on the ground surface in the area of concern. Subsurface movements are measured by the installation of inclinometers. Regular reading of these instruments provides information on lateral and vertical movement within the slope;

where distress to structures occurs to the point that the structural integrity of the structure may be compromised, or there is significant consequence as a result of failure, remedial measures should be implemented in a timely manner. For such remedial measure experienced professional advice should be sought;

it would be prudent for Redland Shire Council to conduct a public relations campaign to make residents aware of what measures should be carried out to maintain or improve slope stability.

Regular maintenance as indicated above may be suitable for the majority of cases, and increase the severity of a climatic event that would be required to initiate a landslide. However landslides that provide little warning such as rock fall or debris flow can not be monitored in such a way. It is the lack of warning, speed/distance at which they can travel and the force of impact that they are so dangerous. In light of this, throughout areas of prominent geological instability it would be prudent to carry out further study to assess rainfall intensity/duration to indicate the likely triggers in similar geology that cause landslides and for Redland Shire Council to enhance forward planning strategies leading to disaster mitigation. Given the relatively minor occurrence of landslides in the Redland area an assessment to this level of detail is not considered to be a high priority unless higher urban intensity is planned in those areas in the future.

7.4 Guidelines for Assessment in Hazard Rating Categories As indicated above, rigorous analysis and restrictions should apply to development on areas with a High or Very High hazard rating of natural landslides. Given population growth there will become a time where demand on land and land values may make it economically viable to consider development in such areas.

The following gives recommendations as to the level of professional input that should be incorporated in the planning and design of proposed developments to suitably identify, control and manage risks associated with landsliding for each hazard rating. In the case of a property being assessed different hazard ratings may apply across the site. If this occurs the geotechnical guidelines related to the higher hazard rating must be selected in preference, or the developer must demonstrate that the proposed development is located away from such landslide hazard management areas.


7.4.1 Very High Carry out a detailed geotechnical engineering report by a suitably qualified geotechnical

professional (RPEQ qualifications). At a minimum the geotechnical engineering report should comprise:

extensive site investigation including subsurface investigation with groundwater measurements over at least one wet season;

frequency of investigation locations should be no less than 1 location/30mx30m grid with assessment of material strength by appropriate in-situ or laboratory testing. Investigations should establish a comprehensive geotechnical model over the whole site;

installation of groundwater monitoring points with measurements over at least one typical wet season and comparison of groundwater levels to rainfall events should be made;

review potential hazards; and analysis of slope stability using a suitable model appropriate for the site conditions.

Where analysis of slope stability in the above step indicates an unfavourable factor of safety under adverse conditions indicates assess the risks to the community with regards to loss or injury of life and infrastructure. Where unacceptable, these risks should be suitably minimised or measures to stabilize the slope (reduce slope angles, water drainage provisions, rockbolt and/or shotcrete protection) should be implemented.

Undertake comprehensive sighting for the development with regards to potential hazards, including restricting design of major structures and unfavourable earthworks in very high landslide hazard areas where possible.

Extensive design input from a qualified Practicing Engineer, including adoption of good hillside construction practices as provided in sections 7.1 and 7.2 above.

The design must be reviewed and certified by an experienced suitably qualified geotechnical professional (RPEQ qualifications).

Regular maintenance of slopes, cleaning of drainage courses and monitoring of slope for signs of distress.

7.4.2 High Carry out a detailed Geotechnical Engineering report by an experienced qualified

geotechnical professional. At a minimum the geotechnical engineering report should comprise:

site investigation including subsurface investigation with groundwater measurements;

frequency of investigation locations should adequately cover the site and slope in question to provide sufficient information to establish a comprehensive geotechnical model over the whole site, with assessment of material strength by appropriate in-situ or laboratory testing;

installation of groundwater monitoring points with measurements over at least one typical wet season and comparison of groundwater levels to rainfall events should be made;

a review potential hazards; and analysis of slope stability using a suitable model appropriate for the site conditions.

Where analysis of slope stability in the above step indicates an unfavourable factor of safety under adverse conditions indicates assess the risks to the community with regards to loss or injury of life and infrastructure. Where unacceptable, these risks should be suitably minimised or measures to stabilize the slope (reduce slope angles, water drainage provisions, rockbolt and/or shotcrete protection) should be implemented.

Undertake appropriate sighting for the development with regards to potential hazards, including restricting/reducing design of major structures and unfavourable earthworks in high landslide hazard areas where possible.


Considerable design input from a qualified practicing engineering professional, including adoption of good hillside construction practices as provided in sections 7.1 and 7.2 above.

The design must comply with recommendations detailed in the geotechnical engineering report.

Regular maintenance of slopes and cleaning of drainage courses.

7.4.3 Moderate Carry out a geotechnical engineering report by an experienced qualified geotechnical

professional. At a minimum the geotechnical engineering report should comprise: site walkover survey with investigations, as required, to establish a geotechnical

model over the whole site. This may require moderate subsurface investigation and/or testing to provide subsoil material properties;

review potential hazards, and assessment of slope stability using a suitable model appropriate for the site

conditions. Consider the risks to the community with regards to loss or injury of life and infrastructure. Undertake appropriate sighting for the development with regards to potential hazards, Design input from a qualified practicing engineering professional, including adoption of

good hillside construction practices as provided in sections 7.1 and 7.2 above. The design must comply with the recommendations detailed in the geotechnical engineering

report.

7.4.4 Low Provide details of on site conditions including a topographic plan showing the ground

surface slope and any features as listed in section 4.1.2. Review any potential hazard areas on the site or adjacent sites and plan a development

scenario to suit such areas. Design should include adoption of good hillside construction practices as provided in

sections 7.1 and 7.2 above.

7.4.5 Very Low Provide details of on site conditions including a topographic plan showing the ground

surface slope and any features as listed in section 4.1.2. Review any potential hazard areas on the site or adjacent sites and plan a development

scenario to suit such areas. Design should including adoption of good hillside construction practices as provided in

sections 7.1 and 7.2.


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8 CONCLUSIONS

This study comprised a regional qualitative assessment of natural slope stability within the Redland Shire area together with the establishment of an overlay map (Appendix 4) and associated overlay code (Appendix 5) consistent with SPP1/03 and the Draft Redland Planing Scheme. This was based on desktop study and limited inspection and assessment of specific sites.

Historical evidence indicates that the majority of landslides are triggered during or after intense storm events, with almost half the landslides recorded in Australia resulting on slopes altered by human activity. Thus consideration must be given to both natural occurring landslides and ones created by human activities.

The results of this study have indicated that given the generally low lying terrain with shallow depths of overburden soils (with exception of North Stradbroke Island) and from the results, it is considered reasonable that the occurrence of natural landslides is relatively minor. The results indicated mainly shallow landslides in soil strength material with soil creep observed and confined to:

the steeper slopes in the Naranleigh-Fernvale geological formation, overlain by colluvium, around the Mount Cotton range,

the steeper slopes within the basalt flows that occupy the coastal areas near Wellington Point, Cleveland, Victoria Point and Redland Bay. In such areas residential developments located on the edge of such plateaus comprise the levelling off of the building site from cut and fill operations. This has resulted in steepening of the natural slope and the alteration of natural surface drainage has / can further contribute to the potential for landslide hazard if not undertaken thoroughly,

the steeper slopes of North Stradbroke Island, in particular where there is little vegetation cover.

Such areas have been identified as Landslide Hazard Management Areas in accordance with the Overlay Map.

For developments within these identified Landslide Hazard Management Areas, there will be some element of risk, dependent on the level of hazard, elements at risk (such as people and houses) and the consequence associated with the occurrence of the hazard.

The risk will become more acute as building developments become larger, urban density is increased and there is further modification of natural slopes to accommodate population growth. Assessment of the risk in these areas should be adequately considered and where unacceptable, the risk should be suitably minimised and/or controlled. Consideration should be given to risk located off the site as a landslide occurrence can impact on both upslope and downslope of the immediate site.

For proposed developments in identified landslide hazard management areas the implementation of the Overlay Code will impose requirements for applicants to achieve this outcome. It will also allow Council to more effectively manage the risk associated with landslide hazard and provide a tool to guide future planning strategies.

With the exception of the one death recorded in the study area from a landslide at Amity Point in 1936, the Redland shire has had minimal landslide occurrences with minimal impact upon the community. It could be argued that the risk associated with existing developments is acceptable or it may be the case that an adverse event has not yet occurred in such areas or the consequence has not been present.

It is also reasonable to assume that Redland will be exposed to higher risk if there is increased urban development in areas identified as Landslide Hazard Management Areas.

In light of this it would be diligent for Redland Shire Council to undertake their own risk assessment of existing development in some of the higher risk areas to satisfy duty of care.. These areas include the populated areas of North Stradbroke Island, and previously identified areas of instability within the basalt flows that occupy the coastal areas. These areas should be given preference as they pose the higher risk with respect to elements at risk and associated consequence.


With reference to existing developments in Landslide Hazard Management Areas there is little that can be done to reduce the elements at risk or consequence so attention should be directed at reducing the hazard component to reduce the risk to acceptable levels.

Slope remedial measures are typically associated with high economic costs, and would be difficult to enforce, particularly given the limited impact on such areas to date.

It is recommended that Redland Shire Council consider implementation of an educational program to educate the residents in the more severe areas on regular maintenance and upkeep that can reduce factors that contribute to slope instability, and where signs of distress emerge then implement timely and cost effective remediation measures where required.

Planners should also be aware of future developments on sloping sites, even in areas not indicated as a Landslide Hazard Management Area. Developments which comprise considerable cut/fill operations, material change and increased water concentration into the slope have the potential to increase the likelihood of a landslide created by human activity. This should be controlled by the use of effective planning tools, the required level of engineering input and use of good engineering hillside practices as provided in this report.

This study is the first step in establishing a detailed assessment of landslide hazard areas within the shire. It is recommended that Redland Shire Council undertake a more extensive search of local records to identify previous landslide events in and around the area with additions to the Overlay Map as appropriate. This should include reference to council records, relevant geotechnical data such as geotechnical reports or consultation with geotechnical companies and will strengthen this document as a planning tool. Future landslides should also be recorded including the events that trigger them (commonly rain intensity and duration). This may enable some crude correlation with rainfall events for use as a warning system in the future and would serve to strengthen disaster mitigation response.


9 REFERENCES

Queensland Government, State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and Landslide” (SPP 1/03), May 2003

Queensland Government, State Planning Policy 1/03 Guideline “Mitigating the

Adverse Impacts of Flood, Bushfire and Landslide” (SPP 1/03), June 2003

Redland Shire Council, “Draft Redland Planning Scheme”, www.redland.qld.gov.au • Australian New Zealand Standard AS/NZ 4360-1999 “Risk Management” Australian Geomechanics Society Sub-Committee on Landslide Risk Management,

“Landslide Risk Management Concepts and Guidelines”, Australian Geomechanics Journal, Volume 37 No. 2, May 2002.

McGregor and Taylor “A Method of Zoning Landslide Hazards”, Australian

Geomechanics Journal, Volume36 No. 3, Sept 2001.

AGSO-Geoscience Australia, “Natural Hazards and the risk they pose to South-East Queensland”, AGSO Cat. No. 37282, 2001

Gold Coast City Council, “Guidelines for Control of Slope Instability within the City

of Gold Coast”

33529 Redland Landslide Hazard Assessment: Final Report: September 2005

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APPENDIX 1 : LIST OF PREVIOUS LANDSLIDES FROM NATIONAL DATABASES

Appendix 1 details the findings of previously recorded landslide information from the Geosciences Australia landslide database, together with a screen grab showing their approximate location.

This data is subject to Geoscience Australia's disclaimer and copyright statements.

Searching criteria:

Date From 01/01/1842 To 15/03/2005 (dd/mm/yyyy)

Latitude between -27.1147 and -27.8176

Longitude between 152.8004 and 153.5188

87 landslides found

YEAR MONTH DAY LOCATION CLASS FATALS INJURIES SYNOPSIS CUSTODIAN

1855 unknown unknown The roadway to North Brisbane Ferry

insufficient information 0 unknown This landslip occurred along part of the roadway to North Brisbane Ferry, making the

bank more precipitous and endangering the safety of passengers during a dark night. GA

1885 2 unknown

near Brisbane, at the Queensland Mercantile and Agency Company property, Bullimba

insufficient information 0 unknown

A portion of about 70 feet of river bank alongside the new wharfs of Queensland Mercantile and Agency Company gave way, carrying with it a strong retaining wall which had been built with the view of extending the wharfs.

GA

1887 2 unknown Brisbane, North Quay. insufficient information 0 unknown A landslip occurred on the riverbank in Brisbane, affecting a road and a retaining wall. GA

1890 2 7 North Quay, Brisbane insufficient information 0 unknown

This landslip occurred on the river banks near North Quay, Brisbane. A morgue, a jetty, a sanitary wharf and sheds were erected on the ground that subsided or slipped into the river, resulting in considerable loss. Gas & water pipes were interfered with.

GA

1936 9 2 Amity Point, North Stradbroke Island, Qld


A 20 foot high sea cliff collapsed at Amity Point, burying two children under several tonnes of sand. One child died. They had been tunnelling into a sand cliff formed some months before by marine erosion.

GA

1949 unknown unknownVictoria Street, Bowen Hills, Brisbane, Queensland

insufficient information 0 0

A section of road, 20m wide, over a filled gully in Neranleigh-Fernvale metasediments was destroyed. The instability was ongoing since about 1949. The slope was 30-45 degrees.

GA


1967 unknown unknown Fanny Street, Annerley, Brisbane, Queensland debris slump 0 0

On a slope of 15-20 degrees, a slip circle failure about 25 m wide destroyed a house (demolished about 1968 because of landslide damage). The land was not suitable for rebuilding. The landslide also affected a road. A crib wall was built to reinstate the roadway. The cause was high groundwater and a weak clay layer 5m deep. The geology was fill over weathered shale.

GA

1970 unknown unknown Fanny Street, Annerley, Brisbane, Queensland debris slump 0 0

A roadway that had failed in 1967 failed again in 1970. The type of landslide was not stated but was probably a debris slump. The cause was high groundwater and a weak clay layer 5m deep. A system of subsurface drainage was completed in 1975, but the wall for the roadway still had to be reinstated. The estimated cost in 1976 dollars was $34,950.

GA

1970 unknown unknown

Aaron Ave, Hawthorne, on right bank of Brisbane River, Queensland

insufficient information 0 0 Two landslides affected a sewer in the 1970s and late 1980s. GA

1971 1 unknown

Henderson Street, Strathburn Street, Howard Street, Oxley, Brisbane, Queensland

rock block glide 0 0

Heavy rain in November 1970, and possibly also water from broken pipes, triggered a large landslide, depth 9ft, width about 200ft, in shale, sitlstone and ferruginous sandstone, overlying hard claystone. Rock is Tertiary Corinda or Darra Formation. One house in Henderson St was extensively cracked and the fence affected, and there were cracks from Henderson St to Strathburn St and Howard St. This was the start of visible movement that continued for a number of years.

unknown

1971 12 unknownCrompton Road, Kenmore Hills, Brisbane, Queensland


A landslide occurred in a 45 degree cutting in white fissured clay on a newly-constructed roadway. The lithology was Moorooka Formation - Mesozoic sandstone/siltstone.

