GEOTECHNICAL REPORT... · 2019. 2. 12. · GEOTECHNICAL REPORT 2 & 2A Tizard Road Development Birkenhead, Auckland Prepared for Young + Richards Ltd JOB No. 166115 DATE 19/09/2017

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GEOTECHNICAL REPORT

2 & 2A Tizard Road Development Birkenhead, Auckland

Prepared for Young + Richards Ltd

JOB No. 166115

DATE 19/09/2017 ISSUE No. B

AUCKLAND OFFICE Level 1, 61-73 Davis Crescent, Newmarket

Auckland, New Zealand PO Box 99388, Auckland 1023

T +64 9 972 9038 F +64 9 972 3394

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2 & 2A Tizard Road Development

Birkenhead, Auckland Issue B Geotechnical Report

Date: 19/09/2017 Job No: 166115

QUALITY CONTROL Title 2 & 2A Tizard Road Development

Client Young + Richards Architect Ltd

Job No 166115

Issue B

Date 19 September 2017

Limitations

This report has been prepared at the specific instructions of Young + Richards Ltd to support the Resource Consent application for the proposed development. No liability is accepted by this company or any employee of this company with respect to its intended use by any other person or persons. The subsurface soil conditions and the interpretations reported are those identified at the soil test locations at the time of the geotechnical assessment and are subject to the limitations of the investigation methods. The test results represent only a small sample of the total subsurface soils. Soil conditions may vary between the test locations and interpretation of the soil information and test results must take into account the spacing and plan location of the tests. If subsurface conditions encountered on the site during construction appear to vary from those inferred from the information contained in this report, Kirk Roberts Consulting Engineers Ltd requests that it be notified immediately. This report is only valid for the proposal as outlined in the introduction and the information and interpretation of the content in this report may not be relevant if the proposed development is altered in any way. If the recipient of this report wishes to contact Kirk Roberts Consulting Engineers Ltd, either Email: [email protected] or Phone: 09 972 9038.

Prepared By Name: Steven Roberts

CPEng (Geotechnical), MIPENZ, IntPE(NZ)

Signature:

Reviewed By Name: Chris Hewitt

Senior Geotechnical Engineer

Signature:

Authorised By Name: Jordan Walker

CPEng (Geotechnical) MIPENZ, IntPE(NZ)

Signature:

mailto:[email protected]

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Date: 19/09/2017 Job No: 166115

TABLE OF CONTENTS

QUALITY CONTROL ..........................................................................................................................................................

1 INTRODUCTION ...................................................................................................................................................... 1

1.1 GENERAL ......................................................................................................................................................... 1 1.2 SITE DESCRIPTION ............................................................................................................................................ 1 1.3 PROPOSED DEVELOPMENT ................................................................................................................................ 1

2 PUBLISHED GEOLOGY ......................................................................................................................................... 2

3 PREVIOUS SITE INVESTIGATIONS ...................................................................................................................... 2

3.1 GEOTECHNICAL INVESTIGATIONS ........................................................................................................................ 2 3.2 GROUNDWATER MONITORING............................................................................................................................. 2

4 INVESTIGATION RESULTS ................................................................................................................................... 3

4.1 GROUND MODEL ............................................................................................................................................... 3 4.2 EAST COAST BAYS FORMATION RESIDUAL SOIL................................................................................................... 3 4.3 EAST COAST BAYS FORMATION COMPLETELY WEATHERED MUDSTONE AND SANDSTONE ....................................... 3 4.4 EAST COAST BAYS FORMATION MUDSTONE/SANDSTONE ROCK .......................................................................... 4

5 GEOTECHNICAL RISK ASSESSMENT ................................................................................................................. 4

5.1 RISK ASSESSMENT DEFINITIONS ........................................................................................................................ 4 5.2 RISK ASSESSMENT OF SITE-SPECIFIC GEOTECHNICAL HAZARDS BEFORE AND AFTER THE PROPOSED DEVELOPMENT 5.3 RISK ASSESSMENT OF GEOTECHNICAL ISSUES CREATED BY PROPOSED DEVELOPMENT ....................................... 7

6 GEOTECHNICAL RECOMMENDATIONS ............................................................................................................ 11

6.1 GENERAL ....................................................................................................................................................... 11 6.2 FOUNDATION OPTIONS ..................................................................................................................................... 11

6.2.1 Shallow strip and pad foundations .......................................................................................... 11 6.2.2 Basement walls ........................................................................................................................ 11

6.3 SITE PREPARATION AND EARTHWORKS ............................................................................................................. 12

7 FURTHER GEOTECHNICAL INVESTIGATIONS FOR BUILDING CONSENT APPLICATION .......................... 12

8 CONCLUSIONS ..................................................................................................................................................... 12

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Date: 19/09/2017 Job No: 166115

1 INTRODUCTION

1 . 1 G e n e r a l

Young + Richards Ltd(Y+R), on behalf of Bernard Reuters, have engaged Kirk Roberts Consulting Engineers Ltd (KR) to carry out a review of previous geotechnical investigations and reporting completed by Engineering Geology Ltd, and confirm if the geotechnical information is still valid for supporting a Resource Consent application for the proposed development.

This geotechnical report covers the following scope:

• Review of the concept design drawings for the current development scheme, provided by Young + Richards Ltd, dated 17 May 2017.

