GEOTECHNICAL DESIGN FOR OPEN PITS AT … · The South African Institute of Mining and Metallurgy International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering

The South African Institute of Mining and Metallurgy International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering Kevin Holley

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GEOTECHNICAL DESIGN FOR OPEN PITS AT TANJIANSHAN, CHINA

Mr Kevin Holley1, Mr Paul Skayman2, Prof Huang Zhiwei3

ABSTRACT

In 1989 The First Brigade for Geology and Mineral Exploration of Qinghai Province discovered gold at Jinlonggou and Qinlongtan, Tanjianshan. Between 1992 and 2002 small scale mining was carried out at these sites using underground mining methods at Jinlonggou and an open pit at Qinlongtan. During 2003 and 2004 geotechnical investigations were carried out in support of a bankable feasibility study. An important objective of this investigation was to meet both internationally accepted standards and codes of practice, and also to satisfy the Chinese requirements. Extensive use of local experience was utilised. Where possible geological drilling information was incorporated into the assessment. Extensive geotechnical mapping of surface and underground exposures, and index testing to gain an appreciation of rock intact strength were used to characterise the sites. Rock mass classification (MRMR) was used to assess indicative overall slope angles and appropriate bench stack heights for the proposed pits at Jinlonggou and Qinlongtan. Measured discontinuity orientations were used in kinematic analysis to assess stability of possible pit slope geometry at a batter and overall scale to provide preliminary recommendations for the pit slope configuration. Numerical modelling was also carried out to improve the understanding of anticipated pit behaviour. Site investigation and preliminary geotechnical design details, that incorporate site specific conditions for the formation of open pits at Tanjianshan, are discussed in this paper. 1 INTRODUCTION 1.1 The site The Tanjianshan Gold Project (TJS) is located in Qinghai Province, Haixi Prefecture, in northwest China (Figure 1). The project site is situated at latitude 38°15’N and longitude 94°32’E, approximately 75 km northwest of Dachaidan which is the closest town. Tanjianshan is in the Saishiteng Mountains which have an elevation of more than 4,000 m to the north of the Chaidamu Basin.

1 Principal Geotechnical Engineer, SRK Consulting – Level 9, 300 Adelaide St, Brisbane, 4000. Telephone (+617 3832 9999). Email [email protected]. 2 China Manager, Eldorado Gold Corp. 3 Professor and Deputy Dean Resource and Environment Department, Wuhan University of Technology


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Figure 1: Location of the Tanjianshan Project, Qinhai Province, P.R. China (after Couture and Siddorn, 2003). The landscape at the site comprises rugged mountains with natural slopes of 45° to 55° that are separated by piedmonts of alluvial fans. The site is very arid and there is almost no vegetation in the mountains as shown in Figure 2.

Figure 2: Photograph Showing the Site Character (Jinlonggou) The site has a dry continental climate with low rainfall and high evaporation. Winters are long and summers short, and there is a large diurnal temperature range. At Dachaidan the average annual temperature is 1.6°C and annual rainfall is 200 mm. The maximum monthly temperature occurs in July and August (21°C) and the minimum monthly temperatures are typically in December and January (-15° C). Maximum monthly rainfall occurs in June and July (40 mm) and, typically, no precipitation is recorded in November, December or January.


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At Tanjianshan there are two areas within which mining is to be carried out. Jinlonggou, is approximately 12 kilometres from the asphalt road between Ge’ermu and Dunhuang. Qinlongtan is about 20 kilometres to the northwest of Jinlonggou. Access between the areas is along an un-surfaced all-weather track from Jinlonggou. There have been previous mining campaigns within both areas, and this needed to be taken into consideration during the geotechnical evaluation. During the various phases of geological exploration that have been undertaken there have been a considerable number of boreholes drilled. This has required the formation of roads to provide access for the drill rigs. During the formation of the access roads there was a requirement to excavate into the mountain side. These excavations provided an important source of geotechnical data. 1.2 Geological Setting TJS is located in the Chaidamu Northern Uplift Zone of the Kunlun Variscan Geo-synclinal Fold Belt. This is part of the Northern Chaidamu Para-platform, which is bounded to the north by the Qilian Geo-syncline Fold Belt. The Tanjianshan gold deposits are hosted in mid-Proterozoic Wandonggou Group carbonaceous phyllite and diorite porphyries of late Variscan age. The simplified stratigraphy is summarised in Table 1. Table 1: Simplified Stratigraphy

Period Group Lithology Age Comment Quaternary Alluvium and

Colluvium Unconsolidated conglomerates

Occurs in river valleys and alluvial fans. Occurs both sides of the Aolaohe River

Tertiary - Alluvium and Colluvium

60Ma to present

Jurassic Sandstone, siltstone and mudstone with carbonate and organic rich shale.

