Revised Dam Design

Preliminary Tailings Dam Design

Doris North Project, Hope BayNunavut, Canada

Prepared for:

Miramar Hope Bay LimitedSuite 300, 889 Harbourside Drive

North Vancouver, BC V7P 3S1Canada

Prepared by:

SRK Project No. 1CM014.006

October 2005

Preliminary Tailings Dam Design, Doris North Project, Hope Bay

Nunavut, Canada

Miramar Hope Bay Limited 300 - 889 Harbourside Drive

Vancouver, BC, Canada. V7P 3S1

SRK Consulting (Canada) Inc. Suite 800, 1066 West Hastings Street

Vancouver, B.C. V6E 3X2

Tel: 604.681.4196 Fax: 604.687.5532 E-mail: [email protected] Web site: www.srk.com

SRK Project Number 1CM014.006

October 2005

Authors Michel Nol, M.A.Sc., P.Eng.

Senior Geotechnical Engineer

Maritz Rykaart, PhD., P.Eng Senior Geotechnical Engineer

Reviewed by Cam Scott, P.Eng.

Principal

SRK Consulting (Canada) Inc. Preliminary Tailings Dam Design, Doris North Project, Hope Bay, Nunavut, Canada Page i

MN/spk PreliminaryTailingsDamDesign.Report.1CM014.006.emr.20051012.doc, Oct. 21, 05, 3:02 PM October 2005

Table of Contents Table of Contents ..........................................................................................................................i List of Tables ............................................................................................................................... iii List of Figures.............................................................................................................................. iii List of Appendices ....................................................................................................................... iii

1 Introduction .................................................................................................................. 1 1.1 Scope of Work..................................................................................................................... 1 1.2 Dam Design Review............................................................................................................ 1 1.3 Report Organisation ............................................................................................................ 2

2 Background Information ............................................................................................. 3 2.1 General ............................................................................................................................... 3 2.2 Location............................................................................................................................... 3 2.3 Topographic Maps and Terrain Model ................................................................................ 3 2.4 Site Layout and Logistics .................................................................................................... 3 2.5 Dam Locations and Operating Intent .................................................................................. 4 2.6 Tailings Discharge............................................................................................................... 5 2.7 Tailings Properties .............................................................................................................. 5 2.8 Spillway ............................................................................................................................... 6 2.9 Tailings Impoundment Closure ........................................................................................... 6 2.10 Concept of a Frozen Core Dam .......................................................................................... 7 2.11 Meteorological Data ............................................................................................................ 8 2.12 Climate Change .................................................................................................................. 8 2.13 Subsurface Investigations ................................................................................................. 10 2.14 Foundation Conditions ...................................................................................................... 11

2.14.1 North Dam .............................................................................................................................11 2.14.2 South Dam.............................................................................................................................12

2.15 Ground Temperature and Permafrost ............................................................................... 13 2.16 Freezing Temperature....................................................................................................... 13 2.17 Unfrozen Water Content ................................................................................................... 14 2.18 Strength............................................................................................................................. 14 2.19 Seismicity .......................................................................................................................... 15

3 Preliminary Dam Design............................................................................................ 17 3.1 General Layout.................................................................................................................. 17 3.2 Design Criteria .................................................................................................................. 17

3.2.1 Dam Classification.................................................................................................................17 3.2.2 Design Earthquake................................................................................................................18 3.2.3 Design Capacity ....................................................................................................................18 3.2.4 Design Freeboard..................................................................................................................19 3.2.5 Design Flood .........................................................................................................................19 3.2.6 Stability ..................................................................................................................................19 3.2.7 Seepage ................................................................................................................................20 3.2.8 Freezing Temperature...........................................................................................................20 3.2.9 Construction Temperature.....................................................................................................20 3.2.10 Climatic Data and Climate Change .......................................................................................20

3.3 Upset Conditions............................................................................................................... 21 3.3.1 Warm Climate........................................................................................................................21 3.3.2 Over-Topping ........................................................................................................................21