GA


Strathburn St, Cliveden Ave, Oxley Terrace, Oxley, Brisbane, Queensland


Intense rainfall triggered three main landslides in the Tertiary Darra Formation with a bedding angle of 10 degrees. Sliding took place between 1971 and 1974, but dated back to 1954. The slide surfaces were up to 10 m deep.

GA

1971 unknown unknownNewland Street, Fig Tree Pocket, Brisbane, Queensland

insufficient information 0 0 In early 1971, two landslides, 25 and 50 m wide, in alluvium were triggered by intense

rainfall. unknown

1972 2 unknownBalis Street, Holland Park, Brisbane, Queensland

insufficient information 0 0 Cyclone Daisy triggered a landslide about 20 m wide in fill overlying alluvium on a

creek bank. unknown


1972 11 unknownKooringal Drive, Jindalee, Brisbane, Queensland

insufficient information 0 0 High groundwater and undercutting of a creek bank caused a landslide 50 m wide and

30 m long on a slope of 10 degrees. It displaced stormwater and sewer pipes. unknown

1972 unknown unknownCliveden Avenue, Oxley Terrace, Oxley, Brisbane, Queensland

insufficient information 0 0 A minor slip occurred in the Tertiary Darra Formation. unknown

1974 1 unknownRadnor Street, Indooroopilly Reach, Brisbane, Queensland

rock slide 0 0 Rapid drawdown following floods triggered earth and rock slides. The earth slides were stabilised by weighting the toe. unknown

1974 1 unknownBoundary Street, West End, Brisbane River, Brisbane, Queensland

insufficient information 0 0 Rapid drawdown following floods triggered a small slide in alluvium. It was stabilised

by weighting the toe. unknown

1974 1 unknown

Newlands Street, Fig Tree Pocket, Brisbane River, Brisbane, Queensland

insufficient information 0 0 A major landslide in alluvium affected 10 residentail blocks and a street, destroyed part

of a serviced subdivision. unknown

1974 1 unknown

Cliveden Ave, Strathburn St, Oxley Terrace, Henderson St, Howard St, Ardoyle St, Seventeen Mile Rocks Rd, Oxley, Brisbane, Queensland

rock block glide 0 0

Increased pore water pressure, unfavourably inclined weak sedimentary rocks (Tertiary Darra Formation, bedding 10 degrees), and lack of adequate surface drainage caused landslides over a distance of 1.6 km. Ten houses were evacuated and 12 others threatened. It was difficult to visualise any remedial measure that would provide a permanent, effective solution. At Cliveden Ave, there was movement of 30-100 mm per day on a slide 10 m deep.

unknown

1974 2 2

Coronation Drive, between Sylvan Road and Patrick Lane, Brisbane, Queensland


Rapid drawdown following floods triggered a 53 m wide landslide in alluvium. Coronation Drive was closed to traffic from 2 February to August 1974 when movement ceased. Closure was because of cracks and subsidence in the road. A high brick property wall was tilted, and a sewer sunk 11 inches. 36,000 cu m of rock fill was placed in the river to stabilise the landslide.

unknown

1974 3 unknown

Brisbane, Oxley, Cliveden Ave, Strathburn St, Oxley Terrace, Howard Street, Ardoyle St, Seventeen Mile Rocks Rd,


Landsliding has threatened numerous houses in Brisbane, many of which have been evacuated. The sliding is occurring as a result of susceptible rock type and formation and inadequate drainage.

GA


1974 unknown unknownBrisbane, at the bank of the Brisbane river at Coronation Drive


This landslide in the bank of the Brisbane River at Coronation Drive in mostly tidal deposits resulted from rapid drawdown following river bank scouring during the 1974 flood.

GA

1974 unknown unknownRiver bank at Crosby on old coal station side, Brisbane, Queensland

earth slump 0 0 A flood triggered slumps about 10 m high in a large fly ash embankment. unknown


Brisbane River, William Jolly Bridge to Bremer River Junction, Brisbane, Queensland

debris slump 0 0

Numerous landslides happened during the 1974 floods and earlier. Landslide types included small and large scale, simple and composite, rotational failures in alluvium, shallow slides and block falls in soil or weathered rock, and local erosion failures. The landslides in the river bank were caused by drawdown following flooding, tidal effects, heavy rainfall, and river bed scour undercutting the slope.

unknown


Roghan/Neville Road, Bridgeman Downs, on bank of south Pine River, Brisbane, Queensland


Cracking occurred along the edge of the road. The landslide was 50 m wide on a bank about 12 m high in Petrie Formation Tertiary mudstone. A house was demolished because the slip was very close to it. The year was probably 1974.

unknown

1975 unknown unknownCentenary Bridge, Fig Tree Pocket, Brisbane, Queensland

insufficient information 0 0 In late 1975, a landslide in fill over weathered shale, on a slope of 15-25 degrees,

affected the sewer. unknown

1981 2 7Blackheath Road, Oxley, Brisbane, Queensland

earth slump 0 0

The slide is a rotational slump in clay on a slope of 10-15 degrees, and is a slow moving landslide caused by pore water pressure from rainfall and high groundwater. The lithology is Tertiary mudstone (Corinda or Darra Formation). One house was severely affected and was demolished by voluntary labour on 21-22 February 1981. Another house was threatened and its sewer pipe ruptured. The path adjacent to a third house was buckled.

unknown

1981 11 unknown

Creek Road, near Newnham Road, Carina, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered an 80 m wide landslide in a fill embankment 4-5 m high, on a

slope of 35-40 degrees. unknown

1982 3 6Forrester Terrace, Bardon, Brisbane, Queensland


A landslide 10 m long and 25 m wide occurred in a fill slope (in-filled gully up to 3 m deep) in probably Bunya Phyllite. It affetced two properties, and a fallen tree cut power to one house. It was probably triggered by rainfall.

unknown


1982 unknown unknown Kedron Street, Kedron, Brisbane, Queensland


In 1982 or 1983, flood drawdown caused a landslide, 25 m long and 50 m wide, in fill over alluvium in a filled drain in a creek bank 4 m high. A bike way, back fence, and back part of a property were affected. The instability was remediated by a rock fill buttress and drainage.

unknown

1982 unknown unknownLochinvar Road, Upper Kedron, Brisbane, Queensland

debris flow 0 0 A 20 m wide debris flow occurred in saturated fill in a 9 m high road embankment on a slope of 35 degrees. unknown

1983 5 unknownMeiers Road, Indooroopilly, Brisbane, Queensland


A leaking water main and ponded water in a table drain caused a 10 m wide landslide that affected the roadway. It is possible that movement of the slip caused the leak in the water main, which then caused reactivation of the slip.

unknown

1984 unknown unknownCoronation Drive, Brisbane City, Queensland


A landslide in fill happened during construction of a bike way and a grader was lost. The locality was near the old Bennetts Creek Bridge, at the original mouth of Bennets Creek.

unknown

1985 unknown unknownBotticelli Street, Fig Tree Pocket, Brisbane, Queensland

insufficient information 0 0 In 1985-1987, a slump with a deep slip circle affected the sewer near the Pony Club. It

dropped down ?5 m?. unknown

1987 unknown unknownCadogan Street, Carindale, Brisbane, Queensland

insufficient information 0 0 In late 1987, a high water table and undercut creek caused a 30 m wide landslide in

alluvium in park land on a slope of 10-15 degrees. unknown

1988 3 unknownWestlake Drive, Westlake, Brisbane, Queensland


Intense rainfall triggered a landslide, about 10 m wide, in an unstable fill platform for a tennis court. The fill had been placed over alluvium on a creek bank in the outlet for Westlake.

unknown

1988 unknown unknown Lanena St, Jindalee, Brisbane, Queensland


A landslide in alluvium was caused by scour of the river bank in mid 1988. Six properties were affected, but no dwellings were in immediate danger. There may have been previous landsliding in 1975.

unknown

1988 unknown unknownDerby Street, Highgate Hill, Brisbane, Queensland

rock fall 0 0 In early 1988, localised rock falls happened in an oversteepened road cutting in Bunya Phyllite. unknown

1988 unknown unknownTimaru Close, Westlake, Brisbane, Queensland

insufficient information 0 0 In early 1988, a high water table and erosion of the toe caused a landslide, in fill over

alluvium, in a bank of about 35 degrees on an easement. unknown

1989 4 25 Brisbane, Queensland insufficient information 0 0 120.7 mm of rain fell in 2 hours. Landslides blocked city streets, walls collapsed and

power lines were down. unknown


1989 4 25Windsor State School, Windsor, Brisbane, Queensland

rock fall 0 0 120.7 mm of rain fell in 2 hours in Brisbane. The top of a brick drain, built around 1860, collapsed. The gully was 4 m long, 2 m wide and 1 m deep. unknown

1989 4 26Terrace Street, New Farm, Brisbane, Queensland

rock fall 0 0 High pore pressure from heavy rainfall caused the collapse of a 2.3 m high brick retaining wall, 20 m long, behind a block of flats. It crushed a tennant's car. In the newspaper photo, it seems that only the wall collapsed, not the material behind it.

unknown

1989 4 unknownMoggill Road, Kenmore, Brisbane, Queensland

rock slide 0 0

During Easter 1989, heavy rain fell in Brisbane. The cut slope behind Coles failed, causing $1 million damage. The batter is composed of metasiltstones and phyllite. The rocks soften when wet, and the batter failed on cross foliation. Remedial work and vegetation were just completed in October 1989.

unknown

1989 4 unknown Langside Rd, Hamilton, Brisbane, Queensland rock fall 0 0 Intense rainfall triggered the collapse of a boulder wall, 2.7 m high. The geology is fill

overlying phyllite. unknown

1989 unknown unknownCurwen Terrace, Chermside, Brisbane, Queensland


In mid 1989, high groundwater and an undercut creek bank caused a landslide 35 m wide and 30 m long on a slope of 5-14 degrees. It affected the back wall of a property, but was remediated in the mid 1990s by drains and vegetating.

unknown

1990 2 unknownZambesi Street, Riverhills, Brisbane, Queensland


Intense rainfall triggered a creek bank failure in park land. It occurred in a fill slope of 30-35 degrees, with the fill overlying alluvium. The scarp was 10 m wide and 6 m high and the volume was 100-200 cubic metres. Landsliding also happened in 1989.

unknown

1990 4 8Lochinvar Road, Upper Kedron, Brisbane, Queensland

insufficient information 0 0 A 70 m wide landslide happened on a slope of 35 degrees in saturated fill in a 9 m high

road embankment . unknown

1990 unknown unknown Lever Street, Albion, Brisbane, Queensland


On 2 August 1990, it was noted that landslides had occurred on this site a number of times since 1988. There was a difference of opinion as to whether slippage occurred in the fill or shear planes in the underlying Mesozoic shales.

unknown

1991 unknown unknownIsles Road, Indooroopilly, Brisbane, Queensland

insufficient information 0 0 A 15 m wide failure in a fill embankment (possible creep) caused cracking in the road

way. unknown

1992 12 21 Tingal Road, Wynnum, Brisbane, Queensland


A crib wall constructed to retain slipped fill, subsequently slipped again. The fill probably overlies alluvium. The landslide, triggered by intense rainfall, affected private property. The head scarp was immediately behind houses, and fences were displaced.

unknown

1992 unknown unknownCoronation Drive, near Land Street, Brisbane, Queensland

insufficient information 0 0 A landslide in fill over alluvium affected a bike way. It was remediated with rock fill in

the river. unknown


1993 10 26 Kangaroo Point cliffs, Brisbane, Queensland rock fall 0 0

Runoff discharge at the top of a cutting caused a 1 cubic metre rock fall (wedge failure from the top of a 15 m high cliff in columnar jointed tuff on to the maintenance track. Rocks rolled on to the bike way.

unknown

1993 unknown unknownSt Lucia, on Bank of Brisbane River, Brisbane, Queensland

insufficient information 0 0 In approximately 1993, a cliff failure happened at the University of Queensland

Rowing Club. It was remediated. unknown

1993 unknown unknown Kate Street, Kedron, Brisbane, Queensland

insufficient information 0 0 In early 1993, a 35 m wide scour in a creek bank, 3-4 m high at an angle of 40-45

degrees, affected the road way. The scour was induced by filling on the opposite bank. unknown

1993 unknown unknownFlynn Street, Holland Park, Brisbane, Queensland

insufficient information 0 0 In late 1993, a 7 m wide landslide happened in a 4 m high cutting in colluvium at the

end of a cul-der-sac. unknown

1994 10 unknown Norman Creek, East Brisbane, Queensland


A landslide, caused by bank erosion and possibly tidal drawdown, on a slope of 20 degrees affected the creek bank near the Anglican Grammar School swimming pool. It was 14 m wide by 8 m long.

unknown

1996 1 unknownAshton Street, Wynnum, Brisbane, Queensland


Intense rainfall triggered a landslide in fill over alluvium in a creek bank 6 m high, oversteepened by erosion. It affected kindergarten grounds and was close to the building. Slippage also occurred in December 1992 and December 1993.

unknown

1996 1 unknown Burns Road, Taringa, Brisbane, Queensland rock fall 0 0

Intense rainfall triggered a rock fall in a 6 m high 65 degree cutting in metasediments of the Neranleigh-Fernvale Beds. It had a volume of 10-15 cubic metres and affected the road.

unknown

1996 1 unknownMcPherson Street, Sinnamon Park, Brisbane, Queensland

insufficient information 0 0 Intense rainfall caused a landslide in fill over Moorooka Formation (Mesozoic

sediments). The loose fill became waterlogged. The landslide affected a park. unknown

1996 5 unknownDewar Terrace, Sherwood, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered a landslide, on a 15 degree slope, in fill over weathered

sandstone (Moorooka Formation). It affected a park. unknown

1996 5 unknownBedier Street, Upper Mount Gravatt, Brisbane, Queensland

insufficient information 0 0 Intense rainfall, erosion and drawdown caused a landslide in alluvium in a bank 1.5 -

2.0 m high. unknown

1996 5 unknown Cobalt Street, Keperra, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered a 10 m wide landslide in saturated fill on a drain batter. The

fill was over Enoggera Granite. unknown


1996 5 unknown

Norman Creek, Stones Corner, adjacent to Logan Road, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered a 12 m wide landslide in fill over alluvium in a creek bank 4

m high with a slope of 35-45 degrees. unknown

1996 5 unknownCascade Close, Riverhills, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered a 25m wide landslide in alluvium in a river bank 6-8 m high. unknown

1996 5 unknownEnoggera Creek, The Gap, Ashgrove, Brisbane, Queensland

insufficient information 0 0 Intense rainfall and flood drawdown caused three landslides in fill and/or alluvium in

the creek bank. unknown

1996 5 unknown

Sparkes Hill Reserve, Longford Street, Stafford, Brisbane, Queensland

rock slump 0 0 A cutting 13 m high at an angle of 40 degrees on weathered rock (Neranleigh-Fernvale Beds) behind a reservoir slumped on to the roof of the reservoir. The site was subsequently remediated.

unknown

1996 5 unknownCarnegie Street, Westlake, Brisbane, Queensland

insufficient information 0 0 The 20 m wide landslide was in alluvium in a river bank 4 m high. unknown

1996 5 unknownHagen Street, Upper Mount Gravatt, Brisbane, Queensland

insufficient information 0 0 Intense rainfall triggered a 10-15 m wide landslide in a creek bank 2-3 m high. The

geology is fill over residual sandstone. unknown

1996 unknown unknownAinsworth Street, Salisbury, Brisbane, Queensland

insufficient information 0 0 A 25 m wide landslide occurred in a 6 m high creek bank in fill over alluvium. It

probably happened in late 1996. unknown

1997 unknown unknown Rode Road, Chermside, Brisbane, Queensland

insufficient information 0 0 In early 1997, high groundwater caused a 10 m wide landslide in fill over alluvium in a

creek bank oversteepened by filling to about 45 degrees. unknown

1997 unknown unknownFairfield Road, Yeerongpilly, Brisbane, Queensland


In mid 1997, bank erosion and possibly high groundwater caused a 20 m x 20 m landslide in a 25-30 degree oversteepened fill batter behind a business. It was remediated. The geology was fill over alluvium.

unknown

1998 3 unknownIvory Street, Brisbane City, Queensland, @ Boundary Street

rock fall 0 0 Intense rainfall triggered a rock fall from an old, near vertical rock cutting, 12 m wide x 9 m high, in Neranleigh-Fernvale Beds. It affected the road way but was remediated. unknown


1998 9 unknown Guanaba, Qld insufficient information 0 0

Landslides in the hillside and in road fill ocurred during the period 2-4 September 1998. The toe from one landslide upslope partly ran out on to the access road. The trigger was probably heavy rain.

