information request 4.3information request 4.3.4 slope stability of open pits references: eis...

Information Request 4.3.4

Slope Stability of Open Pits

References: EIS Guidelines - Section 2.2.3.1, p. 15 (PDF 20) - Section 2.2.3.2, p. 15 (PDF 20) - Section 2.7.6, p. 70 (PDF 75) EIS Main Report - Section 1.4.3, p. 1.55 (PDF 124)

- Section 6.3.2.16, p. 6.163 (PDF 669)

CEAR Doc # 326 “Recommendations for Open Pit Rock Slope Design Marathon PGM-Cu Project.” Golder (2007) Contributing IRs: NRCan-18 Rationale:

SCI is proposing five (5) open pits, consisting of one primary pit and four satellite pits. In the EIS,

SCI indicated that the environmental impact assessment was based on “extensive investigation

directed at pit slope design” by Golder (2007). However, Golder (2007) only investigated the

primary pit and one satellite pit and indicates that the design was based on geotechnical drilling

information targeted specifically on two open pit sites, and explicitly indicates that other pits were not

considered.

Information Request:

Explain the discrepancy between the level of investigations regarding pit slope design described in

the EIS and the supplemental document (Golder 2007).

Provide information about the investigation and design of the three satellite pits that were not

included in Golder 2007.

SCI RESPONSE

Recommendations were provided for the Main and South Pits by Golder Associates Ltd. (Golder) in 2007 to support a definitive feasibility study. However, the Satellite Pit(s) were not considered during this work by Golder. Knight Piesold Ltd. (KPL) has reviewed the available geological and geomechanical data for the Project and identified similarities and differences between the pits. The performance of pit slopes is closely tied to a number of factors, including the slope geology, rock mass quality, rock mass structure and dominant discontinuity orientations. REVIEW OF AVAILABLE DATA

Knight Piésold reviewed the following documents as part of the assessment:

Golder Associates Ltd., March 2007, Recommendations for Open Pit Rock Slope

Design Marathon PGM-Cu Project

Marathon PGM Corp., Dec. 2008, Feasibility Study for the Marathon PGM Project, Marathon, Ontario, Canada

Surficial Geology Maps

David Good, 2013. The Marathon PGM-Cu Deposit – A Toolbox for Exploration in the Coldwell Alkaline Complex (PowerPoint)

SCI Reponses to Comment No. 29 (Framing the Geological Context of the Site) for the EA and No. 30 (Structural Characterization of Rock Formations) for the EIS

Exploration drillhole geology logs (2006 to 2008)

Drillhole database including downhole lithology and RQD (includes all drillholes since 2004)

Digital 3D surfaces of the proposed pit configuration for the Main and Satellite Pit(s)

Digital 3D solids of the mineralized zones

Digital Topography

Deposit Geology Geology strongly influences slope performance. The following summary is based on Marathon PGM Corp. (2008) unless otherwise noted.

• Deposit Genesis - The Marathon PGM-Cu deposit is located on the eastern margin of the Port Coldwell Alkaline Complex, which is a layered basic igneous intrusion. Mineralization is related to a large magmatic system consisting of three or more cross-cutting plutons.

• Rock Types - The hanging wall is comprised of Syenite and the footwall is comprised of Archean intermediate metavolcanics. These units were intruded by the Eastern Gabbro Series and Marathon Series, which includes the Layered Gabbro, Fine Grained Gabbro and Two Duck Lake Gabbro. Partial melting along the intrusive contact with the Eastern Gabbros resulted in brecciation. Late quartz syenite and augite syenite dykes cut across all of the gabbros but form a minor component of the intrusive assemblage. No dominant fabric parallel to the intrusion is expected.

• Mineralization - The mineralized zones are hosted by the Two Duck Lake Gabbro and occur as shallow dipping sub-parallel lenses that follow the basal gabbro contact. These lenses form distinct mineral horizon.

The following work was completed to assess the geological similarities between the Main and Satellite Pit(s):

The rock types in each pit were compared on the basis of surficial geology (Figure 1)1. The likely pit wall geology was inferred from this information.