Underlies the unconsolidated conglomerates (unconformable)

Triassic - Clastic Rocks Late Carboniferous or early Permian

- Intermediate Diorite Porphyry Granite Porphyry

294.7 ±3.8Ma

Early Carboniferous

Huaitala Marine

Devonian Maoniushan Marine Cambrian Ordovician

Tanjianshan Andesites & dacites; Ultramafic and mafic dykes and range of clastic rocks

Mid-Proterozoic

Wandonggou Marble, shale, quartzite 1150 ±280Ma

Occurs to west of sites

Early-Proterozoic

Dakendaban Gneiss, schist, marble 1900


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Tanjianshan is located in an arid environment and the weathering profile of the rock mass at both Jinlonggou and Qinlongtan is poorly developed. It is largely confined to the upper 500 mm crust at the surface that has been shattered by frost action. Within this zone, the rock mass was observed to have commonly slumped on discontinuities. Geological exploration has shown oxides to occur to a depth of between about 20 m (south) and 50 m (north) at Jinlonggou. At Qinlongtan the oxides are thought to occur to a depth of about 30 m. A summary of the different rock types (and the lithological codes used in this paper) is given in Table 2

Table 2: Rock Types at Tanjianshan

Lithology Code Rock Type Lithology

Code Rock Type

FBX fault breccia SDB dolomitic marble IDD diorite SMB marble

MGG gabbro SSB siltstone QB quartz breccia SSG graphite schist

QFP qz-fp porphyry SSQ quartzite QZ quartz TPN carbonaceous phyllite

SAS sandstone TPO phyllite (undifferentiated) SAY claystone, mudstone TPO phyllite (undifferentiated) SBL limestone TPP Pellitic phyllite

The interpreted geological conditions at Jinlonggou and Qinlongtan are complex, and quite different, as can be seen by consideration of Figure 3 and Figure 4.


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Figure 3: Interpreted Geological Section Through Jinlonggou (after DevMin, 2004)

Mineralised Zone

Figure 4: Interpreted Geological Section Through Qinlongtan (after DevMin, 2004)

1.3 Groundwater

At the time of the geotechnical investigation (August 2004) very little information was available with respect to groundwater conditions and the groundwater conditions were inferred on the basis of judgement and interpretation of available information. At Jinlonggou the water table was not intersected by exploration boreholes, and it is expected that the permanent water table will not be intersected during the proposed mining activities. From the available information at Qinlongtan it was anticipated that the proposed “starter” pit could intersect groundwater at a depth of greater than about 70 m, and that the proposed underground workings will encounter water. Water


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inflows for both areas were expected to be minimal. The authors did, however, consider that further information is required to confirm these assumptions. 1.4 Insitu Stress and Seismicity In the absence of site specific information the UNAVCO and World Stress Map Project (WSM) data (Reinecker et. al., 2004) have been used to obtain a preliminary indication of the insitu stress regime that is likely to prevail at the Tanjianshan site. The interpreted insitu stress at the site is shown in Figure 5 and Figure 6. Figure 6 shows that there is not a very strong correlation for the data set between depth and stress. However, in the absence of other more reliable information the authors judged that the calculated relationship could be considered as indicative of the likely stress regime at Tanjianshan. This relationship was adopted for the feasibility assessment with intention of generating site specific data during the early stages of mining. From the available data the authors determined that the ratio between the maximum stress and the vertical stress was 3.3 in the vicinity of Tanjianshan. This ratio is higher than the maximum k value that is defined in the relationship that has been postulated by Brady and Brown (2004). However for the purposes of feasibility assessment, and in the absence of site specific data, it is judged that this relationship can be considered as indicative of the potential stress regime at Tanjianshan. Tanjianshan is located in a zone classified as Grade 7 on the Chinese scale of seismicity, as defined by the Chinese Regulations. For this Grade of seismicity, the maximum acceleration recommended for use in the design is 0.10g with a characteristic period of 0.4s. The Chinese Code of Practice does not assign a return period or probability of occurrence associated with the maximum anticipated acceleration.