GA

1998 unknown unknown North Stradbroke Island insufficient information 0 0 A 400m long landslide has occurred near a sand mine. GA

1998 unknown unknownAinsworth Street, Salisbury, Brisbane, Queensland

insufficient information 0 0 A 25 m wide landslide occurred in a 6 m high creek bank. The geology was fill over

alluvium. unknown

1999 2 unknown Guanaba, Qld insufficient information 0 0

Landslides in the hillside and in road fill ocurred during the period 4, 10 and 16 February 1999. An access road was partly destroyed, and a bitumen path and driveway damaged. Landslide debris covered a section of road. The trigger was probably heavy rain.

GA

2000 6 12 Dinmore, west of Brisbane, Qld rock fall 0 0

A section of road larger than a back yard swimming pool collapsed into a disused coal mine, also causing trees to fall in. The hole, about 40 m from houses, was continuing to enlarge, threatening power poles. The subsidence was thought to be caused by dry weather opening cracks on the surface, and then rain.

GA

2004 1 unknownBrook Street, Wooloowin, Brisbane, Queensland


A 5m wide slip on an 8 m high creek bank, at approximately 45 degrees, was triggered by heavy rain and diversion of surface water flow by recent nearby subdivision. The landslide involved fill and alluvium.

GA

2004 1 unknownMacquarie Street, Newstead, Brisbane, Queensland


A failure in an approximately 6 m high rock cutting (Neranleigh-Fernvale Beds) was triggered by heavy rain. It involved 1-2 cubic metres of rock and did minor damage to a parked car.

GA

2004 1 unknownBrunswick Street, New Farm, Brisbane, Queensland

rock fall 0 0 A minor rock fall, with a volume of less than 0.5 cubic metres, from a 2.5 m high cutting in Brisbane Tuff, was triggered by heavy rain. It landed on the footpath but did no damage.

GA

2004 1 unknownTristania Avenue, Bardon, Brisbane, Queensland


Two minor slips, triggered by heavy rain, on a cutting in Bunya Phyllite behind a house had a total volume of about 1 cubic metre. Some debris spilled on to the rear patio. The cutting was 6-7 m high at an angle of 45-60 degrees.

GA

unknown unknown unknownOxley, a suburb in Brisbane, on a small hill


Sliding has occurred after abnormal rains at a small hill in Oxley since 1971. The build-up of high pore pressures above a subsurface clay layer was identified as the main cause of sliding.

GA

unknown unknown unknown Fig Tree Pocket, Newlands Street

insufficient information 0 unknown This major landslide disturbed ten residential blocks and a street. GA


unknown unknown unknownNorth of ferry landing, Dutton Park, Brisbane, Queensland

debris slump 0 0 A large multiple rotational slide in weathered Bunya Phyllite and colluvium was probably caused by stormwater leakage and seepage. GA

unknown unknown unknownMikado Street, Hamilton, Brisbane, Queensland

insufficient information 0 0 A filled gully slipped in the late 1960s and the sewer could not be found or maintained. GA

unknown unknown unknownCliff downstream from Story Bridge, Brisbane, Queensland

rock fall 0 0 A 10-20 cubic metre rock fall from a cliff in Brisbane Tuff left a retaining wall suspended. It happened prior to 1975. unknown

Drawing Status:

FinalSCALE:

NonePROJECT / DRAWING No. 33529-001

Rev:

A

CLIENT:

Redland Shire Council PROJECT TITLE:

Redland Landslide Hazard Assessment Screen Grabs from Geoscience Australia Indicating the Location of Landslides


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APPENDIX 2 : LIST OF PREVIOUS LANDSLIDES FROM COUNCIL RECORDS

The following is a list of potentially unstable areas as provided by Redland Shire Council from their records and experience in the study area. REDLAND BAY Area of Boundary Street, Broadway Terrace and the Esplanade Orchard Beach Estate WELLINGTON POINT Champion Lane Coffey Geoscience report


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APPENDIX 3 : SITE REPORTS

Appendix 3 comprises the detailed site reports for the various selected sites throughout the Redland Shire area, including a Landslide Frequency Assessment Form. This was undertaken in accordance with the procedures detailed in the paper entitled “A Method of Zoning Landslide Hazards”, prepared by McGregor and Taylor.

SITE

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SITE REPORT NO. 1

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: West Mount Cotton Rd, Mount Cotton SITE NO.: 1 EASTING: 520,850mE NORTHING: 6,944,900mN

NATURAL SHALLOW LANDSLIDES

SITE DESCRIPTION: Th to the int

e site is located on the south western face of the Mount Cotton range. Access is via the unnamed road east of West Mount Cotton Road, some 5.1kms in a southerly direction from the Mount Cotton Roadersection.

REGIONAL GEOLOGY: Ge the absence of closer inspection could possible be Colluvial) from

weathering of Neranleigh-Ferndale formation omorphology: Residual soils (in

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by the Neranleigh-Ferndale formation which typically comprises interbedded Graywache (recrystallised sandstone), argillite (recrystallised mudstone), quartzite (recrystallised chert) and greenstone.

NATURAL SURFACE CONDITIONS: Slope Angle: The general topography comprises hill slopes estimated in the order of 30 degrees, with

gullies. steeper slopes experienced in the

Slope Shape: Slopes are typically convex or planar along the crest and upper slopes. Concave in the gullies. Slopes have been cleared for rural farming.

Surface Water Dra lopes), except in gullies and concave, inage: Expect to by good (crest and upper shummocky areas from previous landslides.

Evidence of Groun

dwater: Nil

Evidence of Instability: In the absence of closer inspection, three previous landslides were evident inheads of gullies from the road (some 800m or s

the o away). The landslides exhibit

e characteristics with slumped / hummocky ground appearance and debris ovement could also be anticipated in periods of heavy rainfall.

rotational slidflow. Creep m

SUBSURFACE CONDITIONS: Soil Type: Residual soils (possibly Colluvial) from weathering of Neranleigh-Ferndale formation

Soil Strength/Deptlopes.

Reasonable to assume in the range of 1-3m

h: Informed by the land owner that OTR is reasonably shallow (<1m) at the top of the hillside. Would expect deeper OTR within the gullies and towards the lower s

Ro ation ck Type: Neranleigh-Ferndale form

Substance St N/A

rength:

OTHER:

Landslides not allow access to property.

g dis cated that the two more prominent landslides (photos 3 & 4) occurred during heavy floods in approx. 1974

could not be inspected as land owner would

Durin cussions, the property owner indi

PHOTOGRAPHS

Photo 1: General photo of the hillside. Evidence of previous landslides present in the heads of the gullies was evident from the road. See following photos for more details (looking at gullies from left to right).

Photo 2: Hummocky ground resulted from a previous landslide in the head of the gully.

PHOTOGRAPHS

Photo 3: Distinct rotational/flow slide with extensive debris occurring in the head of the gully. Resulted from water concentration into the steeper slopes of the gullies during intense storm events in 1974.

Photo 4: Signs of a rotational/flow slide with extensive debris occurring in the head of a gully. Resulted from water concentration into the steeper slopes of the gullies during intense storm events in 1974.

UENCY ANALYSIS LANDSLIDE FREQ

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: 21/03/05 LOCATION: NO.: 1a E 00mN

NATURAL SHALLOW LANDS IDES – MAINLAND / GULLY

1 Basic Frequency

Redland Landslide Hazard Assessment DATE:West Mount Cotton Rd, Mount Cotton SITE EASTING: 520,850m NORTHING: 6,944,9

L

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 x Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5

3 Slope Shape Level Factor 7 Evidence of Groundwater Level Factor Crest or Ridge L 0.7 x None apparent L 0.7 Convex / Planar M 0.9 Minor moistness M 0.9 Rough / Irregular H 1.2 Generally wet H 1.5 x Concave H 1.5 Surface springs VH 3.0

4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 2.0 Sedimentary Rock M 1.0 Minor irregularity VH x Major irregularity VH 5.0 Volcanic rock H 1.1 0.0 Soils H 1.5 Scarps VH 1

5 Geomorphology Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 1.5 Residual soil <1m deep L 0.5 3 Slope Shape 1.5 Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 2.0 Colluvial soil <1m H 1.5 6 Concentration of Water 1.2 x Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 5.0 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 18.9

10 Site Frequency (1x9)

Note: The numerical factors allocated to these site features are based on judgment and experience. Prepared by: SH/TR Checked by: TR

LANDSLIDE FREQUENCY ANALYSIS

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: West Mount Cotton Rd, Mount Cotton SITE NO.: 1b EASTING: 520,850mE NORTHING: 6,944,900mN

NA PE TURAL SHALLOW LANDSLIDES – MAINLAND / UPPER SLO

1 Basic Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 x Upper slope M 0.9 x Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5

3 Slope Shape Level Factor 7 Evidence of Groundwater Level Factor Crest or Ridge L 0.7 x None apparent L 0.7 x Convex / Planar M 0.9 Minor moistness M 0.9 Rough / Irregular H 1.2 Generally wet H 1.5 Concave H 1.5 Surface springs VH 3.0

4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 Sedimentary Rock M 1.0 x Minor irregularity VH 2.0 Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 Geomorphology Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 1.5 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 1.5 x Colluvial soil <1m H 1.5 6 Concentration of Water 0.9 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 2.0 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 2.6



SITE REPORT NO. 2

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CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: West Mount Cotton Rd, Mount Cotton SITE NO.: 2 EASTING: 520,500mE NORTHING: 6,941,750mN


SITE DESCRIPTION: The site is located on the eastern sid e t Co n st of th or

he site isola st ou th R d e boundary, however (given e be representative of similar he south east section of th ount

e of W st Moun tto Rd, some 1.7kms northwe e Calif nia Creek Rd intersection. T ted ju tside e e land shir thlimited access) is anticipated to C

slopes of t e M otton Range.

REGIONAL GEOLOGY: Geomorphology: Colluvial soils fro e of e n formation

m weath ring N ra leigh-Ferndale

Geology: Reference to the ig ,0 0 e theunderlain by the N nleig nd e r pically comprises in bedd

e (recrystallised to d mudstone), qu chert) and g ton .

Beenle h 1:100 0 G ological series sheet indicates site is era h-Fer al fo mation which ty ter ed

Graywach(recrystallised

sandsreens

ne), arge

illite (recrystallise artzite

NATURAL SURFACE CONDI NSTIO : Slope Angle: The general topo c s i l r of 20 t g

with possible sli e e e ds the tography

ghtly steompriseper slop

hs

ll sxpe

opes estimated in the orderienced in the gully towar

o 22 dee of the

rees,

slope.

Slope Shape: Slopes are typicall nvex lan r o and upper slopes. The m rity o slope has been red ral i

y co or p a al ng the crest ajo f the clea for ru farm ng/grazing.

Surface Water Drainage: Expect to by od (c nd e to expect slig oordrainage in the gully at the f t here water will concentrat ngrainfall events.

go rest a upper slopes). Reasonabl htly p er toe o he slope, and w e duri

E

vidence of Groundwater: Nil

Evidence of Instability: SoCould p

me signs o inor i la i ( und) towards the mid pe. ossible be sult o w e m ould be the lye of the nd.

f ma re

rreguf slo

ritcr

es ep

hummocky groovement, or c

slo la

SUBSURFACE CONDITIONS: Soil Type: Colluvial soils from eathe of e n w ring N ra leigh-Ferndale formation

Soil Depth/Strength: As evident in e gu ff the h sonably shallo400mm) p e TR toward he low

nab ssum th rang of 1-3m for the mid slope where th minor larities are ev t.

rosion llies o s oulder of the road, OTR is rea w (approx. at the to of th hillside. Would expect deeper O s t er slopes. Reasoirregu

le to aiden

e in e e e

Rock Type: Neranleigh-Ferndale formation

Substance Strength: N/A

OTHER:

PHOTOGRAPHS

Photo 1: Looking across the slope, displaying the typical slope conditions.

Photo 2: Loo rom the edge of the road, down tking f he slope towards the gully.


CLIENT: Redland Shire Council PROJECT NO.: 33529 PR Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: West Mount Cotton Rd, Mount Cotton SITE NO.: 2a EASTING: 520,500mE NORTHING: 6,941,750mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / UPPER SLOPE

1 Basic Frequency

OJECT:

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 x Between 20 and 30 degrees M 1.2 x Upper slope M 0.9 Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 Sedimentary Rock M 1.0 x Minor irregularity VH 2.0 Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 Geomorphology Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 1.2 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 1.5 x Colluvial soil <1m H 1.5 6 Concentration of Water 0.9 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 2.0 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 2.0




CLIENT: Redland Shire Council PROJECT NO.: 33529

L TE NO.: 2b EASTING: 520,500mE NORTHING: 6,941,750mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / MID SLOPE

1 Basic Frequency

PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 OCATION: West Mount Cotton Rd, Mount Cotton SI

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 x Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x 1.5 Low grade Metamorphic rock M 1.0 Trees bent H x Minor irregularity VH 2.0 Sedimentary Rock M 1.0 VH 5.0 Volcanic rock H 1.1 Major irregularity Soils H 1.5 Scarps VH 10.0


(2x3x4x5x6x7x8x9) 3.6



SITE REPORT NO. 3

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CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Hillview Rd, Mount Cotton SITE NO.: 3 EASTING: 523,000mE NORTHING: 6,943,750mN


SITE DESCRIPTION: The site is located on the southern s m the ct

f he site com s ge lop g i ith filled land from the chick farm e slope

ide of Hillview Road, Mount Cotton some 200m fro interse ion o Mount Cotton Road. T prise ntle s in h llside w ento

wards the upper portion of th s.