The position and orientation of the mineralized zones were compared using 3D visualization software (Figure 2).

The proportion of each rock type in each pit was compared based on the exploration drilling database (Figure 3).

The basic geology is generally similar between the Main and Satellite Pit(s). Considerations that could limit the applicability of the Main Pit recommendations to the South Pit include the more prominent exposures of Syenite in the pit slopes and the variation in the composition and orientation of the mineralized zones in the Satellite Pit. The rotation in the orientation of the mineralized zones in the Satellite Pit is noteworthy, as it could influence the orientation of structures within the rock mass.

1 Note: In order to answer the IR the conceptual pit design described in the Main EIS Report has been used. The

updated conceptual pit design does not differ materially.

Rock Mass Quality The available data was used to compare what is known about the rock mass characteristics in the areas of the Main and Satellite Pit(s). Rock mass quality is a fundamental consideration in slope performance and design. Rock mass quality is most commonly assed with rock mass classification systems, such as the Rock Mass Rating (RMR89) system (Bieniawski, 1989) and

NGI-Q (Barton et. al. 1974). RMR89 data was collected by Golder (2007) on the Main and South Pits. The other information available on engineering characteristics of the rock mass are RQD assessments on the exploration core for the Main, South and Satellite Pit(s) from 2006 onwards. No RMR89 data for the Satellite Pit(s) was available at the time of writing the Main EIS Report but with the

commencement of detailed engineering in 2013, a field program to collect RMR89 data was initiated for

the Satellite Pit(s) (March – April 2013). Design values were included in the pre-feasibility slope recommendation report (Golder, 2007). The Main Pit rock masses are characterized as being generally GOOD quality rock, with a mean RMR89 value of approximately 75 and intact rock strengths between 100 and 200 MPa. In the absence of RMR89 data, RQD was used as the basis for comparison between the rock

mass quality in the Main and Satellite Pit(s). The following approach was utilized:

Exploration RQD Database - In order to confidently use the exploration RQD data, a comparison was made between the downhole RQD data recorded by SCI (since 2006) and Golder on the same three drillholes. The results of the comparison (Figure 4) suggest that the general trends are similar, although the relative magnitude of the recorded RQD values varies.

Downhole RQD Distributions – Typical holes within the Main and Satellite Pit(s) were selected and the downhole values and variations in RQD compared (Figure 5). Drillholes that intersected faults were specifically considered in order to assess the effect of the fault on rock mass quality.

Statistical Comparison - All available RQD data was used to statistically compare the Main and South Pits with the Satellite Pit(s) (Figure 3). The influence of lithology was also considered.

The results suggest that the rock mass quality in the walls of the Main Pit and Satellites Pits will be similar. Rock mass quality is reasonably high in both pits, and the RQD values are marginally better in the Satellite Pit(s). In general, rock mass quality is not expected to control pit slope performance, although zones of reduced rock mass quality associated with faulting could result in local instabilities. Although the exploration drilling has limited intersections with the known (and suspected) faults in the area of the deposit, previously mentioned additional geotechnical work is being conducted to confirm slope stability. Rock Mass Structure The characteristics and orientations of rock mass structure is a fundamental consideration in slope performance. Large-scale structures (faults) and small-scale structures (typically joints) can form blocks or wedges that result in slope failures. The pre-feasibility report by Golder (2007) suggests that slope performance in the Main and South Pits will be controlled by the orientation of the small-scale structure. The following summary of rock mass structure is based on Golder (2007), Marathon PGM Corp

(2008), SCI EIS Comment No. 30 and D. Good (pers. comms., 2013).

Large-Scale Structure: Large-scale, persistent structures have been inferred by SCI

based on surface lineaments and drillhole intersections. The dominant features at the

Marathon Project are thought to be steeply dipping faults trending NNE-SSW, E-W and SE-

NW.

Small-Scale Structure: Small-scale discontinuity orientation data was collected by

Golder (2007) during the pre-feasibility site investigations for the Main and South Pits.