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3.3.3 Extended Duration of Storage...............................................................................................21 3.4 Settlement ......................................................................................................................... 22 3.5 Dam Section...................................................................................................................... 23

3.5.1 Justification............................................................................................................................23 3.5.2 Description.............................................................................................................................24 3.5.3 Abutments .............................................................................................................................25

3.6 Construction Materials ...................................................................................................... 25 3.6.1 Material A (20 mm minus) .....................................................................................................25 3.6.2 Material B (150 mm minus) ...................................................................................................25 3.6.3 Material C (run-of-quarry)......................................................................................................26 3.6.4 Impervious Membrane (GCL)................................................................................................26 3.6.5 Thermosyphons.....................................................................................................................26

3.7 Spillway ............................................................................................................................. 27 3.8 Decant System.................................................................................................................. 27 3.9 Quantities .......................................................................................................................... 28 3.10 Thermal Analysis............................................................................................................... 28

3.10.1 Scenarios...............................................................................................................................28 3.10.2 Model.....................................................................................................................................29 3.10.3 Soil Properties .......................................................................................................................29 3.10.4 Calibration .............................................................................................................................30 3.10.5 Predictions - Normal Operating Conditions...........................................................................31 3.10.6 Predictions - Upset Condition................................................................................................34 3.10.7 Discussions and Conclusions................................................................................................34

3.11 Seepage............................................................................................................................ 35 3.12 Stability.............................................................................................................................. 35

3.12.1 Failure Modes........................................................................................................................35 3.12.2 Method of Analysis ................................................................................................................35 3.12.3 Geometry and Input Parameters...........................................................................................36 3.12.4 Results...................................................................................................................................36

4 Implementation .......................................................................................................... 37 4.1 Final Design ...................................................................................................................... 37

4.1.1 Emergency Preparedness Plan ............................................................................................37 4.1.2 Adaptive Management Plan ..................................................................................................38 4.1.3 Engineering Analysis.............................................................................................................39 4.1.4 Additional Field Work ............................................................................................................40 4.1.5 Additional Laboratory Work ...................................................................................................40

4.2 Construction ...................................................................................................................... 40 4.2.1 Methodology..........................................................................................................................40 4.2.2 Equipment .............................................................................................................................41 4.2.3 QA/QC...................................................................................................................................42

4.3 Post-Construction Activities .............................................................................................. 42 4.3.1 Monitoring..............................................................................................................................42 4.3.2 Site Inspection.......................................................................................................................43 4.3.3 Maintenance..........................................................................................................................43

5 References.................................................................................................................. 45

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List of Tables

Table 1: Correlated ambient temperature, average, cold and warm values..................................... 8 Table 2: Probabilistic seismic ground motion analysis ................................................................... 15 Table 3: CDA dam classification in terms of consequences of failure............................................ 17 Table 4: Minimum factors of safety ................................................................................................ 20 Table 5: Estimated quantities to construct the North and South Dams.......................................... 28 Table 6: Thermal properties used in the thermal model................................................................. 29 Table 7: Input parameters used in the stability analyses ............................................................... 36 Table 8: Summary of critical factors of safety for North Dam......................................................... 36 Table 9: Checklist of work to be carried out prior to completing detailed design of dams.............. 37

List of Figures

Figure 1: Location Map Figure 2: Overall Site Infrastructure Layout Figure 3: Site Plan of Tail Lake Figure 4: North Dam, Layout Plan Figure 5: South Dam, Layout Plan Figure 6: North Dam Alignment, Longitudinal Section A-A Figure 7: South Dam Alignment, Longitudinal Section C-C Figure 8: Ground Temperature and Permafrost Characteristics Figure 9: Unfrozen Water Content, Laboratory Results Figure 10: North Dam, Section and Details Figure 11: South Dam, Section and Details Figure 12: Recommended Gradation Envelope, Material A (Core) Figure 13: Recommended Gradation Envelope, Material B (Transition) Figure 14: Estimated Unfrozen Water Content Figure 15: Calibrated Ground Temperature Profile Figure 16: Thermal Model Geometry, North Dam Figure 17: Temperature Predictions, North Dam, Average Climate Figure 18: Temperature Predictions, North Dam, Average Climate with Thermosyphons Figure19: Temperature Predictions, North Dam, Warm Climate with Thermosyphons Figure 20: Temperature Predictions, Comparisons, 40 Year Simulations Figure 21: North and South Dams, Conceptual Instrumentation