REGIONAL GEOLOGY: Geomorphology: Residual soils fro athe of er l formation

m we ring N an eigh-Ferndale


e (recrystallised to d mudstone), qu chert) and g ton .


Graywach(recrystallised

sandsreens

ne), arge

illite (recrystallise artzite

NATURAL SURFACE CONDI NSTIO : Slope Angle: The general topo c s e e n the f

degrees. graphy omprise g ntl hill slopes measured to be i order o 11

Slope Shape: Slopes are typicall nvex lanar o extending d intoes e been cleared f r .

y co or p al ng the upper slopes and own the gullies. Slop hav o rural farming

Surface Water Drainage: Expect to b od (c nd a little more wconcentration in th ully a to f h urface drainag erallappears to be good

y go rest a upper slopes). Might get ater e g t the e o t e hillside, however s e gen y .

E ence of Groundwater: Nil vid

Evidence of Instability: Nil.

SUBSURFACE CONDITIONS: Soil Type: Residual soils from heri of Neranl weat ng eigh-Ferndale formation

Soil Depth/Strength: As no ccould not

uttings ere presen e t depth o OTR be appr . B on y a ea, it isle to exp TR d i th o

etc.. weciated

t n t

arpic

o the site an indication of thel subsoil conditions for the ar

f ased

reasonab ect O epths n e rder of 1-3m

Rock: Neranleigh-Fernd rmaale fo tion


OTHER:

PHOTOGRAPHS

Photo 1: General photo of the hillside. Looking outh lo g e gully from Hillview Road. s a n th


CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Hillview Rd, Mount Cotton SITE NO.: 3 EASTING: 523,000mE NORTHING: 6,943,750mN


1 Basic Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 x Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 x No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 Sedimentary Rock M 1.0 Minor irregularity VH 2.0 Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 Geomorphology Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 0.8 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 x Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 0.9 Colluvial soil <1m H 1.5 6 Concentration of Water 1.2 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 0.8 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 0.4



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SITE REPORT NO. 4

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Seaview Rd, Mount Cotton SITE NO.: 4 EASTING: 522,000mE NORTHING: 6,944,600mN


SITE DESCRIPTION: Th section of Sea

e site is located on the southern side of Seaview Road, Mount Cotton some 900m from the interMount Cotton Road. The site comprises the slopes on the southern side of the valley (to the south of view Road), and form part of the Mount Cotton Range.

REGIONAL GEOLOGY: Ge om weathering of Neranleigh-Ferndale formation

omorphology: Colluvial soils fr


NATURAL SURFACE CONDITIONS: Slope Angle: The general topography comprises slopes measured to be in the order of 20 to 25

degrees, with occasional steeper portions estimated to be up to some 30 degrees.

Slo anar along the upper slopes and extending down into

ible a result of bush fire, and vegetation regrowth has resulted.

pe Shape: Slopes are typically convex or plthe valley. There are some irregular and/or concave areas where signs of previous instability have occurred. It appears that a portion of the slope has been cleared at somestage, poss

Surface Water Drainage: Expect to by good (crest and upper slopes), with higher water concentration in thvalley and minor irregularities encountered on the hillsid

e e.

Evidence of Groundwater: Nil

Evidence of Instab slumps on the steeply sloping valley way, and an overall creep of colluvium down slope. ility: Multiple small

SUBSURFACE CONDITIONS: Soil Type: Colluvial soils from weathering of Neranleigh-Ferndale formation

Soil Depth/Strength: As no cuttings etc.. were present near to the site an indication of the depth of OTR could not be appreciated. Based on typical subsoil conditions for the area, it is reasonable to expect OTR depths in the order of 1-3m

Ro ation ck: Neranleigh-Ferndale form

Substance St N/A

rength:

OTHER:

As scarps o overall hill side these have been

bserved were small, to get a better representation of the treated as major irregularities in place of scarps.

PHOTOGRAPHS

Photo 1: General photo of the hillside taken from Seaview Road.

Photo 2: A closer view of one of the several small slumps which were evident in the steeper slopes of the valley.


CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 L 4 EASTING: 522,000mE

NORTHING: 6,944,600mN NATURAL SHALLOW LANDSLIDES – MAINLAND / LOWER SLOPE

1 Basic Frequency

OCATION: Seaview Rd, Mount Cotton SITE NO.:

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 x Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 x Lower slope H 1.5

3 Slope Shape Level Factor 7 Evidence of Groundwater Level Factor Crest or Ridge L 0.7 None apparent L 0.7 x Convex / Planar M 0.9 x Minor moistness M 0.9 Rough / Irregular H 1.2 Generally wet H 1.5 Concave H 1.5 Surface springs VH 3.0

4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 Sedimentary Rock M 1.0 Minor irregularity VH 2.0 Volcanic rock H 1.1 x Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0


(2x3x4x5x6x7x8x9) 14.6



SITE REPORT NO. 5

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CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Shore Street, Cleveland SITE NO.: 5 EASTING: 528,130mE NORTHING: 6,955,400mN


SITE DESCRIPTION: The site is located adjacent to the cu c r e d. Theo l slope some in w i s the higher plateau of the m land

site is resen of th co onditions experienced adjace o thea ist d W area exhibits some of the more steeper

sl

l-de-sa at the easte n nd of Shore Street, Clevelan site c mprises a small natura 4.0m height h ch separate ainfrb

om the mud flats below. The salt flows in the Cleveland, Orm

repon an

tativeellingt

e astal cs, and

nt t on

opes found in such areas.

REGIONAL GEOLOGY: G omorphology: Red brown, brown lty clay a o s from weathering of basalt flows e si . Residu l s il

Geology: Reference to the B leigh 0,0 0 e heet indicates the si s derlain by the B t Flo

een 1:10 0 G ological series s te iun asal ws.

NATU CE CONDITRAL SURFA IONS: Sl pe Angle: The general topogr hy com rises m ll slopes, some 4m in height measured be in

the order of 30 to r e to ap p s a to

32 deg ees in th s eeper portions.

Slope Shape: Slopes are typivegetation. Som

call nvex om a lopes with m r e m or irr rit s d andslides were evide

y coin

becegula

ing plie

nar towards the lower s signs of small former l

ino an nt.

Surface Water Drainage: pud

Expect to by good (crest and upper water concentrdles on the low lopes e e

slopes), with somee in photos 2.

ation and er s , as can b s


Evidence of Instability: 2No. small sl s (ro na li w is flow and transverse with e umping) were evi t on ope, as from the following photos.

ump tatio l s p ith debr somsl den the sl can be seen

SUBSURFACE CONDITIONS: Soil Type: Residual soils from eathe of s w ring ba alt flows

Soil Depth/Strength: In the ait would

bsence of rther i rmatio , with imity of the site to e oceanbe reason to a e t e uld be in excess o ome

fuable

nfossum

n th

the close proxdepths of OTR wo

thf s

hat

3m

Rock Type: Basalt Flows


OTHER:

Slope is located on council lan jacen o resi ti ing milder sl es.

Note: some of the residential properties located on the edge of such plateaus (more commonly m cut and fill

r ften associated with some types of retaining structures) resulting in stepening of the ope and alteration of natural surface drainage which can further contribute to the potential

l lide hazard if not undertaken thoroughly. In other areas of gentler longer running slopes stepped construction to suit the lye of the land has been implemented.

d ad t t den al developments exhibit op

Wellington Point and Ormiston) comprise the leveling off on the building site froope ations (onatural slfor ands

PHOTOGRAPHS

Photo 1: Small rot g at the top of the small slope. ational slump and debris flow occurrin

Photo 2: Small translational landslide with some slumping at the toe of the slope, along with puddles / water concentrations after minor rainfall.


CL Redland Shire Council PROJECT NO.: 33529 PR LOCA SITE NO.: 5 EASTING: 528,130mE NORTHING: 6,955,400mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / LOWER SLOPE

1 Basic Frequency

IENT: OJ CT: Redland Landslide Hazard Assessment DATE: 21/03/05

TION: Shore Street, Cleveland E

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 x Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 x Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 1.5 Low grade Metamorphic rock M 1.0 Trees bent H 2.0 Sedimentary Rock M 1.0 Minor irregularity VH x 5.0 Volcanic rock H 1.1 x Major irregularity VH Soi 10.0 ls H 1.5 Scarps VH

5 Geomorphology Level Factor SUMMARY Factor 5 Rock at surface VL 0.1 2 Slope Angle 1. 9 Residual soil <1m deep L 0.5 3 Slope Shape 0. 1 Residual soil 1-3m deep M 0.9 4 Site Geology 1.x 5 Residual soil >3m deep H 1.5 5 Geomorphology 1. C 5 olluvial soil <1m H 1.5 6 Concentration of Water 1. 7 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0. 0 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 5. Fill (slope regarding) VH 5.0 .7 9 Relative Frequency

(2x3x4x5x6x7x8x9) 11


Note: PrChecked by: TR

The numerical factors allocated to these site features are based on judgment and experience. epared by: SH/TR

SITE REPORT NO

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. 6

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Douro Road, Wellington Point SITE NO.: 6 EASTING: 522,850mE NORTHING: 6,959,950mN


SITE DESCRIPTION: The site is located at the end of the cul-de-sac at the western end of Douro Road, Wellington Point. The site comprises a natural slope in excess of some 6.0m in height which separates the higher plateau of the mainland from the mud flats below. The site is representative of the coastal conditions experienced adjacent to the basalt flows in the Cleveland, Ormiston and Wellington areas, and exhibits some of the more steeper slopes found in such areas.

REGIONAL GEOLOGY: Geomorphology: Red brown, brown silty clay. Residual soils from weathering of basalt flows

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by the Basalt Flows.

NATURAL SURFACE CONDITIONS: Slope Angle: The general topography comprises natural slope, in excess of some 6m in height

of 12 to 18 degrees, with slope angles up tions of similar areas.

atypical slopes were measured to be in the orderto some 22 degrees measured in the steeper por

Slope Shape: Slopes are typically convex becom ng planar towards the lower slopes with minor vegetation and landscaping occurr g in some areas of the slopes. Some of the landscaping works (i.e benched levels) has resulted in changing the natural surface draining which could result in water concentration into the slope.

iin

Surface Water Drainage: Expect to by good (crest and upper slopes), with some water concentration and puddles on the lower slopes and in some of the landscaped areas.


Evidence of Instability: Not apparent, recent landscaping of slope would have hidden scarps if present.

SUBSURFACE CONDITIONS: Soil Type: Residual soils from weathering of basalt flows

Soil Depth/Strength: In the absence of further information, with the close proximity of the site to the ocean it would be reasonable to assume that the depths of OTR would be in excess of some 3m

Rock: Basalt Flows


OTHER:

Note: some of the residential properties located on the edge of such plateaus (more commonly Wellington Point and Ormiston) comprise the leveling off on the building site from cut and fill operations (often associated with some types of retaining structures) resulting in stepening of the natural slope and alteration of natural surface drainage which can further contribute to the potential for landslide hazard if not undertaken thoroughly. In other areas of gentler longer running slopes stepped construction to suit the lye of the land has been implemented.

PHOTOGRAPHS

P o 1: Looking from the bottom of t e slope t wards t cul-de-sac at the end of Douro Road. lthoughhot h o he A no photo, landscapin s e d ti round ha et evident in the g work for the r si en al properties in the backg s result d in ch ge coul rease slope. anging of natural surface draina and d inc water concentration into the

CY ANALYSIS LANDSLIDE FREQUEN

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 21/03/05 LOCATION: Douro Road, Wellington Point SITE NO.: 6 EASTING: 522,850mE NORTHING: 6,959,950mN

NATURAL SHA OWER SLOPE

1 Basic

LLOW LANDSLIDES – MAINLAND / L

Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 x B 8 etween 10 and 20 degrees L 0.8 Crest M 0. B 0.9 etween 20 and 30 degrees M 1.2 Upper slope M B 1.5 Mid slope H 1.2 etween 30 and 45 degrees H M VH 2.0 x Lower slope H 1.5 ore than 45 degrees

3 vel Factor Slope Shape Level Factor 7 Evidence of Groundwater Le Crest or Ridge L 0.7 x None apparent L 0.7 x Convex / Planar M 0.9 Minor moistness M 0.9 rregula 1.5 Rough / I r H 1.2 Generally wet H Concave Surface springs VH 3.0 H 1.5

4 S 8 Evidence of instability Level Factor ite Geology Level Factor .8 High grade Metamorphic rock L 0.9 x No signs of instability L 0 eta 5 Low grade M morphic rock M 1.0 Trees bent H 1. Sedimentary Rock M 1.0 Minor irregularity VH 2.0 x Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 Geomorpholog Factor y Level Factor SUMMARY VL 0.1 2 Slope Angle 0.8 Rock at surface Residual soil <1m deep L 0.5 3 Slope Shape 0.9 1.1 Residual soil 1-3m deep M 0.9 4 Site Geology x Residual soil >3m 5 deep H 1.5 5 Geomorphology 1. Colluvial soil <1m H 1.5 6 Concentration of Water 1.5 C 2.0 7 Evidence of Groundwater 0.7 olluvial soil 1-3m deep VH oil >3 nce of instability 0.8 Colluvial s m deep VH 4.0 8 Evide Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 1.0


N located to these site features are based on judgment and experience. PChecked by: TR

ote: The numerical factors alrepared by: SH/TR

SITE

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SITE REPORT NO. 7

CL 29 PROJE LOCA E NO.: 7 EASTING: 522,600mE NORTHING: 6,957,050mN


IENT: Redland Shire Council PROJECT NO.: 335CT: Redland Landslide Hazard Assessment DATE: 22/03/05

TION: Old Cleveland Road, Birkdale SIT

SITE DESCRIPTION: The site is located on Old Cleveland Road, Birkdale some 900m east of the Randell Road intersection. The site is located to the immediate north of the Birkdale Landfill with nearby residential land. The site comprises some of the steeper of the gentle slopes encountered throughout the Alexandria Hills area and is representative of steeper slopes throughout the Woogaroo Sub-group geological formation.

REGIONAL GEOLOGY:

Geomorphology: Brown, yellow brown, red brown silty clay with some sand. Residual soils.

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by the Woogaroo Sub-group of the Bundamba formation which typically comprises sandstone, siltstone, shale and conglomerate.

NATURAL SURFACE CONDITIONS:

Slope Angle: The general topography comprises gentle slopes measured to be in the order of 4 to 5 degrees.