Review of the summary stereonets suggests that there are five common discontinuity

orientations:

A flat-lying joint set. This is the most prominent orientation A steeply dipping joint set trending NW-SE, with dips varying across the vertical o A

steeply dipping joint set trending NE-SW, with dips varying across the vertical o A moderately dipping (50°) joint set trending E-W

A moderately dipping (50°) joint set trending NW-SE The flat-lying and vertical joint sets are roughly orthogonal. There appears to be local variation in

the discontinuity orientations as all of the joint sets were not encountered in every drillhole. This

variation can be partly explained by differences in drillhole azimuth, which tend to exaggerate

some orientations at the expense of others. The average discontinuity spacing for all rock masses

ranges from 0.6 m to 11.1 m. No small-scale orientation data is currently available for the Satellite

Pit(s) but is being collected to support detailed engineering.

The following approach was used to compare the rock mass structure expected in the Main and

Satellite Pit(s):

The orientation of large-scale structure (faults and mineralized bodies) was compared

to the orientation of small-scale structure in the Main and South Pits (Figure 6) with the

objective of establishing a correlation between the two sets. The results of the comparison

do not suggest a strong correlation.

The orientations of the large-scale structures are expected to be similar in all of the pits. The small-scale structure is expected to be the most important factor in determining achievable

slope configurations. The results of the review suggest that the orientation of the small-scale

structure in the Satellite Pit(s) could not be inferred from the orientation of the large and small-

scale structures in the Main and South Pits. As a result, a comparison of the orientation of the

small-scale structures was not possible. However, the changing orientations of the mineralized

zones in the Satellite Pit(s) suggest that there could be local variation in the orientation of the small-

scale structure.

OTHER CONSIDERATIONS There are other factors that influence slope performance that should be considered when assessing

the applicability of the Main Pit recommendations to the Satellite Pit(s).

Pit Geometry and Slope Height: Increased slope height generally results in shallower

achievable slope angles and greater consequences if slope instabilities occur. The slope

heights proposed for the Satellite Pit(s) range from approximately 50 m to a maximum slope

height of 160 m in Satellite Pit #3 (Malachite Hill). These slope heights are substantially

shallower than those proposed for the Main Pit (370 m) and the South Pit (140 m).

Groundwater Conditions: Elevated pore water pressures within pit slopes can adversely

impact slope stability. Previous work (Golder, 2007) concluded that groundwater is not expected to impact slope performance. Given the similarities in geology, rock mass quality and surface water bodies between the Main and Satellite Pit(s), it is thought that this conclusion is also appropriate for the Satellite Pit(s).

Stress Conditions: For deep open pits, stress-induced disturbance is expected to influence rock mass quality and the long-term slope performance. This type of disturbance will occur when the load applied to the pit walls exceeds the strength of the rock mass at that location. Stress-induced disturbance is expected to be minimal in the Main Pit and negligible in the Satellite Pit(s).

Precedent Practice: Pit slope stability and performance depends on a variety of site-specific factors that makes it difficult to provide direct comparisons with other operations. However, it is still valuable to review both the successes and wall performance issues that have been encountered at other open pit operations. A summary plot of pit depth vs. slope angles achieved in various operations is illustrated on Figure 7 (Lutton, 1970; Hoek and Bray, 1981; Sjoberg, 1996; Read and Stacey, 2009). The plot suggests that the slope configurations proposed for the Satellite Pit(s) have been achieved at a number of operations with a Factor of Safety >1.3 .

SUMMARY AND CONCLUSIONS A series of comparisons were undertaken on the rock masses expected to form the Main, South

and Satellite Pit(s) in order to assess the applicability of the existing Main Pit slope

recommendations to the design of the Satellite Pit(s). These comparisons were based on the

available geological and geomechanical data for the Marathon Project. The results of these

analyses are summarized below.

Geology - The pit wall geology is expected to be similar. There will likely be some variation in

the orientation and composition of the mineralized zones in the Satellite Pit(s) that is not

present in the Main Pit. In addition, the Syenite is expected to be more prominent in the

West Walls of Satellite Pit #3.