List of Appendices

Appendix A EBA Engineering Consultants Letter Report by Mr. Don Hayley, P.Eng. Appendix B SRK Technical Memorandum re: Water Cover Design for Tail Lake Appendix C SRK Technical Memorandum re: Doris North Project Tailings Properties Appendix D Summer 2004 Geotechnical Field Investigation Appendix E Winter 2005 Geotechnical Field Investigation Appendix F SRK Technical Memorandum re: Wave Run-up Calculations Appendix G Larger Scale Drawings of Thermal Modeling Results Appendix H Detailed Slope Stability Results

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1 Introduction 1.1 Scope of Work

SRK Consulting (Canada) (SRK) has been working with Miramar Hope Bay Limited (MHBL) since October 2001, on various aspects of the Hope Bay Doris North Project (from here on referred to as the Doris North Project), including completion of a Preliminary Assessment in February 2002 (SRK 2002a) and a Feasibility Study in February 2003 (SRK 2003a). The Doris North Project is a two year underground mining operation, located in Nunavut Canada. Ore will be extracted and processed on site, requiring an on-site tailings disposal facility. The preferred tailings disposal approach is sub-aqueous tailings disposal in Tail Lake (SRK 2005f). Two earth dams are required to contain Tail Lake during the operational period, and SRK developed a series of technical reports documenting preliminary designs of these dams (SRK 2003b; 2005f). The first preliminary dam design (SRK 2003b) consisted of an unprecedented frozen core design constructed from locally sourced marine clays and silts. Subsequently the design was revised to reflect the more common frozen core dam designs adopted at the Ekati Diamond MineTM. Based on additional field data that has been collected, as well as comments received from interveners during technical discussion in Yellowknife in August 2005, MHBL requested that SRK update and replace the April 2005 (SRK 2005f) Preliminary Dam Design report. This report therefore replaces in its entirety all previous dam designs for the Doris North Project. The report was prepared by Mr. Michel Nol, P.Eng. (BC) and Mr. Maritz Rykaart, Ph.D., P.Eng. (BC, SK, NT/NU, YT). The report was reviewed internally by Mr. Cam Scott, P.Eng. (BC, NT/NU).

1.2 Dam Design Review The preliminary dam designs presented in this report have been reviewed by Mr. Don Hayley, P.Eng., from EBA Engineering. Mr. Hayley is a recognized world expert in the design and construction of dams in the arctic, and played a pivotal role in the design and construction of the Ekati Diamond MineTM frozen core dams. Mr. Hayley is a Registered Professional Engineer in Nunavut Territory. The complete review comments presented by Mr. Hayley are included as Appendix A. In his review, Mr. Hayley confirmed that the Tail Lake dam locations have particularly complex permafrost stratigraphy, and that There is no direct precedent for design and construction of a frozen core dam on saline marine soils such as identified at this site. However, Mr. Haley concludes that A frozen core dam remains the most appropriate structure for the environmental conditions and operating parameters at this location. The level of site characterization and design analysis will need to be elevated in order to deal with uncertainties identified in this review Mr. Hayley proceeded to make recommendations as to how the dam design should be modified to accommodate the complex permafrost stratigraphy, and also suggested additional site characterization and analysis which should be conducted prior to the final design stage of the dam.