Figure 5: Orientation of Horizontal Principal Stresses (after Reinecker et.al., 2004)


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Figure 6: Maximum Stress near Tanjianshan (data from Reinecker et.al., 2004) The results of the Global Seismic Hazard Assessment Project (GSHAP) were used to confirm the appropriate seismic design criteria for the Tanjianshan Area. GSHAP show that there is a documented record of large magnitude earthquakes around the site (but not at the site). GSHAP has generated a seismic hazard map (Figure 7) taking the distribution of known seismic events and also the structural geological conditions into account. At 38º15’N and 94º32’E (Tanjianshan Site) GSHAP have calculated that the site can be expected to be subjected to an earthquake induced maximum acceleration of 0.19g, with a probability of exceedance of 10% in 50 years.


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Tanjianshan

Figure 7: The seismic hazard map of Asia depicting peak ground acceleration, given in units of m/s2, with a 10% chance of exceedance in 50 years (GSHAP, 1999). Taking both the Chinese Codes of Practice and the results of GSHAP into account, the seismic criteria given in Table 3 were adopted to assess preliminary design configurations as judged to be appropriate. Table 3: Assumed Seismic Criteria

Seismic Event Maximum Acceleration

Duration Main Period

OBE Operating Basis Earthquake

0.10g 30 s 0.4 s

2 SITE INVESTIGATION Table 4 presents a summary of the type of fieldwork that was done for the purposes of geotechnical assessment at Jinlonggou and Qinlongtan. Surface outcrops were selected by the lead author to provide geotechnical data for rock mass classification from a spread of locations across both of the sites. When surface outcrops were mapped they were subdivided so that markedly different geotechnical properties or outcrop orientations were distinguished between.


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Table 4: Scope of Site Investigation

Site

Surf

ace

Win

dow

M

appi

ng

Und

ergr

ound

Win

dow

M

appi

ng

Geo

tech

nica

l Bor

ehol

e L

oggi

ng o

f (P

re-d

rille

d)

Geo

logi

cal H

oles

Geo

tech

nica

l Bor

ehol

e L

oggi

ng o

f H

oles

Sit

ed

for

Geo

tech

nica

l In

form

atio

n

Insi

tu S

tren

gth

Inde

x T

esti

ng

Lab

orat

ory

Tes

ting

Jinlonggou � � � � � � Qinlongtan � � � � � �

A considerable amount of geological exploration drilling has been carried out at Jinlonggou and Qinlongtan. The quality of the geological core that was stored at site was assessed, and whilst it was recognised that the condition of the core was not ideal for geotechnical assessment (core had been split and was not drilled to the same standards/codes of practice needed for geotechnical assessment), it was judged that useful geotechnical information could be obtained from the boreholes. A number of boreholes were selected for geotechnical logging across the Jinlonggou site, taking into account the anticipated extent of the open pit. Representative boreholes were selected (for the purpose of obtaining geotechnical data) at Qinlongtan on the basis of holes to be drilled during the 2004 drilling season. There were some areas at Jinlonggou that were judged to have insufficient information to make a geotechnical assessment at the feasibility level of detail. Three holes were therefore planned to be drilled at Jinlonggou, with an objective of obtaining information from the proposed pit wall areas. These holes were planned with the geological department and finally sited at locations that would be of benefit to both the geological and geotechnical investigation programs. When the site conditions were known it became apparent that it would be difficult to collect and transport samples to a laboratory for testing without causing damage to the samples. As a substitute to extensive laboratory strength testing, and also to provide a means of correlating field strength descriptions, index tests were carried out at the site. This included the use of a Schmidt Hammer and also a Point Load Testing machine. A limited number of samples were collected from locations within the underground mines at Jinlonggou. These samples were transported to a laboratory in Wuhan by WUT. The authors concerns with respect to sample integrity were proven to be correct