• Review of previous site-specific geotechnical investigation information.

• Geological ground modelling for assessment of soil-structure interaction and potential site-specific geotechnical hazards.

• Geotechnical monitoring during basement excavation

• Construction methodology for basement excavation and ground retention works.

• Groundwater assessment and potential dewatering issues during basement construction

• Geotechnical risk assessment pertaining to the site and proposed development.

1 . 2 S i t e d e s c r i p t i o n

The setting is two adjoining Fee Simple titles located at 2 & 2A Tizard Street, Birkenhead with legal descriptions of Lot 1 & 2 DP 403136 respectively, and a total site coverage of approximately 3056 m2.

The combined site is flanked by a nature reserve along the east boundary, a residential site to the west boundary and a cliff top to the south. Tizard Road defines the north boundary and provides access to the site.

The land slopes to the south and west from Tizard Road at a gradient of between 5H:1V to 3H:1V increasing to about 3H:1V towards the crest of the cliff. The crest of the cliff is partially covered in bush and the topographic plan indicates that the cliff is 22 m high below the western side of the site increasing to about 30m on the eastern side toward the nature reserve. There is an existing palisade wall about 20 m long at the south-eastern corner of the site constructed for a previous building proposal that did not proceed. Based on the proposed basement floor level in relation to the existing ground level, it is very likely that this palisade wall will be completely removed during the basement excavation works.

1 . 3 P r o p o s e d d e v e l o p m e n t

Y+R’s 2017 concept drawings indicate a terraced three-level apartment complex supported below by a basement structure occupying a footprint area of 1037 m2. The site topography dictates the required excavation depth for the proposed apartment complex, with excavation depth varying from 12.4 m at the north-east corner of the basement footprint, reducing to about 5.4 m at the south-east corner and about 0.5 m toward the south-west corner.

The proposed development occupies about 34% of the total combined site area of 3056 m2 from 2 & 2A Tizard Road. The proposed development is positioned about 1.2 m off the east boundary, 3 m off the north boundary, where the excavation depth is a maximum at 12.4 m, and 4 to 8 m off the west boundary. The deep excavation, combined with close proximity to the council reserve boundaries will require temporary retaining of the cut face during construction of the basement structure.

Refer to appendix D for basement set-out plan of proposed development.

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Date: 19/09/2017 Job No: 166115

2 PUBLISHED GEOLOGY

The published geology of the area1 shows the site to be underlain by East Coast Bays Formation (ECBF), comprising alternating beds of mudstone and sandstone of the Waitemata Group1. The mudstone and sandstone readily weather to form residual soils typically described as stiff to very stiff silts.

The published geology is generally in agreement with the results of Engineering Geology Ltd’s 2013 and 2014 site specific investigations.

3 PREVIOUS SITE INVEST IGATIONS

3 . 1 G e o t e c h n i c a l i n v e s t i g a t i o n s

Site specific geotechnical investigations were undertaken in 2013 under the instruction of Engineering Geology Ltd (Report Titled: Proposed Residential Development Lots 1 &2, 2A, 2B & 2C Tizard Road, Birkenhead Geotechnical Investigation, Dated: 5 March 2013, Ref: 4452C). A subsequent investigation comprising Machine Drilled Boreholes was undertaken in 2014 (Engineering Geology Ltd Ref 4452D). These investigations comprised:

• Site walkover inspection complete with nine hand auger boreholes and shear vane testing to 5 m depth.

• 3 No. rotary machine boreholes (MBH1 to MBH3) to depths of 15.5 to 23 m below existing ground level, complete with discrete SPT tests at regular 1.5m intervals to full depth of boreholes.

The machine boreholes were completed by Engineering Geology Ltd.’s subcontractor Pro-Drill Ltd using a rotary drill rig.

The investigation locations are presented drawing 166115_SK-1_rev A; Appendix A, and copies of the borehole logs (BH1 to BH9 and MBH1 to MBH3) are included in Appendix B.

The above investigations are considered to be of sufficient density and depth to provide adequate subsurface ground information to fully characterise the site geology for the purpose of assessing the proposed development complete with associated geotechnical issues. Accordingly, no further testing is deemed necessary at this stage for a resource consent application. The previous site investigation information has been reproduced in this report to assess site-specific and building specific geotechnical risks.

3 . 2 G r o u n d w a t e r m o n i t o r i n g

Ground water was encountered in BH’s 4 and 6 to 9 and water levels measured at depths of between 1.0m(BH7) and 4.3m (BH9) the day following drilling. No water table was encountered in the remaining hand auger boreholes. A piezometer was installed in MBH2, with the water table measured over a period of about a week and the water table level gradually dropped in the borehole to 13.0 m depth.

The deeper water table is most likely to be representative of the prevailing regional water level beneath the entire site, whereas the shallower water table encountered at BH4 to BH9 is consistent with a perched water table confined to within the residual soil mantle.

The perched water depths have been converted to reduced levels (RL) to assess the water surface gradient between BH’s 4 and 6 to 9. The perched water table RL depths are summarised in Table 3.2 below.

1 Edbrooke, S.W (compiler) 2001. Geology of the Auckland area. Institute of Geological and Nuclear Sciences 1:250 000 geological map 3. 1 sheet + 74 p. Lower Hutt, New Zealand: Institute of Geological and Nuclear Sciences Ltd.