Slope Shape: Slopes are typically convex at the crest and planar along the remainder of the slopes. Residential development occupies the majority of slopes in the area with some vegetation. Stepped construction to suit the lye of the land, with minor cut / fill operations and retaining walls has been implemented

Surface Water Drainage: Expect to by good (crest and upper slopes). Might get a little more water concentration towards the lower portions of the slope and Tarradarrapin Creek where land flattens out. However surface drainage generally appears to be good.

Ev

idence of Groundwater: Nil

Ev

idence of Instability: Nil.

SUBSURFACE CONDITIONS:

Soil Type: Residual soils

So il Depth/Strength: Batters cut for the road on the eastern (opposing) hillside indicate the OTR is some1m depth towards the crest/upper portion of the hill (refer to photo 2) and greater than some 2m towards the toe of the hill

Rock Type: Woogaroo Sub-group of the Bundamba formation

Su

bstance Strength: N/A

OTHER:

Despite the sites close proximity to the Birkdale Landfill, due to extensive landfill operations this site report is not representative of such an area.

PHOTOGRAPHS

Photo 1: General photo of the hillside. Looking east along the Old Cleveland Road.

Photo 2: Photo of the batter cut for the road on the eastern (opposing) hillside indicates the OTR is some 1m depth towards the crest/upper portion of the hill.


CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Old Cleveland Road, Birkdale SITE NO.: 7 EASTING: 522,600mE NORTHING: 6,957,050mN


1 Basic Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor x Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 x No signs of instability L 0.8 Low grade Metamorphic rock M 1.0 Trees bent H 1.5 x Sedimentary Rock M 1.0 Minor irregularity VH 2.0 Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 eomorphology Level Factor U MARY Fa or G S M ct Rock at surface VL 0.1 2 Slope Angle 0.5 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 x Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 0.9 Colluvial soil <1m H 1.5 6 Concentration of Water 1.2 Colluvial soil 1-3m deep VH r 2.0 7 Evidence of Groundwate 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 0.8 Fill (slope regarding) VH 5.0 9 R

(2x3elative Frequency

x4x5x6x7x8x9) 0.27


No The numerical factors allocated to these site features are d ce. repared by: SH

te: base on judgment and experienPChecked by: TR

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SITE REPORT NO. 8

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Howlett Road, Capalaba SITE NO.: 8 EASTING: 517,700mE NORTHING: 6,953,550mN


SITE DESCRIPTION:

ThweThIps

e site is located behind the residential properties to the south of Howlett Road, Capalaba some 900m st of its intersection with Mount Cotton Road and is bounded by the Tingalpa Reservoir to the south. e site exhibits some of the steeper slope angles encountered in the immediate area and within the wich Coal Measures of the Redland shire area. The majority of the hillside is densely vegetated.

REGIONAL GEOLOGY:

Ge

omorphology: Residual soils.

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by the Ipswich Coal Measures which typically comprises conglomerate, sandstone, shale, tuff and coal seams.


Slo gentle slopes measured to be in the order of 12 to 15 degrees. Steeper slope angles may be possible on some of the densely vegetated slopes.

pe Angle: The general topography comprises

Slope Shape: Slopes are typically planar.

Surface Water Drainage: Expect to by good with surface runoff into the Tingalpa Reservoir.

Evidence of Groun toe dwater: Nil, reasonable to expect groundwater to be close to reservoir level towardsof slope.



Soil Type: Residual soils

Soil Depth/Strength: Batters cut for Howlett Road indicate that towards the crest of the hill, the PTR is within 1m from the surface, reasonable to assume this slightly thicker depths of OTR

ns of the slopes, reasonable to assume would be in the order of towards the lower portio1 to 3m.

Rock Type: Woogaroo Sub-group of the Bundamba formati on


OTHER: The landslide frequency analysis was carried out on a cleared section of land at the back of a

residential property as access to the more densely vegetated slopes (more typical of the steeper was not possible.

slopes in the area)

PHO OT GRAPHS

Photo 1: General photo of the hillside, showing the densely vegetated slopes more typical of the steeper slopes of the area.

Photo 2: Photo shown the cleared apportion of the slope at the back of a residential property where the frequency analysis was undertaken. Looking at the slope from the shore of the Tingalpa Reservoir


CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Howlett Road, Capalaba SITE NO.: 8 EASTING: 517,700mE NORTHING: 6,953,550mN


1 Basic Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 x Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor L 0.8 High grade Metamorphic rock L 0.9 x No signs of instability Low grade Metamorphic rock M 1.0 Trees bent H 1.5 x Sedimentary Rock M 1.0 Minor irregularity VH 2.0 Volcanic rock H 1.1 Major irregularity VH 5.0 Soils H 1.5 Scarps VH 10.0

5 Geomorphology Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 0.8 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 x Residual soil 1-3m deep M 0.9 4 Site Geology 1.0 Residual soil >3m deep H 1.5 5 Geomorphology 0.9 Colluvial soil <1m H 1.5 6 Concentration of Water 1.2 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.7 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 0.8 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 0.44


Note: The numerical factors allocated to these site features are based on judgment and experience. Prepared by: SH Checked by: TR

SITE REPORT NO. 9

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CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Avalon Road, Sheldon SITE NO.: 9 EASTING: 517,800mE NORTHING: 6,946,750mN


SITE DESCRIPTION:

The site is located along the power e h rse to t ome 700m n west g t w easement from its intersectio ith

heldon ad be s site exhibits some of the steeper l imme te are

line eas ment running t rough the conservation reve the eas of Avalon Road, Sheldon s orth alon he po er line n wAs

valon road (north of where Sope angles encountered in the

Rodia

comea.

Ford Road). The

REGIONAL GEOLOGY:

Geomorphology: Colluvial soils fro e of e n formation



e (recrystallised to d mudstone), qu chert) and green ton .


Graywach sands ne), argillite (recrystallise artzite (recrystallised s e

NATURAL SURFACE CONDI NSTIO :

Slope Angle: The general topo c s lo s der of 16degrees, with ste p rienced

graphyeper slo

omprisees expe

s pe measured to be in the orin gullies.

to 20

Sl call v r s t e cre becoming pl r es. Except f t for the power line ease t and

ess tracks, the s es are sel v e extensive signs osioand scour due to su ce ru on e le to 2).

ope Shape: Slopes are typitowards the lower slop

y conca e towa dor

hhe

st and upland cleared

per slopes, anamen

acc lop den y eg tated, and there are of er n rfa noff th c ared areas (see pho

Surface Water Drainage: Expect to by oderate to good r t and upper slopes, and poor with potential for significant water concent ibiting erosion, gullies and towards the lower slopes.

m foratio

the cresn in areas exh

Evidence of Groundwater: Nil.

Evidence of Instability: Nil, surface , and signs of insta from erosion (water runoff). scars bility resulting


Soil Type: Colluvial soils from eathe of e n ation w ring N ra leigh-Ferndale form

Soil Depth/Strength: From evidence i all c gs an r TR is typowards t est o hill (only n some areas),

some 2m ards the toe f

n sm uttin d e osion gullies the depth of O ically less than 1m t he cr f the a few hundred millimetres iand upto tow o the slope and in the gullies

Rock Type: Neranleigh-Fernd rmaale fo tion


OTHER:

Due to potential for significant water concentration into the slopes resulting from erosion gullies, sis.

n prevented close inspection of the hill slopes for signs of previous landslides. would be reasonable to expect some small landslides and possible slow creep movement

of the colluvium on some of the steeper angles in the gullies.

these have been treated as minor irregularities in performing the frequency analy

Dense vegetatioHowever

PHOTOGRAPHS

Photo 1: Looking south east along the cleared power line easement at the general hillside.

Photo 2: Loo at the opposite side of the hill shown above. Erosionking gullies, over some 1.0m depth in areas have been cut in the hill side and gullies resulting from surface water runoff. Occurred mostly in cleared land for access tracks.


CL Redland Shire Council PROJECT NO.: 33529 PR 5 LOCA SITE NO.: 9a EASTING: 517,800mE NORTHING: 6,946,750mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / UPPER SLOPE

1 Basic Frequency

IENT: OJ CT: Redland Landslide Hazard Assessment DATE: 22/03/0

TION: Avalon Road, Sheldon E

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 x Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 x Upper slope M 0.9 Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5


4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x 1.5 Low grade Metamorphic rock M 1.0 Trees bent H 2.0 Sedimentary Rock M 1.0 x Minor irregularity VH 5.0 Volcanic rock H 1.1 Major irregularity VH Soi 10.0 ls H 1.5 Scarps VH

5 Geomorphology Level Factor SUMMARY Factor 8 Rock at surface VL 0.1 2 Slope Angle 0. 9 Residual soil <1m deep L 0.5 3 Slope Shape 0. 0 Residual soil 1-3m deep M 0.9 4 Site Geology 1. 5 Residual soil >3m deep H 1.5 5 Geomorphology 1.x C 9 olluvial soil <1m H 1.5 6 Concentration of Water 0. 7 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0. 0 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 2. Fill (slope regarding) VH 5.0 4 9 Relative Frequency

(2x3x4x5x6x7x8x9) 1.



The numerical factors allocated to these site features are based on judgment and experience. epared by: SH


CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 L .: 9b EASTING: 517,800mE NORTHING: 6,946,750mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / LOWER SLOPES

1 Basic Frequency

OCATION: Avalon Road, Sheldon SITE NO

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 x Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 Mid slope H 1.2 More than 45 degrees VH 2.0 x Lower slope H 1.5

3 Slope Shape Level Factor 7 Evidence of Groundwater Level Factor Crest or Ridge L 0.7 x None apparent L 0.7 Convex / Planar M 0.9 Minor moistness M 0.9 x Rough / Irregular H 1.2 Generally wet H 1.5 Concave H 1.5 Surface springs VH 3.0

4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x Low grade Metamorphic rock M 1.0 Trees bent H 1.5 Sedimentary Rock M 1.0 x Minor irregularity VH 2.0 .0 Volcanic rock H 1.1 Major irregularity VH 5 Scarps VH 10.0 Soils H 1.5


(2x3x4x5x6x7x8x9) 4.0



SITE REPORT NO. 10

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0 CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Mount View Road, Mount Cotton SITE NO.: 10 EASTING: 521,700mE NORTHING: 6,945,100mN


SITE DESCRIPTION:

The site is located on the north east o o th tow ew Road, M Cot e site e ypical characteristics of the cleared

a of the steeper s s o M otton Range.

ern face of the M unt C tton range, some 200m sou ards thetermination of Mount Vi ount ton. Th xhibits tl

nd for rural farming on some lope f the ount C

REGIONAL GEOLOGY:

Geomorphology: Colluvial soils fro e of e n formation



e (recrystallised to d mudstone), qu chert) and green ton .


Graywach sands ne), argillite (recrystallise artzite (recrystallised s e


Slope Angle: The general topo c s i l r of 20 t gwith steeper slop r e u

graphyes expe

ompriseienced in

h th

ll s g

opes estimated in the ordellies.

o 25 de rees,

Slope onvex or in the ifica ortio the o e cleared for rural farmin

Shape: Slopes are typically cgullies. A sign

planar along tn of

he crest s have been

and upper slopes. Concavent p sl p g.

Surface Water Drainage: Expect to b od (c nd onchummocky areas f prev landsl e

y go rest a upper slopes), except in gullies and c ave, rom ious id s.


Evidence of Instability: 2gu

No. disturb ummo ou ous slips towards the headllies. Both exh w teristics.

ed / hibiting d

cky gebris flo

r a

nd from prevind creep charac

s of the


Soil Type: Colluvial soils fro eathe of e n formation m w ring N ra leigh-Ferndale

Soil Depth/Strength: In absence of fu r info ion, i o to assume similar conditixperienced thr hout t ou t bly shall OTR

(<1m) at the top of the hillside, and deeper OTR in the range of 1-3m within the gullies rds the lo slop

rthe rmat t w uld be reasonable ons as e oug he M n Cotton Range. That is reasona ow

and towa wer es.

Rock Type: Neranleigh-Fernd rmaale fo tion


OTHER:

PHOTOGRAPHS

Photo 1: Looking at the general hillside of the north eastern face of the Mount Cotton Range. Evidence of pre olluvium in the heads of the gullies. Smaller, more vious landslides with debris flows and creep of cprominent signs were also evident in the valley to the west.

Photo 2: A closer view of the hummocky ground resulting from a previous landslide in the head of the gully.


CL N 529 PR ard Assessment DATE: 22/03/05 LOCATION: Mount View Road, Mount Cotton SITE NO.: 10

EASTING: 521,700mE NORTHING: 6,945,100mN

NATURAL SHALLOW LANDSLIDES – MAINLAND / GULLY

1 Basic Frequency

IE T: Redland Shire Council PROJECT NO.: 33CT: Redland Landslide HazOJE

2 Slope Angle Level Factor 6 Concentration of Water Level Factor Less than 10 degrees VL 0.5 Ridge L 0.7 Between 10 and 20 degrees L 0.8 Crest M 0.8 x Between 20 and 30 degrees M 1.2 Upper slope M 0.9 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 More than 45 degrees VH 2.0 Lower slope H 1.5

3 Slope Shape Level Factor 7 Evidence of Groundwater Level Factor Crest or Ridge L 0.7 x None apparent L 0.7 Convex / Planar M 0.9 Minor moistness M 0.9 Rough / Irregular H 1.2 Generally wet H 1.5 x Concave H 1.5 Surface springs VH 3.0

4 Site Geology Level Factor 8 Evidence of instability Level Factor High grade Metamorphic rock L 0.9 No signs of instability L 0.8 x 1.5 Low grade Metamorphic rock M 1.0 Trees bent H 2.0 Sedimentary Rock M 1.0 Minor irregularity VH 5.0 Volcanic rock H 1.1 x Major irregularity VH Soi 10.0 ls H 1.5 Scarps VH

5 Geomorphology Level Factor SUMMARY Factor 2 Rock at surface VL 0.1 2 Slope Angle 1. 5 Residual soil <1m deep L 0.5 3 Slope Shape 1. 0 Residual soil 1-3m deep M 0.9 4 Site Geology 1. 0 Residual soil >3m deep H 1.5 5 Geomorphology 2. C 2 olluvial soil <1m H 1.5 6 Concentration of Water 1.x 7 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0. 0 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 5. Fill (slope regarding) VH 5.0 .1 9 Relative Frequency

(2x3x4x5x6x7x8x9) 15



The numerical factors allocated to these site features are based on judgment and experience. epared by: SH/TR

SITE REPORT NO

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. 11

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 L 11 EASTING: 529,500mE NORTHING: 6,936,250mN


OCATION: Rocky Passage Road, Redland Bay SITE NO.:

SITE DESCRIPTION:

The site is located to the west of Rocky Passage Road, Redland Bay some 1.6kms from the intersection of Beenleigh – Redland Bay Road. The slope occupies the residential / rural properties of 91-117 Rocky Passage Road, and exhibits some of the steeper slope angles in the Redland Bay area.

REGIONAL GEOLOGY:

Geomorphology: Residual soils from weathering of Neranleigh-Ferndale formation



Slope Angle: The general topography comprises hill slopes measured in the order of 18 to 20 degrees on some of the steeper slopes, with milder slope angles towards the crest and lower slopes.

Slope Shape: Slopes are typically convex along the crest and upper slopes becoming planar. The slopes have been cleared for rural/ dential purposes, however the majority of slop s resi ein the area are still densely vegetated.

Surface Water Drainage: Expect to by good over the majority of the slope, may get some minor water concentration in the valleys at the toe of the slopes where the slope angles are milder.




Soil Type: Residual soils from weathering of Neranleigh-Ferndale formation

Soil Depth/Strength: Inspection of the batters cut for Rocky Passage Road indicate that rock structure is evident at some 1m depth towards the upper slope areas, and reasonable to expect deeper OTR in the range of 1-3m towards the lower slopes

Rock Type: Neranleigh-Ferndale formation


OTHER:

PHOTOGRAPHS

Photo 1: Looking west from Rocky Passage Ro at t n ral hillside. Te site exhibits some o he ad he ge e f tst les in the Redland Bay area esidential purposeeper slope ang . The slopes have been cleared for rural/r es, ho r s n t ea re tiwever the majority of the steepe lopes i he ar a s ll densely vegetated.

CY ANALYSIS LANDSLIDE FREQUEN

CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05 LOCATION: Rocky Passage Road, Redland Bay SITE NO.: 11 EASTING: 529,500mE NORTHING: 6,936,250mN

NATURAL D / GULLY

1 Basic

SHALLOW LANDSLIDES – MAINLAN

Frequency

2 Slope Angle Level Factor 6 Concentration of Water Level Factor L 7 ess than 10 degrees VL 0.5 Ridge L 0.x B 0.8 etween 10 and 20 degrees L 0.8 Crest M B Upper slope M 0.9 etween 20 and 30 degrees M 1.2 Between 30 and 45 degrees H 1.5 x Mid slope H 1.2 M VH 2.0 Lower slope H 1.5 ore than 45 degrees

3 r Level Factor Slope Shape Level Factor 7 Evidence of Groundwate Crest or Ridge L 0.7 x None apparent L 0.7 x Planar 0.9 Convex / M 0.9 Minor moistness M Rough / Irregula r H 1.2 Generally wet H 1.5 Concave 3.0 H 1.5 Surface springs VH

4 Site Geology Level Factor 8 Evidence of instability Level Factor H x No signs of instability L 0.8 igh grade Metamorphic rock L 0.9 x eta 1.5 Low grade M morphic rock M 1.0 Trees bent H Sedimentary Ro ularity VH 2.0 ck M 1.0 Minor irreg ck 5.0 Volcanic ro H 1.1 Major irregularity VH Soi .0 ls H 1.5 Scarps VH 10

5 Geomorpholog Factor y Level Factor SUMMARY 0.8 Rock at surface VL 0.1 2 Slope Angle Residual soil <1 0.9 m deep L 0.5 3 Slope Shape x Residual soil 1-3 0.9 4 Site Geology 1.0 m deep M H 1.5 5 Geomorphology 0.9 Residual soil >3m deep Colluvial soil <1m H 1.5 6 Concentration of Water 1.2 0.7 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater Colluvial soil >3m deep VH 4.0 8 Evidence of instability 0.8 F 5.0 ill (slope regarding) VH 0.44 9 Relative Frequency

(2x3x4x5x6x7x8x9)


Note: The numerical f ce. P H Checked

actors allocated to these site features are based on judgment and experienrepared by: S

by: TR

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SITE REPORT NO. 12

CL : 33529 PR E 5 LOCA 12 EASTING: 530,600mE NORTHING: 6,943,400mN


IENT: Redland Shire Council PROJECT NO.OJ CT: Redland Landslide Hazard Assessment DATE: 22/03/0

TION: Wilson Esplanade Drive, Redland Bay SITE NO.:

SITE DESCRIPTION:

The site is located to the east of Wilson Esplanade Drive, Redland Bay some 150m from the intersection of School of Arts Road. The site is situated at the rear of the residential properties and council land and comprises natural slopes of some 8-10m in height which separates the higher plateau of the mainland from the mud flats along the coast to the east. The site is representative of the coastal conditions experienced along the basalt flows in Redland Bay and Victoria Point (similar to that observed in Cleveland, Ormiston and Wellington Point areas), and exhibits some of the more steeper slopes found in such areas.

REGIONAL GEOLOGY:

Geomorphology: Red brown, brown silty clay. Residual soils from weathering of basalt flows

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by the Basalt Flows.


Slope Angle: The general topography comprises natural slopes of some 8-10min height typical slope angles are of the order of 10 to 15 degrees, with the steeper slope angles measured to be in the order of 20 to 22.

Slope Shape: Slopes are typically convex becoming planar towards the lower slopes with varied levels of vegetation and landscaping along the slopes. Some landscaping works (i.e benched levels, small retaining structures) has resulted in changing the natural surface draining which could increase the likelihood of water concentration into the slope.

Surface Water Drainage: Expect to by good (crest and upper slopes), with some water concentration on lower slopes and in some of the landscaped areas.

the

Ev


Ev

idence of Instability: Nil.


Soil Type: Residual soils from weathering of basalt flows

Soi ean

l Depth/Strength: In the absence of further information, with the close proximity of the site to the ocit would be reasonable to assume that the depths of OTR would be in excess of some3m

Rock Type: Basalt Flows

Su


OTHER:

PHOTOGRAPHS

Photo 1: Looking up at the hillside from the mad flats towards the rear of the residential properties along Wilson Esplanade Drive. Some landscaping and small retention wall has altered natural drainage paths.

Photo 2: Looking at a cleared section e hi m h owards the rear of the residentia of th llside fro t e mad flats t l properties along Wilson Esplanade Drive.

LANDSLIDE FREQUENCY A LNA YSIS

CLIENT: Redland Shire Council PROJECT: Redland Landslide Hazard Assessment DATE: 22/03/05

O ilson Esplanade rive dl d TE NO.: TING: 5 ,600

ORTHING: 6,9 400URAL SHALLOW ND D S / LOWER SL

cy

PROJECT NO.: 33529

L CATION: W D , Re an Bay SI 12 EAS 30 mE N 43, mN

NAT LA SLI E – MAINLAND OPE

1 Basic Frequen

2 Slope Angle Level F ion of Water Level F actor 6 Concentrat actor Less than 10 degrees VL 0.5 Ridge L 0.7 x Between 10 and 20 degrees L 0.8 Crest M 0.8 Between 20 and 30 degrees lope M 1.2 Upper s M 0.9 Between 30 and 45 degrees 1. id slope 1.H 5 M H 2 More than 45 degrees e VH 2.0 x Lower slop H 1.5

3 Slope Shape Level F roundwater Level F actor 7 Evidence of G actor Crest or Ridge L 0.7 None apparent L 0.7 x Convex / Planar M 0.9 x Minor moistness M 0.9 Rough / Irregular H 1.2 Generally wet H 1.5 Concave H 1.5 Surface springs VH 3.0

4 Site Geology L F Level F evel actor 8 Evidence of instability actor High grade Metamorphic rock o signs of instability 0 L 0.9 x N L .8 Low grade Metamorphic rock M 1. 0 Trees bent H 1.5 Sedimentary Rock M 1.0 Minor irregularity VH 2.0 x olcanic rock 1 VH 5 V H .1 Major irregularity .0 Soils H 1.5 Scarps VH 10.0

5 Geomor holop gy Level Factor SUMMARY Factor Rock at surface VL 0.1 2 Slope Angle 0.8 Residual soil <1m deep L 0.5 3 Slope Shape 0.9 Residual soil 1-3m deep M 0.9 4 Site Geology 1.1 x Residual soil >3m deep H 1.5 5 Geomorphology 1.5 Colluvial soil <1m H 1.5 6 Concentration of Water 1.5 Colluvial soil 1-3m deep VH 2.0 7 Evidence of Groundwater 0.9 Colluvial soil >3m deep VH 4.0 8 Evidence of instability 0.8 Fill (slope regarding) VH 5.0 9 Relative Frequency

(2x3x4x5x6x7x8x9) 1.3



SITE REPORT NO. 13

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3 CLIENT: Redland Shire Council PROJECT NO.: 33529 PROJECT: Redland Landslide Hazard Assessment DATE: 23/03/05 LOCATION: Mining Company Road, Dunwich SITE NO.: 13 EASTING: 540,550mE NORTHING: 6,957,750mN


SITE DESCRIPTION:

The site is located adjacent to Mini p D w Mitch cprises a sm lly o h une to the east of Dunwich, d sho

o ative o axim nd n le d dune materials and lea tree

ng Com any Road, un ich some 150m east of the ell Cres ent intersection. The site com all gu at the toe f t e sand d an ws s

me signs of instability, indic f m um sa a g s in the san ning s.

REGIONAL GEOLOGY:

G omorphology: As below e

Geology: Reference to the B leigh 0,0 0 e heet indicates the si s derlain by Dune of ter r

een 1:10 0 G ological series s te iun Sand Qua na y Age.


Slope Angle: Slope angles in t es in areas wherethere was no veg w s d e ear their

ngl f 37 t de rees with thicker etati

he gullyetation,

were measurith the

eds

to be some 30 to 35 degrexhibiting characteristics n

an angle of

repose. Slope a es o o 43 g were measured in areas veg on.

Slope Shape: Typically convex t rd th per slopes generally planar toward e lowpes. Some irreg sect and o small gully.

owa e up becoming s th er slo ular ions c ncave areas with in the

Surface Water Drainage: Given the presence o e n rate surface dra witseepage of surface ter in e groundwater table.

f dun sa ds, would expect mode inage h wa to th

E ence of Groundwater: Nil vid

Evidence of Instability: In areas of th lly, w th v e y or thin, slope anglg features icati ey e / e of repose were enco tered.

slopping trees with g l r

e gu here e eg tation was patch e exhibitinSeveral

ind ng thin the

arul

aty a

near their angle also evident.

un


Soil Type: Dune sand: Light g , whi ne ra erey te, fi g in d sand

Soil Depth/Strength: Greater than 3m

Rock Type: N/A


OTHER:

Surface samloose sta

ples were taken at this site. The angle of repose of this material was measured in a very te to be in the order of 30 to 38 degrees.

PHOTOGRAPHS

Photo 1: Looking rds the small gully at the toe of the sand dune. east from Mining Company Road towaSlope exhibits some signs of instability, indicative of maximum sand angles in the sand dune, fallen (po . ssible cleared for access track) and leaning trees

Photo 2: Looking south along the access track at sloping trees present in the small gully at the toe of the slope.

SITE

REP

OR

T N

O. 1

4

SITE REPORT NO. 14

CLIENT: Redland Shire Council PROJECT NO.:PROJECT: Redland Landslide Hazard Assessment DATE: 23/03/05 LOCATION: Mt Hargrave, North Stradbroke Island SITE NO.:

33529

14 EASTING: 544,650mE

NORTHING: 6,958,000mN NATURAL SHALLOW LANDSLIDES

SITE DESCRIPTION:

The site is located to the north of Mining Company Road, some 600m east from the turnoff to North Stradbroke Island Golf Club. The site comprises the western slope of Mount Hargrave. At an elevation of 219m, Mt Hargrave is one of the highest sand dunes on North Stradbroke Island with slope angles up to some 37o measured.

REGIONAL GEOLOGY:

Geomorphology: As below

Geology: Reference to the Beenleigh 1:100,000 Geological series sheet indicates the site is underlain by Dune Sand of Quaternary Age.


Slope Angle: Slope angles were measured to be in the order of 28 to33 degrees over the majority of the slope with occasional steeper slopes up to some 37 degrees recorded. The slopes are well vegetated, with exception of an access track cut through the vegetation.

Slope Shape: Typically convex toward the upper slopes becoming generally planar towards the mid and lower slopes.

Surface Water Drainage: Given the presence of dune sands, would expect moderate surface drainage with seepage of surface water into the groundwater table.

Ev


Ev

idence of Instability: Nil


Soil Type: Dune sand: Light grey, white, fine grained sand

Soi

l Depth/Strength: Greater than 3m

Rock Type: N/A

Su


OTHER:

PHOTOGRAPHS

Photo 1: Looking east from Mining Company Road towards Mount Hargrave.

Photo 2: Looking east from Mining pany t ave. Com Road owards Mount Hargr

SITE

REP

OR

T N

O. 1

5

SITE REPORT NO. 15

CL land Shire Co cil OJECT NO.: 33529 : Redland Landslide Hazard Assessm TE: 23/03/05

OCATION: East Coast Rd, North Stradbroke Island SITE NO.: : 5

ING: 6,9 500NATURAL SHALLOW LANDSLIDES

IENT: Red un PRPROJECT ent DAL 15 EASTING 544,1 0mE NORTH 63, mN

SITE DESCRIPTION:

The site comprises the hillside occupy the g lease t oast Road, som ms o st from the Beehive Road intersection e Aran rawai Creek and associated coastal plains arei e toe of the hillside.

ing minin o the north of East C e 3kss

uth wetuated at th

. Th a

REGIONAL GEOLOGY:

Geomorphology: As below

Geology: Refereunde

nce to the B ne 1 ,00 o he site rlain by Dune nd of ter r of Quate A

g mud, at e e

risba Sa

:100 Qua

0na

Gey A

logical series sheet indicates tge, with coastal planes also

isrnary ge

comprisin sand and peat th to of the hillside.


Slope Angle: Slope angles of the illside w re e m te 20 degrees. In a batteslope cut in the toe of the hillside, for pro d, slope angles at/near the angle of repose was observed in slopes measured at 28 to 30degrees.

h e sti a d to be in the order of vision of an access roa

r

Sl pe Shape: Typically convex to ard the pper e anar towards e loweo w u slop s becoming generally pl th r slopes.

Surface Water Drainage: Given the presence of dune sands, would expect moderate surface drainage with seepage of surface water into the groundwater table.

Evidence of Groundwater: None on the hillside, shallow groundwater table expected towards the lower slopes and coastal plains.

Evidence of Instability: signs of slope angles at/near the angle of repose in a batter cut for the provision of an access road where the vegetation is patchy.


Soil Type: Dune sand: Light grey, white, fine grained sand

Soil Depth/Strength: Greater than 3m

Rock Type: N/A


OTHER: Surface samples were taken at this site. The angle of repose of this material was measured in a very

loose state to be in the order of 33 to 38 degrees.

PHOTOGRAPHS

Photo 1: Looking Aranarawai Creek towards the lower slopes of north east from East Coast Road acrossthe hillside.

Photo 2: A b (slightly south to the picture showatter n above), cut for the provision of an access road exhibits slope es at/near the angle of repose in areas angl where the vegetation is patchy.