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in that a number of samples became damaged during the transportation to the laboratory, despite having been carefully prepared by WUT for shipping. Additional laboratory testing of representative rock samples is intended to be carried out during the early mining stages to confirm assumptions that have been made at the feasibility stage. 3 Evaluation of the Rock Mass and Open Pit Design 3.1 Rock mass Classification The rock mass classification system described by Laubscher (1990) was used for the empirical evaluation to obtain indicative design parameters. This Mining Rock Mass Rating (MRMR) Classification System, is an extremely useful and robust method of utilising relevant rock mass parameters to assist with mine design. Pit design parameters determined using rock mass classification are considered by the authors to present a good starting point in the design process. The MRMR can be related to an Indicative Overall Slope Angle, IOSA (top crest to bottom toe), and an Indicative Bench Stack Angle, IBSA. In general terms, the parameters that can be expected to influence stability of a rock mass include:

• Length and spatial distribution of geotechnical zones • Rock Quality Designation (RQD) • Rock Mass Defects i.e. faults, shear zones, intense fracturing and zones of

deformable material • Intact rock strength/hardness (IRS) • Degree and nature of rock weathering • Relative orientation of structures • Spacing between the sets of structures (Js) • Total number/density/frequency of structures (FF) • Condition of structures i.e., roughness, wall alteration and infilling (Jc) • Groundwater conditions

The parameters listed above are assessed in accordance with the MRMR system, and are allocated ratings up to the limits shown in Table 5 to determine the in-situ Rock Mass Rating (RMR). Depending on the source of data, there is the alternative available to use either RQD plus Js or FF. In practice, the average value determined for RQD/Js and FF is normally used. Table 5: Possible RMR Ratings

Parameter Rating Method 1

Rating Method 2

Intact Rock Strength Rating 20 20 Discontinuity Spacing Rating


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RQD Rating 15 NA Js Rating or 25 NA FF Rating NA 40

Joint Condition and Water 40 40 MAXIMUM POSSIBLE RMR 100 100 The in-situ RMR is adjusted to take account of the expected mining environment, namely the influence of weathering, structural orientation, induced stresses and blasting. The adjustments to the in-situ RMR are introduced in recognition of the type of excavation proposed and the time dependent behaviour of the site specific rock mass. The adjusted RMR is called the Mining Rock Mass Rating, MRMR. The possible percentage adjustments are:

• Weathering (w) 30 to 100% • Orientation (o) 63 to 100% • Induced Stresses (s) 60 to 120% • Blasting (b) 80 to 100%

Although the percentages shown above are empirical, the principle has proved to be sound, in that it forces a designer to allow for these important factors during the mapping process. In effect, the anticipated deterioration of the rock mass, once exposed in the mine environment, is provided for by these adjustments. The RMR was adjusted as shown in Table 6 for the rock at Tanjianshan in order to assign a MRMR to the rock mass. Table 6: RMR Adjustments Applied to Tanjianshan

For Open pit For Underground Rock Type

w o s b total w o s b total IDD, MGG, QFP 1 0.8 1 0.94 0.75 1 0.8 1 0.94 0.75

SAS, SAY, SSB, SSQ 0.9 0.8 1 0.94 0.68 1 0.8 1 0.94 0.75

SBL, SDB, SMB 1 0.8 1 0.94 0.75 1 0.8 1 0.94 0.75

SSG, TPN, TPO, TPP 0.9 0.8 1 0.94 0.68 1 0.8 1 0.94 0.75

The Indicative Overall Slope Angle, IOSA, as determined from the MRMR values for each class or type of material that is being evaluated, is obtained from Figure 8.


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Figure 8: Relationship between IOSA and MRMR It is important that the terminology with respect to pit wall design elements is properly understood. The terminology used in this paper is summarised in Figure 9.

Figure 9: Terminology Used to Describe Pit Wall Configuration

The interpreted MRMR and IOSA for rock masses at Jinlonggou and Qinlongtan are summarised in Table 7 and Table 8.

Table 7: Interpreted Rock Mass Ratings, Jinlonggou Weighted Average Data origin Rock type

RMR MRMR

IOSA (º)

Core Logging IDD, MGG, QFP 39 29 44

Core Logging SBL, SDB, SMB 45 33 47

Core Logging SSG, TPN, TPO, TPP 39 26 43

Windows Mapping All 48 32 46


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Windows Mapping IDD, QFP 45 34 47

Windows Mapping TPO 48 32 46

Table 8: Interpreted Rock Mass Ratings, Qinlongtan

Weighted Average Data origin Rock type

RMR MRMR IOSA (º)

