Heap Leach Pad Slope Stability Analysis

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TECHNICAL MEMORANDUM .GOlder:AssociatesGolder Associates Ltd.500 - 4260 Still Creek DriveBurnaby, British Columbia, Canada V5C 6C6

Telephone: 604-296-4200Fax Access: 604-298-5253

TO:FROM:CC:

Western Copper CorporationChristopher AndersonJohn Hull

DATE: March 26, 2007JOB NO: 06-1413-077DOC NO: 028

EMAIL: [email protected]: GEOTECHNICAL STABILITY ANALYSES AND DESIGN

HLF CONFINING EMBANKMENT AND EVENTS PONDCARMACKS COPPER PROJECT, YUKON

The following describes the design criteria, methodology and results for the geotechnicalstability analyses and design performed for preliminary design of the Heap Leach FacilityConfining Embankment, Events Pond, and Sediment Pond for the proposed CarmacksCopper Project.1.0 INTRODUCTIONThe slope stability assessment has been carried out as part of the preliminary design ofthe Heap Leach Facility (HLF), in conjunction with a thermal analysis and a foundationassessment, which were undertaken in order to estimate the response of the foundationsoils to the increase in temperature and pressure resulting from heap construction andoperation. The methodology and results of these related analyses can be found inAppendices I and II for the thermal analysis and foundation settlement assessment,respectively.This assessment has been carried out using the data collected from previous geotechnicalsite investigations and site information obtained by others. No additional siteinvestigations or site visits were undertaken by Golder Associates (Golder) prior tocompleting the analyses for the current preliminary HLF design.1.1 Current Site Conditions and Proposed DevelopmentIt is understood that site development to date has consisted of clearing trees and brushfrom the Heap Leach Pad and Events Pond area, and developing access roads across thesite. The extent of ground disturbance, in particular removal of surficial organic soil, has

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not been quantified, and as such, any potential improvement or disturbance to foundationconditions can not be assessed at this time. However, it is assumed that completeremoval of all organic soils will occur prior to construction, and as such, the presence ofthe organic layer is neglected for assessment of slope stability.The planned development of the Carmacks Copper project calls for construction of a heapleach facility in a valley leach arrangement. The primary structures of the HLF include,the heap leach pad, the Confining Embankment, and the Events Pond. The heap leachpad covers an area of about 38 hectares. The base of the pad will consist of a compositesoil and synthetic liner placed over the site foundation soils. The moderately sloping ofthe valley floor (approximately 17% overall slope) will require a confining embankmentat the down-slope toe of the heap pad, and a retaining berm along the south western toe ofthe heap pad for stability and to provide lateral containment of the pregnant leachatesolution (PLS).The change in elevation from the toe of the confining embankment to the top of the heappad is approximately 155m, with a maximum pile thickness at any location of about90m.The geomembrane liner at the base of the heap provides containment of the PLS,however, it also reduces friction at the base of the stacked ore, reducing the stability ofthe facility. The overall layout and allowable slope angles are directly impacted by thegeomembrane at the base of the leach pad.The Events Pond will be a lined solution storage pond, constructed partially in cut, with adownslope embankment of up to 20 m height. The pond will be lined with ageomembrane liner system, however, this liner system will be exposed at the surface,with no materials on the liner, and as such, the design of the structure is governed bystability of the foundation and slopes of the impounding dam.

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1.2 Existing Subsurface InformationGeotechnical site investigations have previously been carried out by others across theplanned area of the Heap Leach Facility (Knight Piesold, 1995, 1996, and EBA 1997), forwhich borehole, trench, and testpit information was provided to Golder.

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Based on the available information, the subsurface soil stratigraphy across the HLF areagenerally consists of the following soil layers:

Organic and/or Ash Layer; Glacio-fluvialiGlavio-lacustrine silts and clays; Well-graded Glacial Till; Weathered/Frost shattered Bedrock; and Bedrock.

The material properties of the above soil and bedrock materials are described inadditional detail in the main text of the report.For the purposes of this analysis, thaw-weakening of the foundation soils resulting frompermafrost degradation was not considered, as it is assumed that foundation improvementto remove unsuitable soils would be carried out prior to construction. During detaileddesign, it is recommended that detailed foundation stability analyses be carried out toassist determination of required foundation treatments.2.0 DESIGN CRITERIAThis section provides the design criteria used for design and analysis of the Heap Pad,Confining Embankment, Events Pond, and Sediment Pond Dams. The criteria werederived from CDA Dam Safety Guidelines, and discussions with WCC, and based onprevious experience by Golder with similar structures at other mine sites.2.1 Yukon Government Regulations and GuidelinesThe 2005 agreement between WCC and the Yukon Government identifies specificPerformance Standards for the Carmacks Copper Project (Yukon Government, 2005a).Within these Performance Standards are "Critical" Issues related directly to theHeap Leach Pad, including "Physical Stability of heap and associated earth works, suchas berms constructed to constrain leachate". The applicable guideline identified for theissue of physical stability is the Canadian Dam Association's "Dam Safety Guidelines"(1999).The design criteria and methods of analyses utilized herein have been selected in generalaccordance with the CDA Dam Safety Guidelines.