The recommendations suggested by Mr. Hayley have all been fully and unconditionally adopted in the preliminary dam designs presented for the Doris North Project (See page 4 through 7 of the attached letter report in Appendix A). This includes an upstream dam side slope of 6:1 and a downstream side slope of 4:1. These flat slopes are specifically intended to address concerns related to potential deformation. This design is significantly more robust than the Ekati Diamond MineTM designs, to specifically accommodate site specific conditions. The additional thermal analysis suggested on Page 8 of the review letter report has been completed and is documented in this report. This report also acknowledges that additional stability analysis calculations will be conducted at the detailed design phase. The additional field characterization suggested by Mr. Hayley has also been carried out and is documented in this report. This program specifically targeted obtaining high quality samples of overburden from the North Dam foundation.

1.3 Report Organisation Following this introduction, Section 2 provides an overview of background information that is relevant for the dam design. Section 3 presents the preliminary dam design, which includes the design criteria, the definition of upset conditions, the estimation of settlement, the description of the dam sections and related components, the thermal, seepage and stability analyses. Section 4 covers the implementation aspects of the dam such as the final design, the construction, and the post-construction activities.



2 Background Information

2.1 General

This report relies heavily on background information presented in other reports and documents, and where relevant these reports have been appropriately referenced. The focus of this report is on the preliminary design of the two frozen core dams required to isolate Tail Lake as a tailings impoundment for the Doris North Project. Related components associated with the water management for the facility are not included, other than making brief mention of those aspects that has relevance to the dam design criteria. Where appropriate, sections of the October 2003 Preliminary Tailings Impoundment Design Report (SRK 2003b) have been repeated for completeness.

2.2 Location

The Doris North Project is situated approximately 400 km east of Kugluktuk (Coppermine) and 160 km southwest of Ikaluktutiak (Cambridge Bay) in the West Kitikmeot Region of the Territory of Nunavut (Figure 1). The site is approximately 160 km north of the Arctic Circle and 5 km south of the Arctic Ocean, at latitude 67 30 N and longitude 107 W. The nearest communities are Umingmaktok, located 65 km to the west and Kingauk (Bathurst Inlet), located 110 km southwest.

The site is remote and can only be reached via air (float planes in the summer and ice airstrips in the winter) or sea (using ships or barges during the late summer season).

2.3 Topographic Maps and Terrain Model

Topographic contour data for the terrain model was provided by MHBL. The resolution of the contours is 1 m intervals for most of the area and a small portion around Tail Lake has a contour interval of 2 m. The area along both dam alignments were surveyed and incorporated into the terrain model to generate updated topographic contours, which was then used for the dam design. The field survey data was also provided by MHBL and was consistent with the topographic maps. The surveyed elevation was within 0.5 m of the contours shown in the topographic maps.

2.4 Site Layout and Logistics

SRK (2005a, b) provides a complete description of the overall site infrastructure layout; however, this will be summarized briefly, to place the proposed dams in perspective. The proposed mill site is located approximately four kilometres from Roberts Bay, which forms part of the Arctic Coastline. This area is accessible via ships and barges for a short period during summer months only. This will become the main re-supply route for equipment and supplies for the Doris North Project. A jetty will be constructed in the bay as a landing facility for the barges. Equipment will be offloaded and stored in a lay-down area close to the shore. Annual fuel supply will be pumped from the barges to a fuel transfer station, from where trucks will transport fuel to a tank farm located at the mill. These



facilities will be linked to the mill and tailings facilities via all-weather roads. Between the mill and the bay a portion of the road will be widened to act as a permanent airstrip, suitable for mid-sized aircraft that could transport personnel and small freight. The overall site plan is shown in Figure 2.