Core Logging IDD, MGG, QFP 48 36 48

Core Logging SAS, SAY, SSB, SSQ 49 33 47

Core Logging SBL, SDB, SMB 50 38 49

Core Logging SSG, TPN, TPO, TPP 46 31 46

Windows Mapping All 45 31 46 Windows Mapping SMB 45 34 47 Windows Mapping SAY 46 31 46

3.2 Rock Mass Strength and Deformability The rock mass strength for the different grades of rock was assessed using the recommendations given by Hoek et al (2002). The non linear Hoek – Brown criterion was used to assess the internal friction and cohesion of the rock mass based on the Rock Mass Rating (RMR) and laboratory test results. The strength of a rock mass based on the Hoek – Brown failure criteria is defined by the equations given below.

a

cbc sm ⎟⎟

⎠

⎞⎜⎜⎝

⎛++=

σσσσσ 3

31

Where,

⎟⎠

⎞⎜⎝

⎛

−−=

D

GSImm ib 1428

100exp

⎟⎠

⎞⎜⎝

⎛

−−=

D

GSIs

39

100exp

⎟⎠⎞⎜

⎝⎛ −+=

−−3

2015

6

1

2

1eea

GSI

t

c

c

tim

σσ

σσ −=

and, GSI = Geotechnical Strength Index = RMR-5 σt = UTS RMR = Rock Mass Rating σc = UCS D = Disturbance factor Index testing (Point Load and Schmidt Hammer) to assess strength properties of the various rocks (and to calibrate the field logging) was carried out at the site. The results of this testing were used in the evaluation of intact strength properties. A graphical summary of the interpreted intact rock strength at Jinlonggou and Qinlongtan is presented in Figure 10.


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Figure 10: Strength Interpreted from Index Testing The Young’s modulus for the rock mass was assessed using the relationship:

40

10

101002

1−

−=GSI

cDE

σ

The equivalent rock mass friction (φ°) and cohesion (c) have been calculated using the relationships that are defined above and by assuming a linear relationship. The interpreted rock mass properties based on rock mass classification, index testing and limited laboratory tests are summarised in Table 9. These properties were used in numerical analysis to evaluate the overall geotechnical feasibility design. Table 9: Interpreted Rock Mass Properties for Dominant Rock Types

Value for Rock Type

Property Diorite, Gabbro

Phyllite Quartz Feldspar Porphyry

MRMR 34 32 34 Intact Unconfined Compressive Strength (MPa)

200 100 170

Density (kg/m3) 2.6 2.5 2.7 c (MPa) 1.6 0.7 1.3 Friction Angle (º) 46 29 43 Uniaxial Strength (MPa) 2.3 1.1 2.0 Global Strength (MPa) 25 6 19 Modulus of Deformation (MPa) 3655 3450 3655 3.3 Structural Domains


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It was initially attempted to identify patterns in the rock mass fabrics across the site using stereonets produced for the structure mapped at each of the forty-one surface and underground window mapping localities. It was, however, found that no clearly discernible spatial patterns of rock mass structure could be identified. Detailed structural geological interpretations (SRK, 2004) allowed the rock mass at Jinlonggou to be divided into five domains. These domains were defined by the T2 thrust, F7 fault, and F1/F39/F44/F45 fault systems corridor as shown in Figure 11. At Qinlongtan there was limited structural information available, and the geological structure was interpreted to be much simpler that at Jinlonggou. Structural domains were, therefore, defined on the basis of the anticipated open pit orientation for the purposes of the feasibility study. The authors consider that at this site additional work will be required to better define the geological conditions, and that when this is done there will be a need to integrate the geological structural model into the geotechnical design.

T2 Thrust

F30 Thrust

F7 Fault

F1 Fault Systems Domain 1

Domain 2

Domain 3

Domain 5

Domain 4

North

Pit outline is for 49º shell as determined using Whittle Modelling

Figure 11: 3-D View Showing Structure and Domains at Jinlonggou 4 Analysis 4.1 Kinematic Assessment of Structural Data The structural data measured in the field was assessed using stereonets, within each of the domains that were identified, to identify potential modes of failure. An example of this analysis is given in Figure 12.