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2.2 Consequence Categories

The Canadian Dam Association Dam Safety Guidelines requires determination of aconsequence category (Very Low, Low, High, or Very High) for the structure, based onrisk to downstream and upstream facilities, and on potential loss of life. The expectedlife of mine is approximately 10years, with additional activities on site for an unspecifiedperiod of time for decommissioning and closure.For the heap pad and confining embankment, significant downstream facilities arepresent, including the Plant Site and Events Pond. The resulting risk of loss of life,damage to infrastructure, damage to the environment, and loss of process results in aConsequence Category ranking of High for this structure.The Events Pond is located below the plant site, and the primary consequence of failurewould be loss of process and damage to the environment. The resulting ConsequenceCategory ranking is deemed to be High.The sediment pond is not located above other infrastructure, and is not expected to retainhighly contaminated water, resulting in a relatively low risk to the environment. As such,the Consequence Category ranking for the Sediment Pond is considered to be Low.2.3 Regional SeismicityThe seismic risk in the Carmacks Copper Project area has previously been characterizedby a seismic hazard assessment carried out for the project site (Knight Piesold, 1995).The hazard assessment provided probabilistic and deterministic values for the maximumground acceleration. From the probabilistic analyses, the maximum ground accelerationsfor the 475-yr return period earthquake was 0.085g, and for the 1000-yr return periodearthquake was O.l03g. The deterministic method provided a Maximum CredibleEarthquake (MCE) of Magnitude 8.5, at a distance of 250 km, with local, firm ground,peak horizontal acceleration of 0.13 g.

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The CDA Dam Safety Guidelines provides minimum criteria for design earthquakesbased on consequence category, as follows:

Maximum Design EarthquakeConsequence Category (MDE)

Deterministic ProbabilisticVery High MaximumCredibleEarthquake(MCE) 1110,000High 50%to 100%MCE 111,000o 1110,000Low - 11100o 111,000

Based on the CDA guidelines, and considering the relatively short life of mineanticipated, the design earthquake for the High consequence structures (Heap Pad andEvents Pond) should be the greater of 50% of the MCE, or the 111OOO-yrearthquake.The resulting Maximum Design Earthquake (MDE) would subsequently be the111,000-yr event, with a local firm ground acceleration of 0.103g. For the Lowconsequence structures, the design earthquake can similarly be taken as the 1I1,000-yrearthquake for consistency in this preliminary design.The peak horizontal ground acceleration is subject to amplification at the top of earthstructures. However, for this preliminary assessment, no amplification of accelerationhas been applied to the analysis of these embankments.2.4 Geotechnical Design CriteriaThe structural fill portions of the Heap Pad Embankment and Events Pond embankmentwill be constructed primarily of coarse mine waste rock. The specific geotechnicalproperties such as gradation, angle of friction, and hydraulic conductivity, used in theanalyses have been estimated based on limited field data, typical values used in theliterature, and past experience with such structures.The geotechnical criteria used for the design and analyses of the structures aresummarized below:Embankment Height: Determined by required storage capacities (Ore, Solution, orwater).

Heap Pad Embankment: 30 m.Heap Pad: up to 80 m. Y W BEvents Pond Embankment: 22 m.

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Heap Pad Embankment and Events Pond embankment sitesand alignments previously selected by others.Based on requirements for equipment access, stability andconstructability.

- Heap Pad Embankment and Events Pond: 6 m crestwidth.

Alignment:

Slope Angles: Based on requirements for stability, erosion resistance and toreduce maintenance requirements.Heap Pad Embankment:3H: 1V Upstream; 2.5H: 1V DownstreamOverall Ore Slope:3.35H:IV.Inter-Bench Ore Slope:2.5H:IV.Events Pond Embankment:3H: 1V Upstream; 3H:1V Downstream.