Rock Mass Quality - The pits are expected to have similar rock mass qualities. Rock

mass quality is not expected to control to control slope performance, although zones of

reduced rock mass quality associated with faulting could result in local instabilities.

Rock Mass Structure - Kinematic failures on small-scale structure are expected to

limit achievable pit configurations. The large-scale structural environment is expected to be

similar for all pits. In terms of small-scale structure, similar joint sets are expected to occur

in all of the pits. However, the orientations of these joint sets may vary locally (due to

spatial variation and changes in the orientation of the mineralized zones) and cannot be

predicted with certainty in the area of the Satellite Pit(s)

The similarities in geology, rock mass quality and large-scale structure suggest that the slope

recommendations for the Main and South Pits should generally be achievable in the Satellite

Pit(s). Local variations in achievable configurations will likely occur depending on the orientation of

the small-scale structure in the various final pit walls. Further geotechnical work to support detailed

engineering including final pit design is currently underway.

REFERENCES The following are references in addition to those mentioned in the section “Review of Available Data”: Barton, N.R, Lien, R.; Lunde, J. (1974). Engineering Classification of Rock Masses for the Design of Tunnel Support. Rock Mechanics and Rock Engineering 6 (4): 189-236. Bieniawski, Z.T. (1989). Engineering Rock Mass Classifications. Wiley, New York. Hoek, E. & Bray,

J.W. (1981). Rock Slope Engineering, 3rd Edition. London.

Lutton, R.J. (1970). Rock slope chart from empirical design, Trans. Society of Mining Engineers, 247, 160-162. Read, J. and Stacey, P. (2009). Guidelines for Open Pit Slope Design. CSIRO Publishing, Australia. SCI (2012). Personal Communication with D. Good. VP Exploration, Stillwater Canada Inc. Thunder Bay, ON. Sjoberg, J. (1996). Large Scale Slope Stability in Open Pit - A Review, Division of Rock Mechanics - Lulea University of Technology, S-97187.

PRIMARY PIT

SOUTH PIT

MAIN PIT

TERRU LAKE

MALPA LAKE

549,000

549,500

550,000

550,500

551,000

5,403,000

5,403,500

5,404,000

5,404,500

5,405,000

5,405,500

5,406,000

P/A NO.

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NB13-00242REF NO.

FIGURE 1

MARATHON PGM-Cu PROJECTSTILLWATER CANADA INC.

REVIEW OF SATELLITE PIT(S)COMPARISON OF SURFICIAL GEOLOGY

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REV DATE DESCRIPTION DESIGNED APP'DCHK'DDRAWN09APR'13 ISSUED WITH TRANSMITTAL ATJ RAMBDPAS

NOTES:1. BASE MAP: © HER MAJESTY THE QUEEN IN RIGHTS OF CANADA DEPARTMENT OF NATURAL RESOURCES (2009). ALL RIGHTS RESERVED.2. CO-ORDINATE GRID IS IN METRES. DATUM: NAD27 PROJECTION: UTM ZONE 163. CONTOUR INTERVAL IS 20 METRES.4. MARATHON PGM-Cu PROJECT BOUNDARIES PROVIDED BY STILLWATER CANADA INC. (NOVEMBER 28 2011).

100 0 100 200 300 400 50050 mSCALE

LEGEND:ROADRIVER/STREAM/DRAINAGELINEAMENTSPROPOSED OPEN PIT

0

ARCHEAN FOOTWALL

FINE GRAINED GABBRO

FINE GRAINED GABBRO WITH TWO DUCK LAKE INTRUSIONS

LAYERED GABBRO SERIES

TWO DUCK LAKE GABBRO

WEHRLITE/TROCTOLITE SILL

SYENITE

SATELLITE PIT(S)

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P/A NO.

NB101-446/5

REVIEW OF SATELLITE PIT(S)COMPARISON OF MINERALIZED

GEOLOGY SOLIDS

FIGURE 2

STILLWATER CANADA INC.