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Figure 12: Example of Kinematic Analysis (Domain 2, Jinlonggou) The kinematic assessment identified potential for wedge and planar failures within some sectors of the proposed open pits, at both bench and multi-bench scales. Bench heights, batter angles and inter-ramp angles were therefore optimised (on a sector by sector basis within domains) to take account of interpreted structure, and to minimise the potential for slope failure. 4.2 Spill Berm Width Where it was determined that there was kinematic potential for failure, analysis was carried out to estimate potential failure volume. Some kinematically unstable features were identified as having a calculated factor of safety of greater than 1.1. In the authors experience, kinematically unstable features commonly start to move when the calculated factor of safety is about 1.1. Therefore, for the purposes of this analysis features with a calculated factor of safety of greater that 1.1 were discounted. As a means of eliminating interpreted potential failures that were either too small (and thus likely to be removed by blasting or the excavation process), or too large (and thus unlikely to actually form as a result of insufficient discontinuity persistence), the largest 20% and smallest 40% by volume of all identified wedges in the data set were discounted from the spill berm width analysis. This method of filtering the data is based on experience and has been used by the authors in a number of different situations. The required spill berm width (SBW) between benches can be calculated using the relationship


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3 5.1*)( VolmSBW =

where Vol is the wedge volume (m3), and the constant 1.5 is the assumed bulking factor. Theoretical spill berm widths were calculated for all kinematically unstable features identified within each domain for pit wall orientations as appropriate to the sector under consideration. An example of the results of this analysis is presented in Figure 13. Engineering judgement was used to select the appropriate slope configuration from the results of the analysis. As a guide, most suitable bench height was judged to be that point on the curve where the wedge volume begins to increase more rapidly. The individual batter angles and spill berm widths that are selected also need to take the overall condition of the rock mass as determined by other methods into account.

Figure 13: Example SBW Analysis (Jinlonggou, Domain 1) 4.3 Finite Difference Analysis At Tanjianshan it was anticipated that the Jinlonggou open pit would attain depths of about 230m. Qinlongtan, however, was expected to achieve a maximum depth of only about 100m before reverting from an open pit to an underground operation. The conditions at Jinlonggou were also better known due to the available information.


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Finite difference analysis was, therefore, considered appropriate only for the proposed Jinlonggou open pit to assess the likely deformation of the overall pit walls. Potential deformation of assumed pit walls due excavation of the pit was assessed using FLAC, Version 4.0. FLAC allows calculation of plastic deformation using assumed rock strength properties. FLAC was also used to calculate the overall pit wall factor of safety (FOS). A number of sections were analysed, such as that shown in Table 10, using the interpreted rock strength and deformation properties (Table 9). Table 10: Example of Section Analysed Using FLAC

Slope Parameter Value Height [m] 230 Overall Slope Angle (OSA) 50º Bench Stack Height [m] 50 Bench Stack Angle (BSA) 56º Berm Width [m] 9 For the section profile described in Table 10, analysis showed the slope to be stable with a calculated overall FOS of 1.58. Figure 14 shows the calculated horizontal displacements after the mining operations, and Figure 15 shows the velocity contours for the FOS calculations. The velocities shown in this figure are not real velocities associated with an assumed failure, they have been used to illustrate the extension of the failure surface associated with the calculated critical FOS. In this section the critical failure surface (with a calculated FOS of 1.58) extends 110m behind the crest of the slope.

Figure 14: FLAC Analysis – Calculated Horizontal Displacement


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Figure 15: FLAC Analysis - Interpreted Mode of Failure The results of the FLAC analysis were found to be in close agreement with the outcomes of the empirical design and structural analysis for the Jinlonggou Open Pit. 5 Geotechnical Design Recommendations 5.1 Open Pit Slope Geometry A summary of the recommended slope configurations, based on the results of geotechnical feasibility assessment, is given in Table 11 and Table 12. Table 11: Recommended Slope Geometry for Jinlonggou

Geotechnical Slope Recommendation

Batter (Bench) Limiting Bench Stack (50m)

Geotechnical Domain

Height (m) Face Angle (°) SBW IRA (°) BSA (°)

1 10 70 4 53 56 2 10 80 4.5 58 62 3 10 70 6 46 50 4 10 70 4.5 51 54 5 (a) 10 70 4 53 56 5 (b) 10 70 5.2 49 52 5 (c) 10 70 4.7 50 53


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Table 12: Recommended Slope Geometry for Qinlongtan

Geotechnical Slope Recommendation

Batter (Bench) Limiting Bench Stack (50m)

Domain

Height (m)

Face Angle (°)

SBW (m) IRA (°) BSA (°)