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Seismic Criteria Based on CDA Dam Safety Guidelines: Greater ofDeterministic (50% of Maximum Credible Earthquake), orProbabilistic (1000 yr Return period) Criteria.Maximum Design Earthquake = Probabilistic Criteria:Maximum Firm-Ground Acceleration = 0.103 g.Seismic Coefficient = 0.0515.Structures must be stable under unfavourable conditionscaused by seepage and earthquake forces. The Factors ofSafety for used for design were:F.S.2: 1.5 for Static conditions with an empty impoundment.F.S.2: 1.15 for Dynamic (pseudo-static) stability.The trial Failure Surfaces considered were:Heap Pad: Sliding along the membrane, and/or through theconfining embankment.Events Pond: Deep-seated rotational surfaces that wouldresult in loss of 50% or more of the crest width.

Stability:

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Dam Fill Materials

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Spillway Design:

Freeboard:Liner System:

Structures will be constructed of non acid generating minewaste rock andlor locally excavated granular materials withthe following assumed soil parameters:

Internal Angle of Friction = 35 degrees.Unit Weight = 20 kN/m3.

Sized to safely pass the 200-year, 24-hour design flow routedthrough the pond with freeboard. Location selected based onhydrologic and stability considerations.1 m minimum during the 200-year, 24-hour design event.Based on BC ML-ARD Guidelines identified in 2005 ProjectAgreement.Heap Pad and Embankment: Soil-Geomembrane interfacefriction angle, ~ = 17.Events Pond: Geomembrane exposed, therefore, no effect onstability.

2.5 Hydrological Criteria

he impoundment volumes for the Events Pond, and details of the spillway have beendesigned to safely convey the flows resulting from the design storm events. Theembankments have been designed with either a 3H: 1V downstream shell in order to resisterosion to the structure in the event of overtopping resulting from exceeding the designprecipitation and runoff event during an extreme or emergency event.Further details regarding the Events Pond design can be found in the main text of thereport.3.0 DESIGN CONSIDERATIONS3.1 Site Location and LayoutThe general locations of the Heap Pad facility and Events Pond been previously selectedby others. No on-site review has been conducted to confirm the adequacy of the overallsite and layout of the structures.

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The heap pad location and configuration was required to provide sufficient capacity forthe required quantity of ore, and to maintain positive drainage at the base of the leach padin order to contain and collect the leaching solution. Re-configuration of the previousleach pad layout was required based on slope stability results, discussed further inSection 4.1.2.The confining embankment alignment was previously selected by others. The adequacyof the size and location of the confining embankment to provide support to the heap padwas confirmed as part of this stability analysis.The Events Pond was required to be located below the heap pad and plant site to allowgravity drainage of solution from both the heap and plant. The layout was modified fromthe preliminary layout provided by M3 in order to obtain the required storage capacityand to minimize the required fill volume.3.2 Internal StabilityBased on requirements of site location, the geotechnical analysis and design of thestructures was based on considerations of internal stability. The internal stability of thestructures is based on the specific geometry of a particular structure including originalground surface topography, overall height, crest alignment, overall and interim slopeangles, internal phreatic surface, foundation conditions, and construction materials.To the extent possible, crest alignments of the embankments were selected based onlocalized topography, such that fill quantities were minimized while providing therequired impoundment volume.

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The foundation conditions of the embankments are subject to localized topographic andsubsurface soil and bedrock conditions. Detailed subsurface soil information was notavailable for most of the embankment locations. Therefore, only a generalizedinterpretation of dam foundation soil conditions was used in analysis. Based on availableinformation, fluvial sand and gravel and glacial till overly bedrock for thicknesses of 2 to20 m in most locations. For the purpose of these analyses, the foundation soils wereassumed to consist of compact to dense sand and gravel extending to depth, which isconsidered conservative.

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Thaw-weakening of the foundation soils resulting from permafrost degradation was notconsidered in this analysis, as it was assumed that foundation improvement to removeunsuitable soils would be carried out prior to construction. It should be noted, however,that long term creep deformation of ice-rich soils under the high loads of the heap leachY W B