MARATHON PGM-Cu PROJECT

REF. NO.

NB13-00242

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REV DATE DESCRIPTION PREP'D CHK'D APP'D

NOTES:1. GEOLOGY SOLIDS, POST MINING TOPOGRAPHY PROVIDED BY STILLWATER CANADA INC. (JAN 2013).

SECTION 1

SECTION 3

SECTION 6

SECTION 2PLAN VIEW

1

2

3

4

65

SECTION 5

SECTION 4

Hanging wall and footwall are steeply dipping to northwest.

Hanging wall and footwall are shallow dipping to west.

FOOTWALL

HANGING WALL

MAG MAIN

WASTE (NORTH)

WALLFORD HORIZON

Mineralized Zones

South Pit

Satellite Pit(s)

Main Pit

Mineralized zones shallow dipping to

northwestMineralized zones steeply dipping to

northwest

Mineralized zones steeply dipping to

northwest

Mineralized zones shallow dipping to

west

Mineralized zones moderately dipping

to west


to west


to west

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N: 5,406,225

N: 5,404,170

N: 5,403,025

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REVIEW OF SATELLITE PIT(S)COMPARISON OF RQD

BY ROCK TYPE AND LOCATION

FIGURE 3



REF. NO.

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NOTES:1. RQD AND LITHOLOGY DATABASE PROVIDED BY STILLWATER CANADA INC. (JAN 2013).2. DIVISIONS BY PIT BASED ON NORTHINGS SHOWN IN PLAN VIEW, ABOVE.3. RQD MEASURED ON A STANDARD 3 m RUN.4. HISTOGRAMS HAVE BEEN NORMALIZED TO REPRESENT A FREQUENCY PERCENTAGE OF THE TOTAL DATA.

Total Length of Core:

Average RQD:

Main Pit

3520 m

86 %

Satellite Pit(s)

2016 m

95 %

0%

20%

40%

60%

80%

100%

Frequency (%)

RQD (%)

1 - Archean Footwall


Average RQD:

Main Pit

17602 m

92 %

Satellite Pit(s)

6986 m

94 %

0%

20%

40%

60%

80%

100%

Frequency (%)

RQD (%)

2 - Eastern Series Gabbro


Average RQD:

Main Pit

7929 m

96 %

Satellite Pit(s)

9460 m

97 %

0%

20%

40%

60%

80%

100%

Frequency (%)

RQD (%)

3 - Two Duck Lake Gabbro


Average RQD:

Main Pit

1041 m

95 %

Satellite Pit(s)

1953 m

98 %

0%

20%

40%

60%

80%

100%

Frequency (%)

RQD (%)

4 - Two Duck Lake Breccia


Average RQD:

Main Pit

1557 m

89 %

Satellite Pit(s)

1513 m

96 %

0%

20%

40%

60%

80%

100%

Frequency (%)

RQD (%)

5 - Syenite

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency (%)

RQD (%)

Main Pit vs Satellite Pit(s)

Main Pit

Satellite Pit(s)

MAIN PITTotal Runs: 10,700

Total Length of Core: 32,023 mAverage: 92%

SATELLITE PIT(S)Total Runs: 7,457

Total Length of Core: 22,270 mAverage: 95%

Legend (Applies to all Histograms)

Plan View

LEGEND

5 - Syenite

1 - Archean Footwall

2 - Fine Grained Gabbro with Two Duck Lake Intrusions

2 - Fine grained Gabbro

3 - Two Duck Lake Gabbro

6 - Wehrlite/Troctolite Sill

2 - Layered Gabbro Series

South Pit

Satellite Pit(s)

Main Pit

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P/A NO.

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REVIEW OF SATELLITE PIT(S)COMPARISON BETWEEN GOLDER AND EXPLORATION RQD LOGGING (MAIN PIT)

FIGURE 4



REF. NO.