West Pit Sector 10 80 5.5 54 58 North Pit Sector 10 80 5.5 54 58 East Pit Sector 10 80 5.5 54 58 South Pit Sector 10 60 6.0 40 43 The pit slope geometry recommendations are based on the assumption that good blasting practices are followed. Pit wall stability may be strongly controlled by geological structure and the authors recognise that adverse structure, not identified during the investigation, could result in bench stack failure. It is common practice to adjust slope angles over the life of a pit to account for local structure and other conditions that are exposed during mining. To minimise the potential for unanticipated large scale failure it was considered important that routine geotechnical mapping and analysis was carried out to assess the need for minor adjustments to the recommended slope configurations. The pit slope geometry recommendations given for Qinlongtan are considered to be aggressive for a pit that will ultimately provide access to an underground operation. It was therefore considered to be important to verify the design assumptions during the initial stages of pit formation. The authors are of the opinion that, if the Qinlongtan open pit does develop into an underground mine, then it is likely that during the latter stages of the pit formation there will be a requirement for push backs to flatten the walls in order that the long term integrity of the underground infrastructure access can be designed with greater confidence. An alternative will be for local flattening and or secondary support of pit walls as determined to be appropriate around the ramp and portal sections. 5.2 Interaction with Abandoned Underground Mine Workings There are existing underground workings that will be mined out by the proposed Jinlonggou operation (Figure 16). From the available information it was anticipated that there would be a greater density of underground workings in the northern side of the proposed Jinlonggou pit.


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Figure 16: 3D Representation of Existing Underground Workings at Jinlonggou It was recognised that presence of existing underground workings would need to be taken into account in the design and operation of the mine. To optimise the operation (safety and cost) the authors considered that it was very important for the location of all workings to be accurately mapped prior to commencement of mining, and that a schedule and plans were drawn up to identify those areas within the pit that will be in close proximity to existing underground workings. There are numerous examples of open pits that are being mined in the immediate vicinity of existing underground workings, and that intersect underground workings. Jinlonggou is, therefore, not presented with a particularly unusual situation. It is, however, important to recognise that mining costs within areas that are in the immediate vicinity of abandoned underground workings are often much higher due to a requirement for additional safety and operation procedures, and also the different breakage (blasting) characteristics of the rock. Some of the precautions that may need to be implemented at Jinlonggou include advanced probing to confirm presence of workings and condition of rock above these workings, the use of safety harnesses for personnel, establishment of “no go” zones, backfilling, or even the use of remotely operated drill rigs. It was judged that, with the available information, the required separation distance between existing underground operations could not be determined reliably at the feasibility stage of investigation. Appropriate separation will be variable depending upon the local condition of the rock mass and also the size and orientation of the underground workings. As a preliminary guide, it was suggested that where the location of underground workings is accurately known a minimum separation of 30 m is adopted. Within a 30 m radius of accurately known existing workings special precautions should be taken to protect personnel and equipment. This recommendations will need to be very carefully reconsidered as mining progresses to take account of the observed performance of the rock and also site conditions.


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5.2 General Considerations The scope of investigation and design was, at this stage, to provide geotechnical recommendations with respect to the feasibility of mining operations at Jinlonggou and Qinlongtan. It was judged that there was sufficient information to commence with detailed design, and that additional information (oriented core, field mapping, laboratory testing) could be obtained during the very early mining stages to verify assumptions that had been made. More detailed numerical modelling for the proposed operations at both Jinlonggou and Qinlongtan was considered to be required to take the improved understanding of conditions into account. Insitu stresses have not been measured at Tanjianshan. The available information, for testing done at other nearby locations, indicates that the site may be subjected to a high horizontal stress. High stress gradients have the potential to impact on mining, especially an underground operation at the site. It was, therefore, considered important to carry out site specific insitu stress measurements to confirm the stress environment at Qinlongtan and Jinlonggou. There will be a requirement for ongoing geotechnical design and assessments through the life of the mines. It is, in the authors opinion, important that procedures are set up early during the mining to obtain geotechnical information (through face mapping, drilling, laboratory testing etc), to document the information and analyse the data. At the feasibility stage it was not known where the waste dumps and stockpiles would be located. It was recognised that careful consideration should be given to siting these facilities to ensure that they do not encroach on the open pit. It was judged that stockpiles and waste dumps should be set back from the crest of the pit a minimum of 100 m from the crest of the pit. Access ramps will need to be designed to accommodate the equipment that is expected to be used at site. A minimum 15 m haul ramp width was recommended, taking the type of anticipated mining equipment into account. It was also recommended that there provision was made for a bund on the pit slope side of the ramp to be constructed to a minimum height that is equivalent to the haul truck wheel diameter, and that there was a minimum 3 m separation between the edge of the pit and the haul road travel surface. On the basis of the authors experience it was anticipated that there would be a requirement to provide secondary support to some locations within the open pits. Provision will be made for this in the detailed design. Spot cable installation was anticipated to be required and it was judged that cables of up to 8 m length will be routinely required. Extreme temperatures in winter are expected to result in frost shattering of the rock. Whilst it is judged that this will not impact on overall stability, it will present a safety issue due to ravelling of loose material. For this reason it is considered important that slope walls are properly cleaned up, and that particular care is taken to maintain the design catch berms. In areas where falling debris presents a safety hazard there may be