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pad and confining embankment could potentially result in delayed distress or failure ofthe facility. Such deformations would not be readily apparent, and would requiremonitoring of the structures in order to detect such deformations. The potential for creepdeformation could be reduced with improvements to the foundation soils. Duringdetailed design, it is recommended that detailed foundation stability analyses be carriedout to assist determination of required foundation treatments.It is understood that the embankments will be constructed using mine waste rock fill.These materials generally provide reasonably high internal shear strength, while alsoproviding relatively high permeability.The Heap Pad was modelled using a thin weak layer to simulate the geomembraneelement. Both the heap pad and the Events Pond also included a layer to simulate a lowerstrength fine grained soil liner material. The filter zone material and drainage blanketmaterial were neglected for these preliminary analyses, as these materials would consistof granular materials of reasonably high strength.The internal phreatic surfaces differ for each structure, based on the conditions of seepageof impounded water/solution through the particular embankment. Each structure wasanalysed for "dry", and "full" conditions, where applicable. For the Heap Pad andConfining Embankment, the maximum height of the phreatic surface within the crushedore was assumed to be the elevation of the spillway for the lower portion of the facility,and a height of 1 m, corresponding to localized mounding, for the upper portion of thefacility. The embankment was assumed to not experience an elevated phreatic surfacedue to the coarse fill that would be used to construct the embankment, and the provisionof a drainage blanket. The Events Pond was similarly analysed for dry and full pondconditions with no elevation of the phreatic surface within the embankment due to theliner on the surface of the embankment, and the presence of a blanket drain.3.3 External Risks

The stability of the embankments, as influenced by the internal factors noted above, mayfurther be affected by external influences such as seismic risk and natural hazards.The regional seismicity of the project area is discussed in Section 2.4 of the main report.The effect of a seismic event on the structures would be to cause cyclical lateralacceleration of the structures and result in a transient lateral forces acting to destabilizethe structures. The seismic risk to the structures has been taken into account in the designof the structures by considering dynamic (pseudo-static) loading on the structures duringslope stability modelling.

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Natural geologic hazards may also potentially have a significant impact on the stabilityand performance of the structures. The topography surrounding the impoundment areasconsists of moderate to steep slopes which may pose a landslide hazard. The EventsPond and Heap Confining Embankment are at less risk because the slopes above theimpoundment areas will be excavated / constructed to a stable design shape.4.0 STABILITY ANALYSIS4.1 Methodology

The principal criterion for the structures is to provide adequate stability during the life ofmine, and for long term post closure conditions. The embankments should have adequatestatic Factors of Safety as well as during a design earthquake event (dynamic condition),and high impounded water levels. The structures should also be stable with respect tofailure through the embankment, or sliding along their foundations. In the evaluation ofthe long-term stability of the structures, multiple scenarios were considered: the absenceof water within the impoundment areas (dry condition), and an elevated water level to thefull height of the spillway representing functioning during design-storm runoff collectionand retention.

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For the Heap Pad and Confining Embankment, the critical failure mode was identified inpreliminary trials to be a block-sliding style failure along the crushed ore / geomembranecontact, rather than the classic rotational failure through the foundation soils, and as such,subsequent analyses did not consider foundation failures. In the block-sliding analysis, anon-circular failure surface was obtained by specifying a failure surface following thethin weak layer used to simulate the geomembrane. Where the confining embankment orberm was present, the failure surface was allowed to either follow the weak layer up theinternal slope of the confining embankment, or to extend through the embankment tosimulate failure of the embankment, by either internal failure, or sliding along the base.For the Events Pond, circular failure surfaces were identified as the critical failuremechanism. In the circular analyses, deep-seated rotational surfaces pose a risk to theintegrity of the dam, whereas more shallow failures typically result only in sloughingrequiring minor maintenance and repair. Therefore, the analysis focused on deep-seatedtrial surfaces, which were defined as a minimum of 50% loss of crest width, and exitingat or beyond the base of the slope.The dam stability analyses were performed with the commercially available computerprogram SLIDE, version 5.0 (RocScience, 2003). The dynamic stability of the dam wasanalysed using a "pseudo-static" approach which applies a horizontal force due to the

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earthquake event in the direction tending to cause failure. This approach of modelling aseismic load is generally considered to be conservative. The dynamic analyses for thisdesign were performed using a design horizontal ground acceleration of 0.132 g. Inaddition, the yield acceleration (defined as the ground acceleration producing a Factor ofSafety of 1.0) was computed for both upstream and downstream failures in both dry andfull conditions for each dam.4.1.1 Static Stability

The proposed modifications to the liner materials for the heap pad consequently reducedthe anticipated interface friction angle between the overliner and geomembrane materials.The resulting reduction in slope stability of the heap, in conjunction with changes to thearrangement of the solution collection sump and riser, required modification of the layoutof the heap pad in order to confirm a stable configuration with adequate capacity forcrushed ore. The typical section and stability results of the resulting centerline profile ispresented in Figure I1I-2.Due to the highly irregular shapes of the original ground surface and ultimate heapconfiguration, a number of sections were analysed, including cross-sections at locationssupported by the confining embankment, supported by the retaining berm, and where thetoe of the heap does not require support, however, results have not been presented at eachof the analysis locations. Results are included for the typical section and stability resultsfor the southwest portion of the heap, where a retaining berm will be required, and arepresented in Figure 111-3.

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The cross-sections presented on Figure I1I-4 and III-5, show typical sections and stabilityresults for the Events Pond, as well as the critical upstream and downstream failuresurfaces for the embankment. Both the empty and full impoundment conditions areillustrated.In all cases, the calculated static Factors of Safety for the block-sliding and deep-seatedfailure surfaces exceed the required minimum value of 1.5 for static stability for both dryand full impoundment conditions.4.1.2 DynamicStability

The Heap Pad, Confining Embankment and Events Pond are to be constructed of minewaste rock, and are expected to be founded on a prepared foundation surface. As such, itwas assumed for this analysis that neither the embankments, nor their foundations aresusceptible to liquefaction. Y W B

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The CDA Dam Safety Guidelines specify a minimum Factor of Safety of 1.15 for thedesign acceleration, whether the impoundment is full or empty. Psuedo-static analyses ofthe structures were performed at a range of horizontal accelerations in order to determineboth the factor of safety at the design acceleration, as well as the actual yield accelerationof the structures for both upstream and downstream trial surfaces. The results of theseanalyses are presented with the static analyses on Figures III-2 through III-5, inclusive.

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In all cases, yield accelerations of at least 0.13 g were determined for all of the structures.It is expected that the structures will perform satisfactorily during the design earthquake.However, for the Heap Pad and Events Pond, where liner systems may be damaged byexcessive crest deformations, deformation analyses due to dynamic loading should becarried out during detailed design.5.0 SUMMARYThe slope stability assessment has been carried out, in conjunction with a thermalanalysis and a foundation assessment in order to estimate the response of the foundationsoils to the increase in temperature and pressure resulting from heap construction andoperation. The following provides a summary of the findings of the slope stabilityassessment:Long term creep deformation of ice-rich soils under the high loads of the heap leach padand confining embankment could potentially result in delayed distress or failure of thefacility. Such deformations would not be readily apparent, and would require monitoringof the structures in order to detect such deformations. The potential for creepdeformation could be reduced with improvements to the foundation soils.Natural geologic hazards, including earthquakes and landslides could potentially impactthe stability and performance of the structures. The landslide hazard would likely behighest during or shortly after high rainfall events, and a landslide into a fullimpoundment could potentially result in a wave of water overtopping the embankment,causing flooding downstream and potentially destabilizing the embankment.From the slope stability analyses, the critical mode of failure of the heap leach pad andconfining embankment was identified to be a block-sliding type failure along thegeomembrane.

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The general arrangement of the heap leach pad was modified to provide a suitable factorof safety against sliding. The resulting arrangement consists of an inter-bench slopeangle of 2.5H : 1V, and overall slope of 3.35H : 1V, with a bench width of 30 m atelevation 850m, and 60 m at elevation 890 m.A retaining berm will be required along the southwest side of the proposed heap leachpad in order to achieve a suitable factor of safety.For the Events Pond the critical failure mode was found to be a classic circular failuresurfaces through the embankment and foundation soils.The general arrangement of the Events Pond that provided an acceptable Factor of Safetywas found to be 3H: 1V upstream and 3H: 1V downstream embankment slope angles.It is expected that the structures will perform satisfactorily during the design earthquake.However, for the Confining Embankment and Events Pond, where liner systems may bedamaged by excessive crest deformations, deformation analyses for should be carried outduring detailed design.It is recommended that a detailed site investigation be carried out in order to providesufficient information regarding foundation and frozen ground conditions to carry outdetailed stability analyses as part of detailed design.CDA/JAH/gs/cmO:\FinalI2006114131061413-077ldoc 028 TM-0326_07 Stability Assessment of HLF. doc

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REFERENCES

Canadian Dam Association, 1999. Dam Safety Guidelines, Canadian Dam Association,Edmonton, AB.EBA, 2005. "Heap Leach Pad Liner Design, Carmacks Copper Project near

Williams Creek, YT", Report submitted to Western Silver Corporation, May 2005.Golder, 2007a. "Surface Water Management", Report submitted to

Western Copper Corporation Ltd., in preparation.Knight Piesold, 1996. "Report on Detailed Design", Report submitted to

Western Copper Holdings Ltd., August 14, 1996.Knight Piesold, 1995. "Report on Preliminary Design", Report submitted to

Western Copper Holdings Ltd., May 1, 1995.RocScience Inc., 2003. Slide, v5.0 ~ Two-Dimensional Limit Equilibrium Slope Stability

Analysis.

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Heap Leach Pad Slope Stability Analysis

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