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Date: 19/09/2017 Job No: 166115

T a b l e 3 . 2 – P e r c h e d w a t e r t a b l e d e p t h

BH location BH Reduced level (RL) in metres (m)

Water table depth (m) Water table reduced level (RL) in metres (m)

BH4 31 1.6 29.4

BH6 36.3 2.4 34

BH7 34 1.0 33

BH8 31 1.7 29.3

BH9 29.5 4.3 25.2

The basement foundation RL at 27.2 m combined with a foundation depth of 0.6 m results in an invert level RL of 26.6 m, which is lower than the perched water table RL levels at BH’s 4 to 8. Therefore, excavation of the basement structure will completely remove the perched water table within the residual soil mantle, except for borehole BH9, where the water table RL is lower at 25.2 m.

4 INVESTIGATION RESULTS

4 . 1 G r o u n d m o d e l

Table 4.1 below presents a typical geotechnical ground model for the site as inferred from the shallow and deep soil testing completed in 2013 and 2014 by Pro-Drill Ltd. Further description of each geological unit is presented in the subsequent sections.

T a b l e 4 . 1 – T y p i c a l g e o t e c h n i c a l g r o u n d m o d e l .

Unit Typical depth to top of unit (m, bgl)

Typical thickness (m)

Typical undrained shear strength (Su)

ECBF Residual Soil 0 5.5 – 7.5 Exceeds 88kPa

ECBF completely weathered mudstone and sandstone

5.5– 7.5 2 – 8 Exceeds 120kPa

ECBF sandstone rock, slightly weathered

9.5 – 15.5 >7.5 -*

*Note: UCS values for sandstone rock are indicated to be within the range of 1 MPa to 5 MPa in accordance with NZGS section 3.0.

4 . 2 E a s t C o a s t B a y s F o r m a t i o n r e s i d u a l s o i l

Fine grained ECBF residual soils were encountered in all boreholes to 5.5 – 7.5 m bgl. The residual soils are generally described as silts to clay, and having moderate plasticity. In situ undrained shear strengths recorded in these materials ranged from 88 kPa to >140 kPa.

4 . 3 E a s t C o a s t B a y s F o r m a t i o n c o m p l e t e l y w e a t h e r e d m u d s t o n e a n d s a n d s t o n e

Completely weathered ECBF sandstone and mudstone soils were encountered beneath the residual soils in MH1 to MH3 from 5.5 m to 15.5 m. The ECBF found in this area of Auckland is generally massively bedded and this is carried through to the weathered soils. The completely weathered soil found at the site can be generically described as grey to orange brown silty clay to silt. Uncorrected SPT blow count values (N60)

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Date: 19/09/2017 Job No: 166115

recorded in the ECBF soils ranged from 15 to 34. The wide variation in ‘N’ values is consistent with the highly interbedded nature of the sandstone and mudstone material.

4 . 4 E a s t C o a s t B a y s F o r m a t i o n M u d s t o n e / S a n d s t o n e R o c k

The ECBF rock encountered at depths of 9.5 – 15.5 m below existing ground surface generally comprises very weak interbedded mudstone and sandstone, thin to moderately thick. The general description of the rock material at this site is ‘very weak’, which suggests a unconfined compressive shear strength of between 1 MPa to 5 MPa in accordance with NZGS2, and is dependent on weathering, grainsize and matrix composition. We have adopted a conservative strength of 1 MPa for the sandstone and mudstone rock.

5 GEOTECHNICAL R ISK AS SESSMENT

We have adopted a generic risk assessment approach to assess if the current geotechnical investigation completed by Engineering Geology Ltd is suitable for the proposed development, combined with the site specific geotechnical hazards identified at this site. The geotechnical hazards are assessed by either qualitative and/or quantitative techniques commonly used in geotechnical engineering. For this site, the following qualitative techniques have been adopted

• Reviewing previous geotechnical investigation reports prepared for the site

• Reviewing reliable published material to assess the site and surrounding geology

• Carry out site walkover to assess site topography and geomorphology

• Review historical aerial photo imagery to assess site instability and cliff face regression from erosion For this site, the following quantitative techniques have been used to assess the geotechnical hazards associated with the proposed development:

• Review of soil strength parameters cited within Engineering Geology Ltd’s 2013 and 2014 interpretive reports for quantitative assessment of soil stability and strength under imposed foundation loads in accordance with NZS 1170.

• Carry out simple slope stability based on ‘theoretical slip failure surface’ from cliff crest

• Application of Mohr-coulomb theory to assess stability of unsupported vertical cuts in soil/weathered rock mantle during basement excavation.

• Develop geological cross sections to identify subsurface soil profile and water table regime.

Based on a simple risk-assessment framework, we have considered both the likelihood of occurrence and consequence of geotechnical hazards on the site before and after the proposed development. The Likelihood (or chance) of each geotechnical hazard occurring combined with the consequence (outcome) has been rated on a percentage/point system as described below.

5 . 1 R i s k A s s e s s m e n t D e f i n i t i o n s

The term Likelihood (or chance) is rated as follows:

• Very unlikely (0 to 20% chance)

• Unlikely (21 to 40% chance)

• Likely (41 to 60% chance)

• Very likely (61 to 80% chance)

• High (81 to 100% chance)

2Section 3.0 of the NZ Geotechnical Society’s ‘Field Description of Soil and Rock’, 2005

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The term Consequence (or outcome) due to identified geotechnical hazard has been rated from 1 to 5 as follows:

• Score of 1 means: (The consequence) would pose no immediate risk to owner/occupier

• Score of 2 means: Would pose some risk to owner/occupier

• Score of 3 means: Would pose moderate risk to owner/occupier

• Score of 4 means: Would pose significant health and safety risk to owner/occupier

• Score of 5 means: Would be fatal to owner/occupier’s health and safety

5 . 2 R i s k A s s e s s m e n t o f S i t e - S p e c i f i c G e o t e c h n i c a l H a z a r d s Be fo re a n d A f t e r t h e P r o p o s e d D e v e l o p m e n t

Soil Inundation: The site is located in a well-established suburb of Auckland with no extreme elevated ground contours to either boundary which may initiate potential soil inundation from slope instability onto the subject property. Therefore, soil inundation is unlikely to be a risk over the subject property. Flood Inundation:

The site is located at the bottom of a sealed road with an established public storm water system to intercept and divert potential surface water runoff from upslope of the subject property. Combined with the micro topography to the east and west side of the subject property, which naturally diverts surface water runoff away for the site, flood inundation is unlikely to be a risk during extreme storm events over the subject property. Localised slope instability

Based on the previous investigations completed at this site, the performance of existing and neighbouring buildings, and observations made on site, it is considered that there is no existing risk of localised slope instability on the site. However, we consider there may be increased risk of surficial slope instability within the residual soil mantle immediately downslope of the proposed development. The risk of surficial soil movement is created by potential ingress of surface water migrating between the perimeter face of the basement wall and the wedge of residual soil overlying the weathered sandstone/mudstone unit. To mitigate shallow instability of the remaining residual soil mantle would require a subsurface drainage channel to divert water to either the established storm water pipe that crosses the site near the west boundary, to a retention tank system towards the south west corner of the site, which should be designed to release the water overland at the ‘green-field’ discharge rate. Specific design of such a system would be part of a building consent application.

Global slope stability

With significant removal of the overlying residual soil mantle for the proposed development and subsequent reduction of overburden weight upslope of the cliff crest, the overall global stability of the site would be improved.

Based on the previous investigations, combined with the observed performance of the neighbouring sites from aerial imagery, the risk of global slope instability on the site is considered to be low. We consider the proposed development increases the site’s global stability due to intercept and diversion of surface water runoff and removal of the weaker overlying residual soil mantle.

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Cliff erosion Engineering Geology Ltd commented in their 2014 interpretive report, that the ‘exposed cliff face comprises interbedded sandstone and siltstone of the Waitemata Group of Tertiary age that dip to the west approximately parallel to the cliff face at 100 ‘. The top edge of the interbedded sandstone and siltstone is protected by a mantle of residual soil which provides effective protection from surface erosion. However, the same erosion mechanisms will gradually erode the exposed cliff face over time. Engineering geology Ltd has considered this type of erosion process along the southern face of the cliff, and concluded with the general setting being within the inner harbour combined with a low energy wave environment, the rate of cliff face erosion and undermining of the cliff would be slow, at about 2 m per 100 years. Review of aerial imagery from 1963 to 2017, suggests that negligible to minor cliff top regression has occurred during the last 54 years, with apparently negligible accumulation of detritus material observed along the west and east side of the coastline adjacent to the cliff toe. Therefore, we concur with Engineering Geology Ltd’s inferred cliff erosion rate at 2 m per 100 years for the south-facing flank of the cliff face. Reference to cross section A-A; Appendix C which shows the inferred cliff face erosion regression line in relation to the building’s basement perimeter along the south boundary. At the closest point between the proposed building and the inferred cliff crest, the eroded cliff face regresses to about 6 m from the basement foundation wall after 100 years of erosion. Table 5.1 below shows the likelihood and consequence of each site-specific geotechnical hazard before and after the proposed development as discussed above: Table 5.1: Risk assessment of site-specific geotechnical hazards before and after proposed development

Geotechnical Hazard

Likelihood of Occurrence Consequence

Before development

After development Before

development After development

Soil inundation Very unlikely Very unlikely 1 2

Flood inundation

Very unlikely Very unlikely 1 3

Localised slope instability

Unlikely Very Likely 1 3

Global slope instability

Likely Very unlikely 3 5

Cliff erosion Likely Likely 2 4

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5 . 3 R i s k A s s e s s m e n t o f G e o t e c h n i c a l I s s u e s c r e a t e d b y P r o p o s e d D e v e l o p m e n t

Based on the same risk-assessment framework adopted in Section 5.1 we have considered the geotechnical issues particular to the design and/or construction of the proposed structure and the likelihood and consequence of each geotechnical issue on the structure in accordance with the New Zealand Building Code.