The proposed development has enough reserves to sustain mining for two years (SRK 2003a). Mining will be underground, and the ore will be transported to surface via an adit. The ore will then be crushed and processed in a plant to produce gold bars as final product. Tailings produced during the milling process will be deposited in Tail Lake (Figure 3) about five kilometres from the proposed mill location. Tailings deposition will be sub-aqueous, requiring the construction of two water retaining structures; the North Dam and the South Dam. The North Dam is designed to retain a maximum hydraulic head of 7.5 m and the South Dam 2.0 m. Both dams are designed to operate for a maximum period of 25 years, after which the North Dam will be breached. The South Dam will not require breaching since the dam coincides with a natural watershed boundary with the adjacent Ogama Lake, and after the North Dam is breached, will impound no water, and not impede any natural flow of water. It should be noted that there is no potential for flow from Ogama Lake into Tail Lake. The normal operating water elevation in Tail Lake is 28.3 m, and that in Ogama Lake is 24.3 m. The lowest point in the saddle between Ogama and Tail Lake is 33 m. Therefore leaving the South Dam in place will not disrupt the natural hydrology under normal or extreme events, especially considering the fact that the normal range of water level for these lakes is in the 0.5 m range.

All construction equipment and supplies will be shipped (or barged) to site during the short summer navigation season. This equipment will be stored in a temporary lay-down area until the winter season when temporary winter roads will be constructed to relocate construction equipment to the desired construction areas. All other surface infrastructure will be constructed during this season, and through the following spring and summer.

2.5 Dam Locations and Operating Intent

The tailings impoundment requires the construction of two dams located at the north and south ends of Tail Lake as indicated in Figure 3. The proposed site layout for the North Dam is shown in Figure 4 and the South Dam in Figure 5.

The tailings impoundment is sized to operate as a zero discharge facility during the two years of operation, if necessary. In addition, under the most conservative water balance assumptions (SRK 2005c) Tail Lake would take just over five years to reach the design Full Supply Level (FSL) of 33.5 m. A permanent spillway will be constructed at this elevation, to prevent the possibility of dam overtopping.

The tailings water management plan (SRK 2005c) stipulates that the maximum water level in Tail Lake would be about 29.4 m, and that within five years after start of tailings deposition (i.e. three years after mining ceases), the natural inflow in Tail Lake would be equal to the amount of annual discharge that could be allowed. This proposed tailings management strategy requires discharge via



active pumping during the operational, closure and post-closure phases of the project. Under this scenario, the dams will never reach FSL, and the spillway will never be used.

The water management plan is based on a water quality model, and detailed sensitivity analysis has been carried out to address any uncertainties in the model. Based on this analysis, the soonest timeframe within which Tail Lake could reach FSL is five years. Furthermore, the longest period of time that Tail Lake would have to be at FSL would be about 22 years. Therefore, the dams containing Tail Lake has been designed for a minimum operational design life of 25 years; however, as an minimum upset condition, the design has been tested to ensure safe operation for at least 40 years.

The design takes into account, that after a maximum of 25 years, the North Dam will be breached, allowing the water level in Tail Lake to return to its pre-mining elevation of 28.3 m. The South Dam will not be breached as it is constructed on the watershed boundary between Tail and Ogama Lakes and is higher than elevation 28.3 m.

2.6 Tailings Discharge

Slurried tailings from the mill will be pumped about 5 km to the Tail Lake tailings impoundment. The tailings will be deposited sub-aqueously and the water level in the impoundment will be regulated through the use of recycle water and summer decant to Doris Creek. Tailings deposition locations will be continuously changed to ensure that tailings is evenly spread over the deepest sections of Tail Lake. Winter deposition will also be sub-aqueous, and will be achieved by pipes though the ice. No on-ice tailings deposition will be allowed. Tailings will not be in contact with any of the two dams.

Appendix B provides the details of the minimum water cover design thickness for Tail Lake, but also includes figures presenting the final tailings deposition plan view for Tail Lake. Although the intent would be to deposit tailings as level as practical, it is understood that there will be some undulations. An annual bathymetric survey of the tailings surface should be conducted to assist in planning of the tailings deposition. Should there be significant undulations in the tailings surface that would compromise the final water cover design requirements, consideration will be given to levelling the surface through dredging.

2.7 Tailings Properties

Since tailings will not be used to structurally or hydraulically enhance the dam design, the tailings properties do not play a role in the dam design; however, for completeness Appendix C contains a detailed description of the tailings properties for the Doris North Project. A complete discussion of the tailings settlement characteristics and tailings geochemistry is presented in SRK (2005c).