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a requirement to install a mesh or other constraining mechanism on the face to contain falling debris. Blasting has the potential to cause damage to pit walls. At feasibility stage, the geotechnical assessment has made allowance for minor blast disturbance. It is important that appropriate blasting practices are implemented to minimise potential for damage. It is important that an appropriate slope management plan is put in place during the early stages of pit formation. This plan should make provision for:

• Monitoring of the groundwater conditions (installation of piezometers) • Installation and monitoring of survey prisms • Monitoring of cracks by means of wire extensometers • Proper surface water control and monitoring • Control of erosion, and • Routine geotechnical mapping and reduction/interpretation of the data.

6 Conclusions The outcome of a geotechnical feasibility study for a proposed mining operation at Tanjianshan has been presented in this paper. The scope of the feasibility study was structured to provide sufficient information to allow commencement of mining in 2005. Mining did commence in 2005 at Qinlongtan as planned (Figure 17). Geotechnical information was obtained during the investigation from field mapping of outcrop and previous workings, borehole core and laboratory testing. Considerable reliance was placed on using the exploration boreholes and rock cuts as a geotechnical data source in order to contain costs and meet the tentative mining schedule. During the course of the geotechnical work there was interaction between the geotechnical and geological personnel in order to gain a good understanding of the structural conditions at the sites. A rock mass classification system was used to develop an understanding of the insitu conditions and to develop conceptual pit geometry design. The conceptual pit geometry designs were optimised by considering interpreted geological structural conditions and rock mass properties at the site. Finite difference modelling was then used to assess the overall performance of the optimised pit geometry design.


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Figure 17: Qinlongtan Open Pit, November,2005 A pit design should be expected to evolve as more information and site specific practical experience is obtained. The approach that was adopted in this study recognised that further information is needed. It also places and emphasis on obtaining this information during the early phases of mining, whilst there is sufficient time to make minor design adjustments and whilst revenue is being earned. REFERENCES BRADY, B.H.G., BROWN, E.T. (2004). Rock Mechanics for Underground Mining. 3rd Edition. Kluwer Academic Publishers. pp626. COUTURE, J.F. AND SIDDORN, J. P. (2003). Structural Geology investigation of the Jinlonggou Gold Deposit, Qinghai Province, P.R. China. Unpublished report to AFCAN Mining Corporation, December 2003. DEVMIN PTY LTD. (2004). Confidential report. Tanjianshan Gold Project. Qinghai Province, China. 2003 & 2004 Work Programmes and Resource Estimation. 16 December 2004. HOEK, E., CARRANZA-TORRES, C., AND CORKUM, B. (2002). Hoek-Brown Failure Criterion – 2002 Edition REINECKER, J., O. HEIDBACH, M. TINGAY, P. CONNOLLY, and B. MÜLLER (2004). The 2004 release of the World Stress Map (available online at www.world-stress-map.org). SRK CONSULTING (2004). Confidential Report titled “Structural Controls on Gold Mineralization, Jinlonggou Deposit, Qinghai Province, P.R., China” (December 2004)”


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ZHANG, P., YANG, Z., GUPTA, H.K., BHATIA S.C., SHEDLOCK K.M. (1999). Global Seismic Hazard Assessment Program (GSHAP) in Continental Asia. http://www.seismo.ethz.ch/GSHAP/eastasia/eastasia.html.


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GEOTECHNICAL DESIGN FOR OPEN PITS AT … · The South African Institute of Mining and Metallurgy International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering