The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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 kholley@srk.com.au.
2
China Manager, Eldorado Gold Corp.
3
Professor and Deputy Dean Resource and Environment Department, Wuhan University of Technology
Page 483
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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.
Page 484
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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 Geosynclinal 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
Quaternary
Lithology
Age
Unconsolidated
conglomerates
Tertiary
-
Jurassic
Triassic
Late
Carboniferous
or early
Permian
Early
Carboniferous
Devonian
Cambrian
Ordovician
MidProterozoic
EarlyProterozoic
-
Alluvium and
Colluvium
Sandstone, siltstone and
mudstone with
carbonate and organic
rich shale.
Clastic Rocks
Intermediate Diorite
Porphyry
Granite Porphyry
Huaitala
Marine
Maoniushan
Tanjianshan
Wandonggou
Marine
Andesites & dacites;
Ultramafic and mafic
dykes and
range of clastic rocks
Marble, shale, quartzite
Dakendaban
Gneiss, schist, marble
Page 485
Comment
Occurs in river
valleys and
alluvial fans.
Occurs both
sides of the
Aolaohe River
Alluvium and
Colluvium
60Ma to
present
Underlies the
unconsolidated
conglomerates
(unconformable)
294.7
±3.8Ma
1150
±280Ma
1900
Occurs to west
of sites
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
FBX
IDD
MGG
QB
QFP
QZ
SAS
SAY
SBL
Rock Type
fault breccia
diorite
gabbro
quartz breccia
qz-fp porphyry
quartz
sandstone
claystone, mudstone
limestone
Lithology
Code
SDB
SMB
SSB
SSG
SSQ
TPN
TPO
TPO
TPP
Rock Type
dolomitic marble
marble
siltstone
graphite schist
quartzite
carbonaceous phyllite
phyllite (undifferentiated)
phyllite (undifferentiated)
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.
Page 486
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
Page 487
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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)
Page 488
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
OBE Operating Basis
0.10g
Earthquake
2
Duration
30 s
Main
Period
0.4 s
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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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
Underground Window
Mapping
Geotechnical Borehole
Logging of (Pre-drilled)
Geological Holes
Geotechnical Borehole
Logging of Holes Sited
for Geotechnical
Information
Insitu Strength Index
Testing
Laboratory Testing
Jinlonggou
Qinlongtan
Surface Window
Mapping
Site
Table 4: Scope of Site Investigation
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
Page 491
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
Intact Rock Strength Rating
Discontinuity Spacing Rating
Rating
Method 1
20
Page 492
Rating
Method 2
20
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
RQD Rating
Js Rating or
FF Rating
Joint Condition and Water
MAXIMUM POSSIBLE RMR
15
25
NA
40
100
NA
NA
40
40
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)
Orientation
(o)
Induced Stresses (s)
Blasting (b)
30 to 100%
63 to 100%
60 to 120%
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
Rock Type
w
o s
b
total
IDD, MGG, QFP
SAS, SAY, SSB, SSQ
SBL, SDB, SMB
SSG, TPN, TPO, TPP
1
0.9
1
0.9
0.8
0.8
0.8
0.8
1
1
1
1
0.94
0.94
0.94
0.94
0.75
0.68
0.75
0.68
w
1
1
1
1
For Underground
o
s
b
total
0.8
0.8
0.8
0.8
1
1
1
1
0.94
0.94
0.94
0.94
0.75
0.75
0.75
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.
Page 493
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
Data origin
Core Logging
Core Logging
Core Logging
Windows Mapping
Rock type
IDD, MGG, QFP
SBL, SDB, SMB
SSG, TPN, TPO, TPP
All
Weighted
Average
RMR MRMR
39
45
39
48
Page 494
29
33
26
32
IOSA
(º)
44
47
43
46
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
Windows Mapping
Windows Mapping
IDD, QFP
TPO
45
48
47
46
34
32
Table 8: Interpreted Rock Mass Ratings, Qinlongtan
Data origin
Core Logging
Core Logging
Core Logging
Core Logging
Windows Mapping
Windows Mapping
Windows Mapping
3.2
Weighted Average
IOSA
RMR MRMR
(º)
48
36
48
Rock type
IDD, MGG, QFP
SAS, SAY, SSB,
SSQ
SBL, SDB, SMB
SSG, TPN, TPO,
TPP
All
SMB
SAY
49
33
47
50
38
49
46
31
46
45
45
46
31
34
31
46
47
46
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.
⎛ σ
⎞
σ 1 = σ 3 + σ c ⎜⎜ mb 3 + s ⎟⎟
⎝ σc
⎠
a
Where,
⎛ GSI − 100 ⎞
mb = mi exp⎜
⎟
⎝ 28 − 14 D ⎠
−20
1 1 − GSI
a = + ⎛⎜ e 15 − e 3 ⎞⎟
⎠
2 6⎝
⎛ GSI − 100 ⎞
s = exp⎜
⎟
⎝ 9 − 3D ⎠
mi =
σt σc
−
σc σt
and,
GSI = Geotechnical Strength Index = RMR-5
RMR = Rock Mass Rating
D = Disturbance factor
σt = UTS
σc = UCS
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.
Page 495
The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
Figure 10: Strength Interpreted from Index Testing
The Young’s modulus for the rock mass was assessed using the relationship:
D σc
E = 1−
10
2 100
GSI −10
40
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
MRMR
Intact Unconfined Compressive
Strength (MPa)
Density (kg/m3)
c (MPa)
Friction Angle (º)
Uniaxial Strength (MPa)
Global Strength (MPa)
Modulus of Deformation (MPa)
3.3
Diorite,
Gabbro
Phyllite
Quartz
Feldspar
Porphyry
34
200
32
100
34
170
2.6
1.6
46
2.3
25
3655
2.5
0.7
29
1.1
6
3450
2.7
1.3
43
2.0
19
3655
Structural Domains
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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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.
Pit outline is for 49º shell
as determined using
Whittle Modelling
F7 Fault
F1 Fault Systems
Domain 1
Domain 2
North
T2 Thrust
Domain 3
Domain 4
Domain 5
F30 Thrust
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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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
SBW ( m ) = 3 Vol * 1.5
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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The South African Institute of Mining and Metallurgy
International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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
Height [m]
Overall Slope Angle (OSA)
Bench Stack Height [m]
Bench Stack Angle (BSA)
Berm Width [m]
Value
230
50º
50
56º
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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Kevin Holley
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
Geotechnical
Domain
Height (m)
1
2
3
4
5 (a)
5 (b)
5 (c)
Limiting Bench Stack
(50m)
Batter (Bench)
10
10
10
10
10
10
10
Face Angle
(°)
70
80
70
70
70
70
70
SBW
4
4.5
6
4.5
4
5.2
4.7
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IRA (°)
53
58
46
51
53
49
50
BSA (°)
56
62
50
54
56
52
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Kevin Holley
Table 12: Recommended Slope Geometry for Qinlongtan
Geotechnical Slope Recommendation
Domain
West Pit Sector
North Pit Sector
East Pit Sector
South Pit Sector
Limiting Bench Stack
(50m)
Batter (Bench)
Height
(m)
10
10
10
10
Face Angle
(°)
80
80
80
60
SBW (m)
5.5
5.5
5.5
6.0
IRA (°)
54
54
54
40
BSA (°)
58
58
58
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:
•
•
•
•
•
•
6
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.
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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International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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.worldstress-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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International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering
Kevin Holley
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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