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NOTES:1. DEPTHS ARE DOWNHOLE IN METRES.2. GOLDER RQD TAKEN FROM PREFEASIBILITY PIT SLOPE RECOMMENDATIONS REPORT BY GOLDER (2007).3. EXPLORATION RQD PROVIDED BY STILLWATER CANADA INC. (JAN 2013).

020406080100 020406080100 020406080100

GD-06-01Golder Exploration GD-06-02Golder Exploration GD-06-03Golder Exploration

EXPLORATION LOGGINGENDS AT 168 m



DownholeDepth (m)

RQD (%)

RQD (%)

RQD (%)

RQD (%)

RQD (%)

RQD (%)

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P/A NO.

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REVIEW OF SATELLITE PIT(S)COMPARISON OF TYPICAL DOWNHOLE RQD

FIGURE 5



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1) MB-08-14 2) M-06-199

5) M-06-189 7) M-06-149 8) M-06-162

PLAN VIEW3) M-07-345 4) M-07-319

NOTES:1. RQD AND LITHOLOGY PROVIDED BY STILLWATER CANADA INC. (JAN 2013).2. PIT SHELL AND POST MINING TOPOGRAPHY PROVIDED BY STILLWATER CANADA INC. (JAN 2013).3. FAULT INTERSECTIONS BASED ON FAULT MODEL AS PROVIDED BY STILLWATER CANADA INC. (JAN 2013).

6) M-06-184

RQD LEGEND

< 70 %

70 - 90 %

90 - 100 %

Fault

4400N Fault

1

2

3

4

6

5

87

5100N Fault

4400N Fault

3100N Fault

Fault 2

South Pit

Satellite Pit(s)

Main Pit

LITHOLOGY LEGEND

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P/A NO.

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REVIEW OF SATELLITE PIT(S)COMPARISON BETWEEN SMALL AND

LARGE SCALE STRUCTURE

FIGURE 6



REF. NO.

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NOTES:1. DRILLHOLE DETAILS, PIT DESIGN AND SHADED RELIEF MAP PROVIDED BY STILLWATER CANADA INC (JAN 2013).2. STEREONETS TAKEN FROM GOLDER (2007).3. DRILLHOLE STEREONETS ARE EQUAL AREA LOWER HEMISPHERE PROJECTIONS.4. JOINT SET WINDOWS BASED ON AVERAGE OF ALL STEREONET CONCENTRATIONS.

EQUAL ANGLEEQUAL AREA

Comparison Between Small Scale Discontinuity Sets and Large Scale Structure

Fault from Structural Model

Lineament (Golder, 2007)

Lineament (Inferred)

Legend

N

GD06-01

GD06-02

GD06-03

GD06-04

GD06-05

GD06-06

TV06-01

TV06-02

TV06-03

LEGEND

Main Pit Fault/Lineament

Satellite Pit Fault/Lineament

4400N Fault

5100N Fault (1)

5100N Fault (2)

3100N Fault

3800N Fault

Fault 1

Fault 2

Lineament 2

4400N Fault

Lineament 1

South Pit

Satellite Pit(s)

Main Pit

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0

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600

630

660

690

720

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780

810

840

870

900

930

960

990

1020

0 10 20 30 40 50 60 70 80 90

SLOPE HEIGHT (ft)

SLOPE HEIGHT (m)

OVERALL SLOPE ANGLE (°)

South Pit

NOTES:1. ORIGINAL DATA POINTS AFTER LUTTON 1970, HOEK AND BRAY 1981, AND SJOBERG 1996.

2. ADDITIONAL DATA FROM KNIGHT PIESOLD PROJECTS AND OTHERS.3. SLOPE HEIGHTS AND SLOPE ANGLES MEASURED ON PIT DESIGN PROVIDED BY STILLWATER CANADA INC. (JAN 2013)