Foundation settlement Reference to engineering geology sections in Appendix C, the excavation depth required to form the basement foundation platform varies from 0.5 m at the south-west (SW) corner to about 12.4 at north-east (NE) corner of the basement footprint. The exposed subgrade material at the basement platform would be completely weathered ECBF rock at the NE corner, transitioning to residual soils towards the SW corner. Applying an average bulk density of 18 kN/m3 (as citied in Engineering Geology Ltd’s 2014 report) translates to a reduced overburden soil stress at cut platform level of 223 kPa at the NE corner, reducing to 9 kPa at the SE corner of the basement footprint. Based on a typical imposed floor weight of 6 kPa (inclusive of permanent reduced live load), we have assessed the building weight at basement level between 20 to 24 kPa. With the basement self-weight taken as 14kPa (for a 0.6m thick foundation), the total imposed foundation weight is between 34 to 38kPa. Most of the excavated soil depth would more than compensate for the maximum imposed building weight, and therefore long-term static settlement within the exposed weathered rock material would be unlikely. However, some static settlement may be expected at the SE corner, considering the overburden removed (9kPa) would not completely compensate for the imposed building weight. To reduce the risk of settlement would require over-excavation of the remaining residual soil layer above the completely weathered rock (see cross section C-C; Appendix C). The magnitude of total static settlement is expected to be within the range of 5 to 15 mm, with corresponding differential settlement of 3 to 8 mm over a horizontal distance of 47 m. This complies comfortably with Section B1/VM4 of the NZBC, which recommends limiting differential settlement to 25 mm over a 6 m span.

Stability during basement excavation We have quantified the cut face stability within the residual soil and completely weathered rock material by applying Mohr Coulomb’s theory. The unsupported cut face height can be expressed by the following simple expression:

Zc = 2c’.sqrt(1/ka)/b ; where Zc is measured in metres

The following soil parameters haven been reproduced from Engineering Geology Ltd.’s 2014 report, for calculation of Zc:

• Within the residual soil unit: c’ = 30 kPa

b = 18 kN/m3

ka = 1.0 (for ’= 0 degrees)

The calculated Zc value for the above soil strength parameters is 3.3 metres

• Within the completely weathered rock unit: c’ = 40 kPa

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b = 18 kN/m3

ka = 0.24 (for ’= 38 degrees)

The calculated Zc value for the above soil strength parameters is 9.1 metres

Excavation into the completely weathered rock would be stable, without any requirement for lateral support during the basement construction phase. However, with excavation of the residual soil mantle exceeding the critical unsupported cut face height of 3.3 m, the residual soil mantle will require lateral support during the construction phase.

Basement retention works

• Along the east boundary Considering the proposed cut face is setback about 1.2 m off the legal boundary, soil retention is required along this boundary. Discussion with the structural designer suggests RC piles propped back with two rows of steel beam whalers and steel screw piles appears to be the most practical solution. Sheet piling would be vibration-installed and driven into the slightly weathered sandstone/mudstone rock surface at 13 m and then tied back with horizontal steel whalers and screw pile anchors as excavation of the soil proceeds. The upper row of screw piles would be drilled horizontally into the soil mantle at about 3 m below existing ground level and extending across the legal boundary line to the adjacent council reserve. A second row of steel screw piles would be required at about 8 m below existing ground level and drilled into the stiffer weathered rock material.

Written permission may be required from the council to permit screw pile anchors being drilled into the subsurface soils within the Council’s public reserve site.

• Along the north boundary Considering the proposed cut face is setback about 3 m off the legal boundary, soil retention is also required along this boundary. Discussion with the structural designer suggests RC piles propped back with two rows of steel beam whalers and steel screw piles appears to be the most practical solution. The RC piles should be drilled at least three pile diameters into the slightly weathered sandstone/mudstone rock surface at 13 m and then tied back with horizontal steel whalers and screw piles as excavation of the soil proceeds.

• Along the west boundary Considering the proposed cut face is setback about 8 m off the legal boundary, soil retention is not required along this boundary, as the upper residual soil mantle can be battered back at 1H:1.6H would remain stable subject to the battered face being adequately covered by polythene sheeting (or similar) to minimise change in the soils’ natural moisture content. Excavation within the underlying completely weathered rock material would not require soil retention, as the maximum exposed cut face height at about 7 m is less than the critical unsupported height (Zc) of 9.1 m (see calculation above). Stability to neighbouring properties during basement construction To the east side of the site, exists a Council reserve which is undeveloped and covered by moderately dense vegetation. The proposed basement excavation and associated retention works is not expected to affect this site. The likelihood of ground subsidence and/or horizontal land movement will be mitigated by specific design of the sheet-pile wall system and horizontal tieback solution discussed above.

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To the west side of the site, exists a well-established residential site comprising single storey dwelling set-back about 3 m off the west boundary. The horizontal distance between the proposed basement excavation line and the existing dwelling is about 11 m. This distance exceeds the minimum required setback of 8 m required to mitigate subsidence risk to the dwelling during the basement construction phase. To the north side of the site, the horizontal distance between the proposed basement excavation line and Tizard road is less than the minimum required setback of 8 m and therefore, susceptible to settlement and/or instability if the basement excavation cut face is not retained by the method discussed above. Subject to the north and east basement excavation cut faces being retained by the propped sheet-pile wall as discussed above, the risk of instability and/or subsidence of the neighbouring sites would be unlikely.

Basement construction methodology Basement construction will require specialist contractors to install:

• Sheet-pile profiles

• Steel screw piles

• Dewatering

• Bulk excavation

In terms of noise and disturbance, the above construction activities are not expected to generate operating noise levels exceeding 85 decibels, and therefore should not cause unnecessary disturbance to neighbouring properties. Sheet piles will need to be installed first before commencing with bulk excavation to at least 2 m below existing ground level. Once achieved, the first row of steel whalers and screw pile anchors should be installed before continuing with excavation. At the same time, the residual soil mantle should be gradually battered to 1V:1.6H (32 degrees) slope along the west boundary to maintain stability during the basement construction phase. Once bulk excavation reaches 8 m depth, the second row of steel whalers and screw pile anchors should be installed, and dewatering should be included to control seepage through the north west cut face.

Dewatering As discussed in Section 3.2 above, a perched water has been identified towards the west side of the site, between hand auger boreholes BH4 and BH 6 to 9. The perched water table is confined to the residual soil mantle overlying the weathered sandstone/mudstone unit. During excavation of the basement structure, the entire residual soil mantle will be excavated, creating the potential seepage from the exposed cut face to the north-west corner of the site. The residual soils comprise very stiff silts and clays with typically low permeability values in the order of 10-5 to 10-7m/s. Therefore, seepage would be quite low during excavation of the basement area, and dewatering would most likely be controlled by a series of single phase submersible pumps. After completion of the basement structure, the active seepage from the north-west cut face should be intercepted by a subsurface drainage system to collect and divert the water to either the established reticulated storm water system that crosses the site near the west boundary, or by construction of a large retention tank system to be located towards the south west corner of the site, which should be designed to release the water into the natural soils at the ‘green-field’ discharge rate. Specific design of such a system would be part of a building consent application.

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Date: 19/09/2017 Job No: 166115

In accordance with Chapter E7 of the AUP: ‘Taking, using, damming and diversion of water’, dewatering and/or diversion of water from the subject property is a permitted activity (i.e.no resource consent required) if the relevant permitted activity standards in Rule E7.6.1 are met. There are two permitted activity standards that are relevant to these activities. If all the relevant standards in these two rules are met then no resource consent is required. The ground water table occurs at 13 m depth (see MBH3; appendix B) and is below the basement excavation depth. Therefore, clause E7.6.1.6 associated with dewatering of the groundwater is not relevant this site.

Intercept and diversion of a perched water table is required during excavation of the basement foundation and accordingly the groundwater depth is not affected by this activity. Accordingly, clause E7.6.1.10: ‘Diversion of groundwater caused by any excavation’ is not relevant to this site. As discussed under Basement Retention Works heading above, during excavation of the basement sheet piling is required to an embedment depth of 13 m, which deliberately coincides with the regional ground water table. Accordingly, items 4a and 4b of clause E7.6.1.10 are satisfied for this site, as sheet piling will not:

• Impede the flow of groundwater and

• Extend below the natural groundwater level. In summary, dewatering of the site associated with a perched water table meets the various standards associated with Activities A17 to A28 of Chapter E7 of the AUP, and accordingly a resource consent is not required for dewatering of the subject property.

Site erosion Approximately 34% of the available site area is to be occupied by the proposed development. The remaining areas will be planted in accordance with a specifically designed landscape plan to enhance the existing area and minimise surface erosion by controlling surface water runoff. Therefore, we consider the proposed development will not worsen or accelerate the current rate of surface erosion. Table 5.2 below summarises the likelihood and consequence of the geotechnical issues created by the proposed development as discussed above:

Table 5.2: Risk Assessment of geotechnical issues created by proposed development

Geotechnical Issue Likelihood Consequence

Foundation settlement Unlikely

2

Instability during basement excavation

Unlikely

5

Instability to neighbouring properties

Likely 3

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Dewatering

Likely

2 to 3

Site Erosion Very Likely

2

6 GEOTECHNICAL RECOMMENDATIONS

6 . 1 G e n e r a l

The recommendations and opinions contained in this report are based upon ground investigation data completed by Pro-Drill Ltd for Engineering Geology Ltd.’s 2014 report. The actual sub-surface conditions may vary from those described in Engineering Geology’s 2014 report, subject to further subsurface investigation.

.

6 . 2 F o u n d a t i o n o p t i o n s

6 . 2 . 1 S h a l l o w s t r i p a n d p a d f o u n d a t i o n s

The intended development will comprise four storeys inclusive of the basement level. The proposed development requires a basement platform cut out to an RL of about 26.6 m and translates to excavation depth of 12.4 m to 0.5 m as dictated by the natural site slopes. The overburden weight removed more than compensates for the imposed building weight and therefore a shallow foundation system combined with a monolithically poured concrete slab on grade would be satisfactory for this development. At this stage, it is envisaged that the shallow strip and pad foundations would be embedded 600 mm below cleared platform level (RL 27.2m) in to the completely weathered ECBF soils.

With reference to the machine borelog (MBH1 to MBH3), the exposed weathered rock material returned an average corrected SPT blow count value (N60) of about 30. By adopting Meyerhof’s equation (1974) below for SPT verses bearing capacity, we can quantify the allowable bearing capacity for a shallow loaded foundation with settlement limited to 25 mm.

Meyerhof’s equation takes the form of: qa = (N55/0.08).((B+0.3)/2)2.(1 + 0.33D/B) in kPa

Adjusting the N60 of 30 to an N55 value of 33 and adopting a footing width (B) of 1.2 m and embedment depth (D) of 0.6 m, we arrive at an allowable bearing capacity of 270 kPa for 25 mm of static settlement. This corresponds to a geotechnical ultimate bearing capacity of 810 kPa.