LEGEND

Site No. Location Site 1 West Mount Cotton Rd, Mount Cotton Site 2 West Mount Cotton Rd, Mount Cotton Site 3 Hillview Rd, Mount Cotton Site 4 Seaview Rd, Mount Cotton Site 5 Shore St, Cleveland Site 6 Douro Rd, Wellington Point Site 7 Old Cleveland Rd, Birkdale Site 8 Howlett Rd, Capalaba Site 9 Avalon Rd, Sheldon Site 10 Mount View Rd, Mount Cotton Site 11 Rocky Passage Rd, Redland Bay Site 12 Wilson Esplanade Dr, Redland Bay Site 13 Mining Company Rd, Dunwich Site 14 Mt Hargrave, North Stradbroke Island Site 15 East Coast Rd, North Stradbroke Island

xx1133 xx1144

.xx1155

xx66

xx55

xx77

88

99

xx

.xx

xx1122

xx1111

.xx22 xx33

xx44 1100

11xxxx

Drawing Status:

FinalSCALE:

NonePROJECT / DRAWING No. 33529-002

Rev:

A

CLIENT:

Redland Shire Council PROJECT TITLE:

Redland Landslide Hazard Assessment Location of Site Inspections


APP

END

IX 4

: HA

ZAR

D A

ND

OVE

RLA

Y M

APS

APPENDIX 4 : HAZARD AND OVERLAY MAPS

Appendix 4 comprises Arcview GIS shape files of the Hazard Map, based on a five level hazard rating system, and the Overlay Map developed from these hazard ratings.

(please see attached cd’s).


APP

END

IX 5

: OVE

RLA

Y C

OD

E

APPENDIX 5 : OVERLAY CODE Appendix 5 comprises the Overlay Code in Redland Shire Councils standard format, for use in conjunction with the Overlay Map. An example of a Planning Scheme Policy drafted for Landslides is also included to assist with the understanding of the Overlay Code. It is understood that Redland Shire Council will develop the Planning Scheme Policy further.

Part 5 - Overlays, Division X - Landslide Hazard Overlay - Page 1

Division X – Landslide Hazard Overlay 5.X.1 Introduction

(1)

(a)

(b)

(1)

(a)

(b)

(2)

This division contains the provisions for the Landslide Hazard Overlay. They are -

The Landslide Hazard Overlay Tables of Assessment, that incorporates - (i) Levels of assessment for development in the Landslide Hazard Overlay (section 5.X.2); (ii) Assessment criteria for development in the Landslide Hazard Overlay (section 5.X.3); (iii) Landslide Hazard Overlay - Table of Assessment for Material Change of Use of

Premises (section 5.X.4); (iv) Landslide Hazard Overlay - Table of Assessment for Other Development not associated

with a Material Change of Use of Premises (section 5.X.5).

The Landslide Hazard Overlay Code, that incorporates - (i) Compliance with the Landslide Hazard Overlay Code (section 5.X.6); (ii) Overall Outcomes for the Landslide Hazard Overlay Code (section 5.X.7); (iii) Specific Outcomes and Probable Solutions applicable to Assessable Development

(section 5.X.8). 5.X.2 Levels of assessment for development affected by the Landslide Hazard

Overlay

Sections 5.X.4 and 5.X.5 identify the level of assessment for development affected by the Landslide Hazard Overlay, as follows -

Section 5.X.4 Landslide Hazard Overlay - Table of Assessment for Making a Material Change of Use of Premises -

(i) column 1 identifies uses that are exempt, self-assessable or assessable; (ii) column 2 identifies the level of assessment for the uses listed in column 1; (iii) where the use is defined in Schedule 3 - Dictionary, Division 1 - Uses and is not listed in

column 1 it is exempt; (iv) where the use is not defined in Schedule 3 - Dictionary, Division 1 - Uses and is not

listed in column 1 it is code assessable.

Section 5.X.5 Landslide Hazard Overlay - Table of Assessment for Other Development not associated with a Material Change of Use of Premises -

(v) column 1 identifies other development that is exempt, self-assessable or assessable; (vi) column 2 identifies the level of assessment for other development listed in column 1; (vii) where the other development is not listed in column 1 it is exempt.

Other Overlays may alter the level of assessment identified in 1(a) and (b)5.10.

5.10 Refer to Part 5 - Overlays to determine the level of assessment for the use or other development where

another Overlay affects the lot and Part 1, section 1.2.5(8)(f) that explains how the highest level of assessment applies.

Page 2 - Part 5 - Overlays, Division 3 - Landslide Hazard Overlay

5.X.3 Assessment criteria for development in the Landslide Hazard Overlay

(1)

(a)

(b)

(2)

Development affected by the Landslide Hazard Overlay is assessed against the assessment criteria listed in column 3 of sections 5.X.4 and 5.X.5, as follows -

acceptable solutions in section 5.X.8 of the Landslide Hazard Overlay Code for self-assessable development; or

specific outcomes in section 5.X.9 of the Landslide Hazard Overlay Code for Code assessable development.

Self-assessable development that does not comply with all the acceptable solutions in section 5.X.8 of the Landslide Hazard Overlay Code is assessable development.

2


5.X.4 Landslide Hazard Overlay – Table of Assessment for Material Change of Use of Premises

Landslide Hazard Overlay – Table of Assessment for Material Change of Use of Premises

column 1 column 2 column 3 Use5.11 Level of Assessment5.12 Assessment Criteria

Aged Persons and Special Needs Housing

Agriculture Airport Animal Keeping Apartment Building Bed and Breakfast Brothel Bulky Goods

Showroom Car Wash Facility Caretakers Dwelling Child Care Centre Commercial Office Community Facility Display and Sale

Activity Drive Through

Restaurant Dual Occupancy Dwelling House Education Facility Emergency Services Estate Sales Office Forestry Funeral Parlour Garden Centre General Industry Health Care Centre Heavy Industry Home Business Hospital Hotel Indoor Recreation

Facility Institution Intensive Agriculture Landscape Supply

Depot Marine Services Minor Utility Mobile Home Park Multiple Dwelling Night Club Outdoor Dining Outdoor Recreation

Facility Park Passenger Terminal Place of Worship

Self- Assessable If complying with the assessment criteria being the acceptable solutions listed in column 3 Code Assessable If not self-assessable

Acceptable Solutions in section 5.X.8 of the Landslide Hazard Overlay Code

Landslide Hazard Overlay Code

5.11 See Schedule 3 - Dictionary, Division 1 - Uses for defined uses. 5.12 See Schedule 3 - Dictionary, Division 2 - Administrative Terms for a definition of level of assessment.


Landslide Hazard Overlay – Table of Assessment for Material Change of Use of Premises

column 1 column 2 column 3 Use5.11 Level of Assessment5.12 Assessment Criteria

Produce Store Refreshment

Establishment Relatives Apartment Retail Warehouse Roadside Stall Rural Enterprise Service Industry Service Station Shop Small Lot House Telecommunications

Facility Temporary Use Tourist

Accommodation Tourist Park Utility Installation Vehicle Depot Vehicle Parking

Station Vehicle Repair

Premises Veterinary Surgery Warehouse




Defined uses not listed in column 1 Exempt

Uses not defined in Part 9 – Schedule 3 – Dictionary, Division 1 – Uses

Code Assessable Landslide Hazard Overlay Code

4


5.X.5 Landslide Hazard Overlay – Table of Assessment for Other Development not associated with a Material Change of Use of Premises

Landslide Hazard Overlay – Table of Assessment for Other Development

column 1 column 2 column 3 Other Development Level of Assessment5.13 Assessment Criteria

Reconfiguration for –

Creating lots by subdividing another lot by Standard Format Plan5.14

Code Assessable


Rearranging the boundaries of a lot by registering a plan of subdivision; or

Dividing land into parts by Agreement; or

Creating an easement giving access to a lot from a constructed road




Building Work for –

Domestic Additions




Domestic Outbuilding

Self- Assessable If complying with the assessment criteria being the acceptable solutions listed on column 3 Code Assessable If not self-assessable



On-site raising or relocating of an existing dwelling unit




Private Tennis Court Self- Assessable If complying with the

Acceptable Solutions in section 5.X.8 of the Landslide Hazard

5.13 See Schedule 3 - Dictionary, Division 2 - Administrative Terms for a definition of level of assessment. 5.14 Whether or not having a Community Management Statement.


Landslide Hazard Overlay – Table of Assessment for Other Development column 1 column 2 column 3

Other Development Level of Assessment5.13 Assessment Criteria

assessment criteria being the acceptable solutions listed on column 3 Code Assessable If not self-assessable

Overlay Code


Private Swimming Pool




Private Waterfront Structure




Operational Work for -

Excavation and Fill




Operational Work for Reconfiguring a Lot (by Standard Format Plan)

Code Assessable


All other development not listed in column 1 Exempt

6


5.X.6 Compliance with the Landslide Hazard Overlay Code

(1)

(a)

(b)

Development that is consistent with the following complies with the Landslide Hazard Overlay Code -

acceptable solutions in section 5.x.8 where self-assessable development; or

specific outcomes in section 5.X.9 where assessable development.

Note – Planning Scheme Policy X – Landslide Hazard will assist in achieving the requirements of the Landslide Hazard Overlay Code. 5.X.7 Overall Outcomes of the Landslide Hazard Overlay Code

(1)

(2)

(a)

(b)

The overall outcomes are the purpose of the Landslide Hazard Overlay Code.

The overall outcomes sought for the Landslide Hazard Code are the following -

to limit the extent of uses and other development to an appropriate level, relative to the area’s landslide hazard risk;

to minimise the landslide hazard risk to people and property through the appropriate siting, design and management of development and uses.


5.X.8 Acceptable Solutions applicable to Self-Assessable Development

Self-Assessable Development Acceptable Solutions

A1.

(1)

(a)

(b)

(c)

(d)

(e)

(f)

(2)

(a)

(b)

Building work and operational work where –

all works are located outside of the Landslide Hazard Management Area; and

no vehicular access or formal pedestrian access occurs through the Landslide Hazard Management Area; and

earthworks does not exceed more than 50m3 (other than the placing of top soil) with a maximum cut and fill depth of 2 metres; and

retaining walls do not exceed more than one vertical metre in height; and

the existing flow direction of surface and/or ground water is retained; and

all structures exceeding a surcharge of more than 5kpa are setback beyond a projected line of 45 degrees from the toe of any retention system (refer to diagram 1 below).

Uses and other development where –

not involving building work and solely contained within an existing building; and

where complying with Acceptable Solution A1(1).

Diagram 1 – Location of structures from a retention system

8


5.X.9 Specific Outcomes and Probable Solutions applicable to Assessable Development

Assessable Development

Specific Outcomes Probable Solutions S1.

General

(1)

(a) (b) (c)

(d) (e) (f)

(g)

(h)

(1) All development and uses proposed on land shown on this overlay map, only occurs where the development will not create or increase the Landslide Hazard Risk to that land or adjoining land as determined by a suitably qualified individual, having regard to –

Built form; Slope; The extent of vegetation removal; Soil type and stability; Earthworks; Alteration of existing groundwater or surface water flow paths; Waste water disposal areas; and Environmental values.

P1.

No probable solution identified.

Note: To assist in achieving S1 – (1) In areas identified on the Overlap

Map as Very High Hazard, the proposed development is supported by a geotechnical report that has been undertaken in accordance with section X.6.1, Planning Policy X.

(2) In areas identified on the Overlap

Map as High Hazard, the proposed development is supported by a geotechnical report that has been undertaken in accordance with section X.6.2, Planning Policy X.

(3) In areas of Moderate Hazard as

shown on this overlay map, the proposed development is supported by a geotechnical report that has been undertaken in accordance with section X.6.3, Planning Policy X

S2.

Community Infrastructure

(1) (1) The community infrastructure is able to function effectively during and immediately after landslide events.

P2.

Community infrastructure is not located in a Landslide Hazard Management Area;

Note: Alternative siting may be considered to address specific outcomes where – (1) The community infrastructure

development - (a) does not result in any new

building work other than an addition to an existing building;

(b) does not involve vegetation clearing;

(c) does not alter ground levels or stormwater conditions.

(2) The development includes

measures that ensure -


(a) the long term stability of the site;

(b) access to the site will not be impeded by a landslide event;

(c) the community infrastructure will not be adversely affected by landslides originating on sloping land above the site

10

Part 11 - Planning Scheme Policy X – Landslide Hazard - Page 1

Planning Scheme Policy X – Landslide Hazard X.1 Purpose (1)

(a)

(b)

(c)

(1)

The purpose of this policy is to –

give guidance relating to the identification of potential slope instability areas;

set out the requirements for preparation and submission of development applications, including technical reports, on land within designated Landslide Hazard Management Area;

provide information relating to good engineering practices with regards to hillside development to assist applicants, engineers and planners in the design and application of appropriate type and form of developments that best reflects the capability of the land.

X.2 Applicability

This policy applies to all development applications under the planning scheme on land within a designated Landslide Hazard Management Area in accordance with the Landslide Hazard Overlay Map.

X.3 What is a Landslide? (1) A landslide is the movement of a mass of rock, debris or earth down a slope. They are the result

of shear failure of the soil and/or rock materials that make up the hill slope and they are driven by gravity.

X.4 Formulating a Development Proposal (1) Every year in Australia landslides damage many houses and cause millions of dollars damage to

the natural and built environment including buildings, roads, railways and pipelines. Historical records indicate some 50% of all landslides recorded are a result of alteration to slopes by human activity.

(2) The planning and design of future development comprising building or other works on sloping

sites should consider the relevance of the slope instability to the type of development proposed and if required the implementation of effective and timely remedial measures. Specific geotechnical requirements will depend on the hazard rating category as discussed in Section X.6.

(3) As a general rule:

(a) Development should be practicable in areas with a High or Very High Landslide Management Area provided rigorous analysis and restrictions apply. Good hillside practices must be adopted. For the majority of situations a risk assessment with respect to landsliding will be required. Remedial measures may be required to reduce/control the risk of slope instability to acceptable levels.

(b) Development should be practicable in areas with a Moderate Landslide Management Area

provided appropriate restrictions apply. Good hillside practices must be adopted. A risk assessment with respect to landsliding would be prudent.

(c) Development is practicable in areas with a Low or Very Low Landslide Management Area

without specific restrictions related to landslide hazard. Good hillside practices should be adopted.

Page 2 - Part 11 - Planning Scheme Policy X – Landslide Hazard

(4) It is strongly recommended that applicants arrange a pre-lodgement meeting with Council to discuss the inherent landslide hazards of a site identified as within the Landslide Hazard Management area prior to the lodgement of a Development Application.

X.5 Landslide Hazard Mapping (1) A regional qualitative study to establish hazard ratings with respect to landslide potential has

been carried out for the Redland Shire area as detailed in the Landslide Hazard Overlay Map. The assessment of the hazard ratings was carried out in accordance with SPP1/03 Guidelines – Mitigating the Adverse Impacts of Flood, Bushfire and Landslide and is consistent with the procedures detailed in the paper entitled “A Method of Zoning Landslide Hazards” prepared by McGregor and Taylor.

(2) The implications of the hazard rating are given in Table 1. This serves as a tool for both planners

and developers to determine the appropriate layouts, type and form of development that best reflects the capability of the land.

Table 1 – Implications of hazard classification

Hazard Rating

Description Implications

VH (Very High)

The event is expected to occur

Extensive investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels.

H (High)

The event will probably occur under adverse conditions

Detailed investigation, planning and implementation of treatment options essential to reduce risk to acceptable levels.

M (Moderate)

The event could occur under adverse conditions

May be acceptable provided treatment plan is implemented to maintain or reduce risk level.

L (Low)

The event might occur under very adverse conditions

Can be accepted. Treatment to maintain or reduce risk level should be defined.

VL (Very Low)

The event is conceivable but only under exceptional circumstances

Accepted. Managed by routine procedures.

(3) For individual sites within a designated Landslide Hazard Management Area, where slope

instability is of concern, or areas that may impact on a Landslide Hazard Management Area as a result of the proposed development similar procedures can be applied for refinement of these hazard ratings, identification of unfavourable site conditions and control/manage such areas with regards to the proposed development.

X.6 Requirements for Preparation / Submission of Development Applications (1) In accordance with the Landslide Hazard Overlay Code site specific assessment is required when

a premises is affected by land designated a Moderate, High or Very High Landslide Management Area.

(2) If an applicant can NOT show reasonable cause that their proposed development is located

outside land designated as a Moderate, High or Very High Landslide Management Area, and does not contribute to slope instability of such areas, the applicant must achieve the intentions of the Sections X.6.1 to X.6.3 for the appropriate Landslide Management Area. Reasonable cause will be subject to the approval of Redland Council, and may require supporting documentation by a suitably experienced geotechnical professional.

(3) In many cases most of the above information on site conditions may be common logic,

supplemented by adoption of good hillside practices, or can be obtained during a walk-over survey by a suitably experienced geotechnical professional. However, it may be necessary to supplement the site observations by subsurface investigations such as boreholes or test pits.


(4) In some cases different Landslide Hazard Management Areas may vary across the site. If this

occurs the requirements for preparation / submission of development application should adopt the higher hazard rating in preference, or the developer must demonstrate that that the proposed development is located a sufficient distance away from such an area and does not impact with regards to contributing slope instability to justify a lower hazard rating.

X.6.1 Development within a VERY HIGH Landslide Management Area (1) The following identifies the level of professional input that should be incorporated in the

assessment, planning and design of proposed developments to suitably identify, control and manage risks associated with development on, or with the potential to impact on, land designated as a Very High Landslide Hazard Management Area:

(a) Carry out a detailed geotechnical engineering report prepared by a suitably qualified

geotechnical professional (RPEQ qualifications). At a minimum the geotechnical engineering report should comprise: (i). extensive site investigation including subsurface investigation with groundwater

measurements over at least one wet season; (ii). frequency of investigation locations should be no less than 1 location per 30m x 30m

grid with assessment of material strength by appropriate in-situ or laboratory testing. Investigations should establish a comprehensive geotechnical model over the whole site;

(iii). installation of groundwater monitoring points with measurements over at least one typical wet season and comparison of groundwater levels to rainfall events should be made;

(iv). a review of potential hazards; and (v). analysis of slope stability using a suitable model appropriate for the site conditions.

(b) Where analysis of slope stability in the above step indicates an unfavourable factor of safety

under adverse conditions indicates assess the risks to the community with regards to loss or injury of life and infrastructure. Where unacceptable, these risks should be suitably minimised or measures to stabilize the slope (reduce slope angles, water drainage provisions, rockbolt and/or shotcrete protection) should be implemented.

(c) Undertake comprehensive sighting for the development with regards to potential hazards,

including restricting design of major structures and unfavourable earthworks in very high landslide hazard areas where possible.

(d) Extensive design input is required from a qualified Practicing Engineering professional,

including adoption of good hillside construction practices as provided in this policy. (e) The design must be reviewed and certified by an experienced, suitably qualified geotechnical

professional (RPEQ qualifications). (f) Regular maintenance of slopes, cleaning of drainage courses and monitoring of slope for

signs of distress.

X.6.2 Development within a HIGH Landslide Management Area (1) The following identifies the level of professional input that should be incorporated in the

assessment, planning and design of proposed developments to suitably identify, control and manage risks associated with development on, or with the potential to impact on, land designated as a High Landslide Hazard Management Area:

(a) Carry out a detailed geotechnical engineering report by an experienced qualified

geotechnical professional. At a minimum the geotechnical engineering report should comprise: (i). site investigation including subsurface investigation with groundwater measurements; (ii). frequency of investigation locations should adequately cover the site and slope in

question to provide sufficient information to establish a comprehensive geotechnical


model over the whole site, with assessment of material strength by appropriate in-situ or laboratory testing;

(iii). installation of groundwater monitoring points with measurements over at least one typical wet season and comparison of groundwater levels to rainfall events should be made;

(iv). a review of potential hazards; and (v). analyse slope stability using a suitable model appropriate for the site conditions.

(b) Where analysis of slope stability in the above step indicates an unfavourable factor of safety

under adverse conditions indicates assess the risks to the community with regards to loss or injury of life and infrastructure. Where unacceptable, these risks should be suitably minimised or measures to stabilize the slope (reduce slope angles, water drainage provisions, rockbolt and/or shotcrete protection) should be implemented.

(c) Undertake appropriate sighting for the development with regards to potential hazards,

including restricting/reducing design of major structures and unfavourable earthworks in high landslide hazard areas where possible.

(d) Considerable design input from a qualified Practicing Engineering professional, including

adoption of good hillside construction practices as provided in this policy. (e) Restrict/reduce design of major structures and earthworks in High Landslide Hazard Areas

where possible. (f) The design must comply with recommendations detailed in the geotechnical engineering

report. (g) Regular maintenance of slopes and cleaning of drainage courses.

X.6.3 Development within a MODERATE Landslide Management Area (2) The following identifies the level of professional input that should be incorporated in the

assessment, planning and design of proposed developments to suitably identify, control and manage risks associated with development on, or with the potential to impact on, land designated as a Moderate Landslide Hazard Management Area:

(a) Carry out a geotechnical engineering report by an experienced, qualified geotechnical

professional. At a minimum the geotechnical engineering report should comprise: (i). site walkover survey with investigations as required establishing a geotechnical model

over the whole site. This may require moderate subsurface investigation and/or testing to provide subsoil material properties;

(ii). review potential hazards; and (iii). assessment of slope stability using a suitable model appropriate for the site conditions.

(b) Consider the risks to the community with regards to loss or injury of life and infrastructure. (c) Design input from a qualified Practicing Engineering professional, including adoption of good

hillside construction practices as provided in this policy. (d) The design must comply with the recommendations detailed in the geotechnical engineering

report. X.7 Characteristics of Landslides X.7.1 What types of landslide occur? (1) Once a landslide is triggered, the material is transported in three main forms;

(i). by sliding along a failure surface; (ii). by falling down a steep slope; and (iii). by flowing as a suspended mass, usually in water for example a mudslide or debris

flow.


(2) Landslides may be classified into the flowing main types:

Translational Slides: where failure occurs on a planar surface or surfaces, usually natural defects in the material such as fissures, joints or bedding. Material within the slide can remain relatively undisturbed. Creep Slides: where failure occurs as a gradual downslope progression (often extremely slow rates) of slope material. The slide area may appear relatively undisturbed and identification of the slide is often reliant on surface features. Rotational Slides: where failure occurs through the material substance commonly on a concave surface. Material within the slide is considerably disturbed. Topple: where failure occurs from the end over end motion of rocks down a slope. Often resulting from closely spaced sub-vertical jointed rock outcrops. Falls: where movement is by free-falling or rolling of fragments on steep slopes with outcrops of closely jointed rock. Flows: where, after failure along a planar or concave surface, the material is transformed into a viscous fluid consisting of soil and rock particles suspended in water. Complex: where there is a combination of one or more of the above mechanisms.

Figure 1 – Common types of landslides

TRANSLATIONAL

CREEP ROTATIONAL

TOPPLE

FALL

FLOW

The above figure is courtesy of the Geosciences Australia Web-site. (www.ga.gov.au) (3)

(1)

(2)

(3)

The rate of landslide movement varies from extremely slow (millimetres to centimetres per year) to a sudden and extremely rapid (metres per second) as with rock fall or debris flow. Sudden and rapid events are the most dangerous because of the lack of warning, the speed at which they can travel down the slope and the force of impact.

X.7.2 What causes landsliding?

The stability of sloping ground is controlled by three main factors: (i). the angle of the ground surface; (ii). the strength of the materials below the ground surface; and (iii). the level of water within the slope.

In Australia intense rainfall is by far the most common trigger of landslides.

Several factors combine to define the complex relationship between the physical environment and land instability, however two basic conclusions can be drawn into the likelihood of their occurrence. Firstly, it is likely that landslides will occur in areas where they have occurred in the past, and secondly they are likely to occur in areas exhibiting similar conditions to these areas.

Landslides can be triggered by both natural causes or by human activity.

(a) Natural causes may include:


(i). saturation of slope material from rainfall or seepage; (ii). undercutting of cliffs and banks by erosion; (iii). prying loose of rock masses from vegetating growth within joints; and (iv). vibrations caused by earthquakes.

(b) Human activities may include:

(i). the modification of slopes by cut and fill activities associated with construction; (ii). interference with or changes to natural drainage; and (iii). leaking pipes (water, sewer); (iv). changes to materials; (v). the removal of vegetation; (vi). mining activities; and (vii). vibrations from heavy traffic, blasting or excavation.

X.7.3 Identification of potential slope instability (1) In comparison to many other countries, much of Australia is subject to minimal landslide activity.

Generally we receive little rainfall and the landscape has minimal influence from the processes of uplift.

(2) There are however certain areas that are more commonly affected by landslides. Such areas

typically comprise cliffs, steep colluvial deposits, or gentler slopes of unstable geology subjected to prolonged or intense rainfall events. Landslide prone areas commonly comprise:

(i). coastal cliffs; (ii). existing or old landslides; (iii). any sloping ground in an area known to have a landslide problem; (iv). areas at or on the base of slopes; (v). within or at the base of minor drainage hollows; and (vi). at the base or top of cut and fill slopes.

(3) In the natural environment the progressive development of hill slopes by weathering and erosion

involves a gradual incision of the stream beds into higher ground and results in the formation of slope surfaces that are essentially uniform, convex or planar. The occurrence of natural landslides on these slopes produces an irregular profile, often concave, accompanied by features reflecting the disturbance that has taken place. In the case of recent landslides these features are usually sharp and distinct. With time, the effects of weathering and erosion modify these features which become indistinct but usually can be recognised by close observation. Individually the features may not be related to landsliding but the presence of several features at one location indicates that some mass movement of material may have occurred.

(a) Features that indicate existing natural slope instability include:

(i). irregular surfaces: areas of hummocky ground and depressions indicating disturbed material;

(ii). benches: anomalous flat areas in uniform sloping areas; (iii). scars: areas where vegetation has been stripped during slope movement; (iv). scarps: linear features showing the location of vertical displacement of the ground

surface; (v). cracks: linear features showing lateral displacement of the ground surface; (vi). debris mounds: deposits of loose soil and rock on or at the base of slopes; (vii). disturbed vegetation: tilted trees; and (viii). seepage: presence of springs and dense vegetation regrowth.

(b) Features that indicate that some lateral mass movement of material may have occurred in

areas that have been developed include: (i). cracking or tilting of walls and retaining structures; (ii). cracking or slumping of embankment slopes; (iii). cracking and fall of material from excavated slopes; (iv). broken/fractured water pipes and underground facilities; (v). tilted powerlines, retaining walls and fences (or offset); and (vi). sunken or cracked road surfaces.


X.8 Implementing Good Hillside Practices X.8.1 What guidelines apply to development applications? (1) Examples of Good and Poor Hillside Engineering Practice are given in Table 1 and Figure 2

below. Table 1 – Guidelines for hillside construction practice



Figure 2 – Illustration of good and poor hillside practices



X.8.2 What guidelines apply to road design over sloping ground (1) Roads on side slopes usually are formed by a combination of cut and fill operations. The design

must incorporate effective drainage, and should incorporate the following good practices: (2) The road cut slope design should incorporate:

(a) The adoption of batter slopes appropriate to the engineering properties of the different materials exposed in the cut face. As a general rule batters in soil should be 2H:1V, in poor rock 1H:1V and in good rock 0.5H:1V.

(b) Where cuttings in rock are proposed, road alignments should be planned as not to coincide

with major jointing orientations of the rock.

(c) The higher cut faces should include the provision of benches at vertical intervals of not greater than 10 m. These benches are required to catch fallen material, to control drainage and to provide access for maintenance of the cut face.

(d) The provision of formed surface drains at the top of the cut slope, on the benches and at the

toe of the cut slope. (e) The provision of slope protection, slope treatment or slope support in areas of potential

concern. Slope protection against erosion may utilise a cover of topsoil and grass. On steeper slopes treatment of erodible and closely joined rock is commonly by a cover mesh and shotcrete with rock bolts providing treatment of areas with adversely oriented jointing. In areas of greater concern slope support can be provided by an engineered retaining wall. The design of the wall depends on the site conditions and cut dimensions but could include gabion crib, masonry and reinforced concrete wall designs.

(3) The road fill embankment design should incorporate:

(a) The removal of all unsuitable material including trees, vegetation and topsoil from the embankment foundation

(b) The preparation of the embankment foundation by the formation of terraces across the

slope. These terraces should be at least 2 m wide with a maximum height of 0.6 m.

(c) The installation of drainage, if required, in the foundation. This drainage may involve trench drains in areas of local seepage or a drainage blanket in an area that is generally wet.

(d) The embankment fill should be placed in an engineered manner. Placement of earth fill

should be in layers – each not thicker than 300 mm and compacted by roller to not less than 95 % relative to Standard Compaction.

(e) The design of compacted earth fill slopes in soil should be no steeper than 1.5H:1V, and

may often be lower subject to retained height, soil strength and maintenance considerations. Surface protection should be by grass or rock.

(f) The provision of drainage at the crest and toe of the embankment as formed drains leading

to an identified disposal area

(4) Examples of how to maintain slope stability for road design is illustrated in Figure 3.


Figure 3 – Possible methods of maintaining stability in road design

X.9 References

Queensland Government, State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and Landslide” (SPP 1/03), May 2003

Queensland Government, State Planning Policy 1/03 Guideline “Mitigating the Adverse

Impacts of Flood, Bushfire and Landslide” (SPP 1/03), June 2003

Australian Geomechanics Society Sub-Committee on Landslide Risk Management, “Landslide Risk Management Concepts and Guidelines”, Australian Geomechanics Journal, Volume 37 No. 2, May 2002.

McGregor and Taylor “A Method of Zoning Landslide Hazards”, Australian Geomechanics

Journal, Volume36 No. 3, Sept 2001.

Documents

Document / Report Control Form - Redland City · 2016-09-02 · Draft 21/04/05 S. Holt T. Rannard, J. Westerman Final 1/06/05 S. Holt J. Westerman Final Rev 1 16/09/05 S. Holt J