0 09APR'13 ISSUED WITH TRANSMITTAL ATJ BDP RAM

DATE DESCRIPTION PREP'D CHK'D APP'DREV

REVIEW OF SATELLITE PIT(S)SLOPE HEIGHT VERSUS SLOPE ANGLEPRECEDENT FOR HARD ROCK SLOPES

FIGURE 7



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Las Encinas, Chile

Bingham Canyon, UT

Chuquicamata, W Sector, ChileChuquicamata, E Sector, Chile

Palabora, S. Africa

Cuajone, Peru

Round Mountain, NV

Island Copper, W Wall, BC

Ruth, Liberty Pit, NV

Highmont, BC

Betze-Post, NV

Toquepala, Peru

Main Cresson, W Wall, CO

Main Cresson, E Wall, CO

Cassiar, BC

Ruth, Kimbley Pit, NV

Highland Valley, Valley Pit, BC

Jeffrey, SE Wall, QC

Highland Valley, Lornex Pit, BC

Ruth, Ruth Pit, NV

Island Copper, S Wall, BC

Kemess South, N Wall, BC

Escondida, Chile

Montana Tunnels, SW Wall, MT

Cyprus Bagdad, AZ

Ruth, Veteran-Tripp Pit, NV

Tesoro, Chile

Montana Tunnels, NW Wall, MT

Jeffrey, N Wall, QC

Nickel Plate, BC

Afton, SW Wall, BC

Carlin Trend Gold, Post Pit, NV

Aznacollar, Spain

Aitik, Sweden

Afton, NW Wall, BCMontana Tunnels, SE Wall, MT

Gold Quarry, E Wall, NV

Brenda, BC

Twin Buttes, NM

Kemess South, W Wall, BCSan Andres, Honduras

1.3 1.0

TREND LINES OF NOMINAL

FACTOR OF SAFETY AFTER

HOEK & BRAY (1981)

LEGEND

Lutton, Hoek & Bray DataStable SlopesUnstable Slopes

Sjoberg DataStable SlopesUnstable Slopes

Other ProjectsStable SlopesUnstable Slopes

Marathon PGM-Cu ProjectSouth Pit

Satellite Pit(s)

Main Pit

Satellite Pit(s)

Main Pit

Information Request 4.3.5

Grouting and Groundwater Quality

References: EIS Guidelines - Section 2.4.3.1, p. 25 (PDF 30)

- Section 2.7.2.3.1, p. 52 (PDF 57) EIS Main Report - Section 6.1.1.2, p. 6.9 (PDF 515) SID#11 - Section 3.1.5, p. 12 (PDF 20) SID#15 - throughout Rationale: The groundwater model has been conducted assuming two different base case scenarios: without grouting under the PSMF dams, and with grouting under the dams. The model shows that grouting results in significant reductions in seepage from the PSMF. However, it is not apparent in either the SID #11 or SID #15 if grouting is the preferred approach. After the end of mine operations and reclamation work associated with the permanent disposal of PAG waste rock in the mined out open pits, the pits will be allowed to fill with water. Once filled, there is a potential for groundwater to flow from the pits and discharge into nearby water bodies, including the Pic River. If there is groundwater flow from the pits to nearby water bodies, and that water is of poor quality, then this could lead to impacts on water quality in the post-closure period. While SID #15 predicts that the flooded main pit would become a groundwater discharge area, more information is needed to substantiate this prediction. Information Request: Identify the preferred approach for mitigating seepage through faults or fractures in the PSMF. If the preferred approach has not yet been identified, discuss the basis on which the preferred approach will be identified and discuss how the risk of seepage will be assessed. Confirm if the necessary data to determine the preferred approach is available, and if so, include the data in the response. Provide additional information on the predicted direction, pathway(s) and rates of groundwater flow out of the flooded pits, taking into account the presence of an east-west trending fault that cuts through the pit and extends into the Pic River valley. Also provide information on the predicted timeframe for any groundwater originating from the open pits to reach the Pic River, Bamoos Lake and any other potentially affected water bodies.

SCI RESPONSE

PSMF Seepage Mitigation The seepage mitigation strategy for the Process Solids Management Facility (PSMF) embankments will include the following:

Installation of a HDPE geomembrane on the upstream face of the embankments.

Anchoring of the HDPE geomembrane to low permeability bedrock via a concrete plinth.