If shallow foundation loads are expected to exceed the capacities given above then specific geotechnical review should be undertaken of the foundation design to determine if greater bearing capacities are available.

6 . 2 . 2 B a s e m e n t w a l l s

As discussed in Section 5.2 above, the basement walls are required to resist lateral earth pressure loads after completion of the development. The very stiff structure will limit lateral movement of the basement walls and therefore, the walls should be designed for ‘at-rest’ (Ko) earth pressure loads. For basement walls supporting

the residual soils, a Ko value of 0.5 is recommended (based on a ’ value of 30 degrees).

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Date: 19/09/2017 Job No: 166115

For basement walls supporting the underlying weathered rock material, a Ko value of 0.38 is recommended

(based on a ’ value of 38 degrees)

The basement walls should be designed for static earth pressure loads to NZS 1170.

6 . 3 S i t e p r e p a r a t i o n a n d e a r t h w o r k s

During earthworks operations all topsoil, buried topsoil, uncontrolled fill including construction wastes (e.g., masonry, concrete, metals), organic matter including tree roots and other unsuitable materials should be removed from the construction areas in accordance with the recommendations of NZS 4431:1989.

Final cut earthworks are expected to form the basement foundation platform for the proposed development.

Localised low volume filling earthworks (i.e., less than 0.6 m thickness) may be required to beneath the basement slab.

Exposure to the elements should be limited for all soils encountered on site. Subgrade excavations should be left proud of the finished subgrade level by 200 to 300 mm if a delay prior to construction is expected. The final cut to grade should be performed immediately prior to foundation construction. Alternatively, these areas can be undercut and rebuilt to formation level with certified granular fill should the subgrade deteriorate due to exposure.

Any exposed surfaces should be graded to allow runoff and to avoid ponding of water. All overland flow should be collected and treated in accordance with Auckland Council TP90 – Erosion and Sediment Control.

7 FURTHER GEOTECHNICAL INVESTIGATIONS FOR B UILDING CONSENT APPLICAT ION

Engineering Geology Ltd.s’ 2013 and 2014 investigations provide sufficient shallow and deep testing data to adequately design the foundations and perimeter basement walls for the proposed development. One of the three deep machine auger boreholes identified the regional water table, which in this case would not influence dewatering of the site during construction of the proposed development.

The shallow boreholes encountered a shallow perched water table within the residual soil mantle and located towards the west side of the development footprint.

Therefore, we recommend additional shallow borehole testing is carried out between BH5 and BH7 to BH9 to assess if the perched water has migrated across the site due to increased transient horizontal seepage through potential discrete layers of sandy silt and sand.

8 CONCLUSIONS

Based on the site-specific investigation data and the currently proposed development scheme, the following geotechnical recommendations are presented.

• The site soil profile comprises a weathered profile of East Coast Bays Formation.

• The proposed development will require significant earthworks to reach the basement platform level.

• The depth of basement excavation will require temporary retaining along the north and east boundaries to maintain stability to neighbouring sites.

• Global stability of the site is enhanced due to improved control of surface water runoff over the site combined with complete removal of the residual soil mantle as a consequence of the proposed development.

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Date: 19/09/2017 Job No: 166115

• The inferred cliff face erosion rate of 2 m in 100 years would not affect the structural performance of the proposed development in accordance with the New Zealand Building Code.

• The proposed four storey development can be founded on shallow pad and strip footings.

• A geotechnical ultimate bearing capacity of 810 kPa (270 kPa allowable) can be adopted for specific design of shallow pad and strip foundations

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Date: 19/09/2017 Job No: 166115

APPENDIX A

• Site Investigation Plan (SK-1_rev A)

APPROVED

DRAWNCHECKED

SCALES :

DESIGNED

CAD

REF

:

DATE

Job No. Sheet No. Rev.

ORIG

INAL

SIZ

E A4

DO N

OT S

CALE

- IF

IN D

OUBT

, ASK

Client:

Site: ACENZ

PROPO

SED BU

ILDING

PR

OP

OS

ED B

UIL

DIN

G

PROPOSED BUILDING

RC CO

NSENT

2B

RC CO

NSENT

2C

RC CONSENT2A

PALISADE WALL

PALISADE WALL

TOP OF CLIFF

ESPLANADE STRIP

ESPLANADE STRIP

TOP OF CLIFF

STAKES - EXISTING CONSENTED

STAKES - NEW PROPOSED

3 4 5 6 7 8 9 10 11 12 13 14 15 16 1718 19

20 212

22

24

23

25

26

27

28

29

30

31

32 33

34

35

36

37

39

38

37

36

35

34

33

3231

3029

282726

2524

24 25

26

2728

29 30

38

2

3 45 6

78

9

10 11 1213

2

34 5 6 7 8 9 10 11 12

1314 15 16 17 18 19

20 21 22 23

Bush ProtectionCovenant

4 Tizard Road,Birkenhead, Auckland.

LOT 22DP 14016

Hinemoa Park

Allot 658 PSH of Takapuna

LOCAL ESPLANADERESERVE

1478 m2

COASTAL INUNDATION

CONTROL

SIGN

IFICANT ECO

LOG

ICAL AREA

SOIL TEST LOCATION PLAN

Not To Scale

AJ - 22.08.2017

Young & Richards Ltd

2 & 2A Tizard Road, BickenheadAuckland 166115 SK-1 A

Site Plan has been re-produced from overlaying Young & Richards Architects Ltd Site Plan and Engineering Geology Ltd's Bore hole Location Plan. Kirk Roberts Ltdhas re-sized the plans in order to provide a visual indicator of the proposed development and the Bore hole testing completed onsite by Engineering Geology Ltd. KirkRoberts Ltd can not verify the accuracy of these drawings and plans used. All soil test locations and cross sections are approximately only.

N

S

S

S

S

S

S

BH2

BH3

BH5

BH1

A

A

C

C

B

B

Outline of Proposed Building

Boundary of the Site

S

S

SMBH3

MBH1

MBH2

LEGEND:

BH 1 - 9 From Engineering GeologyLtd's Geotechnical Investigation Report dated 5th March 2013

MBH 1- 3 From EngineeringGeology Ltd's SupplementaryGeotechnical Report dated 15thSeptember 2014

Cross Sections A, B & C completedby Kirk Roberts Ltd.

SBH7

BH4S

BH9

BH8S

Approximate Locationof perched water table

BH6

D

D

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Date: 19/09/2017 Job No: 166115

APPENDIX B

• Machine borehole logs (MH1 to MH3) reproduced from Engineering Geology Ltd’s 2014 report

• Shallow hand auger boreholes (BH1 to BH9) reproduced from Engineering Geology Ltd’s 2013 report

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Date: 19/09/2017 Job No: 166115

APPENDIX C

• Cross section B – B (ref 166115_SK-2)

• Cross section B – B (ref 166115_SK-3)

• Cross section C – C (ref 166115_SK-3)

• Cross section D – D (ref 166115_SK-4)

N

SR

AJAJ 166115

SK -2 A22.08.2017

CROSS SECTION A -A2 & 2A Tizard Road, BirkenheadAucklandCLIFF TOP APARTMENTS

1:400 @ A3

Elev

ation

in M

etres

(m)

40

10

20

0

30

40

10

20

0

30

??

Approximate Location of New Residential Building

Esplanade Strip

Site Boundary

26.01°

??

Site Boundary

Area to be Excavated

Proposed 2.0 m Cliff Face Erosionin accordance with Engineering

Geology Ltd dated 5th March 2013.

Existing Ground Surfacefrom Contour Information

Section C

Section B

MBH1 Offset 33.2 m due East

MBH3 Offset 15 m due West

??

? ?? ? ?

? ? ? ?? ? ?

? ?

?

CROSS SECTION A - A

BH7 Offset 13 m due West

BH5 Offset 7.5 m due East

SlightlyWeathered Rock

Completely to HighlyWeathered Rock

MBH2Offset 12m due East

??

??

?????? ?

BH2 Offset 14 m due West

Proposed Palisade Wall

Inferred theoretical slipsurface in residual soils.

Inferred Soil /Rock Boundaries

N

SR

AJAJ 166115

SK -3 A22.08.2017

CROSS SECTIONS B & C2 & 2A Tizard Road, BirkenheadAucklandCLIFF TOP APARTMENTS

1:400 @ A3

40

20

30 30

30

20

30

20

40 40

20

40

Section A

BH 3No offset

BH 915 m offsetdue South

??

?

Site Boundary

Site Boundary

Site Boundary

Site Boundary

BH 416 m offsetdue North

BH 215 m offset due South


BH 116.5 m offset

due South

Approximate Location of Proposed Development

Area to be Excavated

Existing Pallisade Wall- Depth unknown


BH 524 m offsetdue South

BH 73.5 m offsetdue South


BH 8No offset

Section A

Approximate Location of Proposed Development


1010

? ? ?? ?

? ? ? ?? ? ?

Area to be Excavated? ??

1010

MBH 315 m offset due South

MBH 12 m offset due North

CROSS SECTION C - C

CROSS SECTION B - B



???????????

MBH22.5 m offset due North



?

? ? ?? ? ?

? ? ?? ? ? ? ?

? ? ? ? ? ? ?

??????

???

?

?????



Section D

BH 422.5 m offset

due North

Section D

??

?? ?

?? ?

? ? ? ?? ?

? ?? ?

? ?? ?

???????

?????

?

??

?

??

??

?? ? ? ? ?

? ?? ?

? ??

?

N

SR

AJAJ 166115

SK -4 A24.08.2017

CROSS SECTION D -D2 & 2A Tizard Road, BirkenheadAucklandCLIFF TOP APARTMENTS

1:400 @ A3

Elev

ation

in M

etres

(m)

40

10

20

0

30

40

10

20

0

30

MBH3 Offset 22 m due South

BH2 Offset 21 m due South


BH8 Offset 13 m due North


BH3 Offset 21 mdue North

Section C

Section B




Approximate Location of New Residential Building


Existing Ground Surfacefrom Contour Information BH6

Offset 3 m due South

InferredGroundwater

SiteBoundary

Proposed 2.0 m Cliff Face Erosionin accordance with Engineering

Geology Ltd dated 5th March 2013.

26.02°

Inferred theoretical slipsurface in residual soils.

Documents

GEOTECHNICAL REPORT... · 2019. 2. 12. · GEOTECHNICAL REPORT 2 & 2A Tizard Road Development Birkenhead, Auckland Prepared for Young + Richards Ltd JOB No. 166115 DATE 19/09/2017