2.8 Spillway

A permanent operational spillway is required, and will be provided as protection against overtopping. The soonest that Tail Lake would reach FSL is just after five years of zero discharge, under the most conservative water balance assumptions. A permanent spillway will be constructed at the North Dam, at the FSL of 33.5 m, to allow through flow for a period of up to 25 years. The spillway will be sized to accommodate the 24-hour storm event with a 1:500 year recurrence interval. The spillway is illustrated in Figure 4 and entails a side-spillway across the northeast abutment of the North Dam. Spillway outflow will enter the original Tail Lake outflow channel approximately 100 m downstream of the North Dam toe, and would thus enter Doris Lake immediately upstream of the Doris Lake outflow point into Doris Creek. A spillway is not required at the South Dam as well due to the fact that both dams operate under the same conditions. If the North Dam cannot overtop due to the presence of an operational spillway, the South Dam will not be able to overtop as long as the freeboard requirements are met.

Considering the fact that the spillway may never be required or seldom used, an alternative approach would be to design the dams without a permanent spillway but to rely on a decant pumping system and incorporate components in the dam design that would support short term overtopping. Short term overtopping of the dam is however not considered a reasonable alternative and are therefore not recommended as part of this preliminary design.

It is however possible to consider delaying the construction of the spillway. Due to the fact that there is up to five years before the FSL is expected to be reached, if at all, it may make sense to delay the construction of the spillway and re-evaluate the situation annually based on actual water level measurements compared to the predicted values. SRK recommends that this alternative be further investigated at the detailed design stage. For the purpose of this report, it has been assumed that the spillway will be constructed at the same time as the dam.

2.9 Tailings Impoundment Closure

The final closure for the Tail Lake tailings impoundment is a permanent water cover of at least 3.0 m above the highest tailings elevation in the impoundment. Research has shown that a minimum stagnant water cover of 0.3 m is sufficient to prevent oxidization of tailings. Tailings can however be re-suspended due to wave action induced by environmental factors, and therefore the rule of thumb is to design a water cover of at least 1.0 m thick. Based on the orientation of Tail Lake, the predominant wind direction, maximum wind speeds, and the particle size of the tailings, the actual minimum water cover depth for Tail Lake has been calculated to be at least 2.0 m thick. A 3.0 m thick water cover was subsequently selected as the design criteria to add an additional factor of safety against unforeseen events. Complete details of the minimum water cover thickness calculations are presented in Appendix B.

For the full design tailings volume over two years, the tailings surface is expected to be below 24.3 m, which implies that the minimum final water elevation in Tail Lake must be at 27.3 m. In



actual fact the existing (i.e. pre-mining elevation of Tail Lake is 28.3 m, which implies that once the water quality in Tail Lake return to background concentrations, the North Dam can be breached to allow Tail Lake to return to its pre-mining elevation. Under this condition there will be a 4.0 m water cover over the tailings, which is a two-fold factor of safety.

2.10 Concept of a Frozen Core Dam

The concept of frozen core dams is to maintain both the core of a dam and the foundation in a frozen state, which will provide an impervious barrier to water. Frozen core dams thus requires that the foundation be saturated and frozen, and that the core of the dam be constructed with soils containing sufficient fines to retain water and enable the pore water to freeze in a quasi-saturated state.

The frozen pore water, once saturated, provides two benefits: strength and an impervious media. The strength is achieved by the frozen pore water that acts as bonding agent to the soil. The frozen pore water will block the pathways for groundwater seepage by filling the air voids, thus enabling impervious conditions over the frozen zone. Additionally, the frozen foundation does not require extensive excavation and grouting because of the rigid and impervious nature of the frozen ground along the foundation. It is however essential that the ground is saturated with pore ice to achieve impervious conditions.