Deposition of fine grained process solids along the upstream face of the PSMF embankments to develop a low permeable blanket/deposit adjacent to the geomembrane, plinth and upstream foundation areas.

Cleaning and treatment of the bedrock surface with slush grout the plinth to fill any discontinuities and irregularities in the bedrock to minimize potential seepage through the near surface bedrock.

Injection grouting will be completed to decrease bedrock permeability (see below) at locations such as faults, shears or deeper fractured bedrock zones.

The injection grouting will be completed in areas where the bedrock permeability has been determined to be greater than 1x10-4 cm/s (1x10-6 m/s). The injection grouting program will be developed to suit site conditions. The program may include drilling and grouting of primary, secondary and tertiary holes depending on the in situ conditions. In situ permeability testing would be completed as part of each grouting stage to confirm the permeability conditions and grouting results. Predicted Groundwater Flow Directions - Open Pit As discussed in SID #15, the flooded pits are not expected to be sources of recharge to the groundwater flow regime. Numerical groundwater flow modeling around the main pit 100 years post closure with a pit water elevation of 258 masl (filled to overflowing) and application of the ZoneBudget module predicts groundwater flow from the pit into the groundwater flow regime to be approximately 3 m3/day, substantially less than the predicted groundwater inflow to the pit of more than 300 m3/day. The only area of the main pit predicted in the model to have water moving from the pit to the subsurface when the pit is full, is along the eastern side at the lowest point along the edge of the pit. Water leaving the pit at this location would migrate east through the ground towards the Pic River. Horizontal groundwater velocity can be estimated with the following formula:

Where v is the horizontal groundwater flow velocity, K is the hydraulic conductivity, i is the horizontal hydraulic gradient and ne is the effective porosity.

For the purposes of this calculation the hydraulic conductivity of the upper layer of the calibrated groundwater flow model is used (1 x 10-7 m/s), which is representative of the overburden and upper bedrock. After 100 years the water level in the main pit is predicted to be 258 masl (SID #15) while the elevation of the Pic River east of the main pit is approximately 175 masl, a difference of 83 m. The distance from the eastern edge of the main pit to the western bank of the Pic River is approximately 1,360 m, resulting in a horizontal hydraulic gradient of 0.06 between the main pit and the Pic River. An effective porosity of 0.05 has been assumed for the upper rock zone. Substituting these values into the equation above produces a horizontal groundwater flow velocity of approximately 4 m/year. As described in IR 24.7 hydraulic conductivity testing of the fault that passes through the main pit shows that the hydraulic conductivity of the fault is similar to that of the surrounding rock; therefore, it is not interpreted to be an important preferential pathway.

The satellite pits are conceptually similar to the main pit and only very small amounts of water would move from the pits into the groundwater flow regime when full. Based on a horizontal groundwater flow velocity of 4 m/year and a distance of 1,360 m between the main pit and the Pic River, it would take centuries for water originating in the pit to migrate to the Pic River. Water from the pits will not migrate to Bamoos Lake. The surface of Bamoos Lake is approximately 295 masl while the predicted water level in the main pit when “full” is 258 masl, meaning water would have to flow “uphill” to migrate from the main pit to Bamoos Lake. Furthermore and in any event, groundwater flow from the pits will not affect surface water quality for the following reasons. When the pits are filling and not yet at their equilibrium water level, they represent groundwater sinks. That is groundwater can only flow into the pits and there will be no outward flow. The water levels will be monitored to determine filling progress and to know when the pit water levels will approach the equilibrium or “full” level. Water quality will also be monitored in each pit. If water quality is unacceptable for release to surface drainage, mitigation will be planned and implemented prior to pit filling. Such mitigation would typically involve batch treatment by pH adjustment with lime. This is a proven and standard approach. The pits will not be allowed to fill to overflow levels if water quality is not acceptable for release to surface water. This will also ensure that groundwater will not be adversely affected and that movement of pit water through groundwater will not affect other surface waters.

information request 4.3information request 4.3.4 slope stability of open pits references: eis...

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