The performance of frozen core dams will be dependent on the sustainability of the frozen conditions in the long-term. The long-term performance should also sustain potential upset conditions. This will often translate in additional backfill material to provide the necessary thermal insulation for the frozen core. Frozen core dams, and in particular the core itself, are normally constructed in winter to lock-in as much cold temperature as possible within the dam.

Frozen core dams often include synthetic liners placed against the upstream face of the core. This relative inexpensive option provides an additional barrier to contain water, thus increasing the margin of safety against potential seepage.

Thermosyphons are sometimes installed during the construction of the dam to provide additional cooling within the dam. The benefit of using thermosyphons is to lower the ground temperature in areas where ground temperature may exceed a threshold value, or as an additional precaution to compensate for uncertainties that may be associated with the dam design and the site conditions. Thermosyphons can either be horizontally placed loops or independent vertically placed systems. The horizontal looped systems are placed during the construction and the vertical thermosyphons generally require drilling and are installed after construction of the dam.

Most recently, five frozen core dams were successfully constructed at the Ekati Diamond MineTM Mine (Hayley et al. 2004). The configuration of these dams consists of a central frozen core that is encapsulated with a rock fill shell. The material consists of processed crushed rock to obtain the required gradations. The central core was constructed with granular material that was adjusted for water content to achieve the proper level of saturation. The construction was performed in winter which enabled the introduction of cold temperatures in the dam, thus establishing the frozen



condition as the dam was built. Thermosyphons were installed at some locations to target potential taliks or as a preventive measure against possible thermal impact to the warm permafrost that was present at those sites.

The construction of the dams is scheduled for winter to benefit from the cold climate, thus achieving the coldest conditions possible. The winter construction is the most economical approach to create a frozen core. It also provides a safeguard against introducing heat into the foundation material and impacting the thermal regime of the existing permafrost.

2.11 Meteorological Data

Some climatic data was collected at the Doris North and the Boston Camp sites during exploration (60 km south of the Doris North Project site). The local climatic data was complemented using three regional weather stations operated by Environment Canada, namely Lupin, Ikaluktutiak (Cambridge Bay) and Kugluktuk (Coppermine) (AMEC 2003a, b). The climatic data collected at the Doris North Project and Boston Camp sites was then used to develop correlations for the Doris North site using Environment Canada weather stations. The correlated average ambient temperature from the Environment Canada weather stations over a 30-year period (1974 to 2003 inclusive) is summarised in Table 1. It also shows temperature values representative of extreme cold and warm conditions. These extreme values are based on the three coldest and warmest annual ambient temperatures over the 30-year period.

Table 1: Correlated ambient temperature, average, cold and warm values

30 year average

Cold Condition (3 coldest years)

Warm Condition (3 warmest years)

Mean annual air temperature, MAAT (C): -12.0 -13.9 -10.0 Ambient temperature annual amplitude of monthly average (C):

20.2 20.2 19.0

Air freezing index (C-days): -5,105 -5,566 -4,461 Air thawing index (C-days): 754 536 860

The total annual precipitation is in the order of 207 mm, with about 80 mm as rain and 145 mm as snow water equivalent. Wind speed data reported for the Boston area (Rescan 2001) indicates predominant wind directions ranging from northwest to northeast, with wind speed in the order of 5 to 7.5 m/s. Calm conditions (wind speed below 1 m/s) occur about 6 to 9% of the time.

2.12 Climate Change

The Department of Indian and Northern Affairs of Canada (INAC) commissioned a technical report on the Implication of Global Warming and the Precautionary Principle in Northern Mine Design and Closure (BGC 2003). This report highlights the importance of including the impact of climate changes into the design for mine development. The issue of climate change becomes more critical as the geographical location approaches the southern boundary of continuous permafrost. Although the



Doris North Project is well within the zone of continuous permafrost, climate change will impact its thermal regime, as discussed below.

Many climate change studies have indicated that global warming is expected to be most pronounced in the Polar Regions (e.g. Cohen 1997, Houghton et al. 1996; NRCan 2004). Several papers (e.g. Harvey 1982; Hansen et al. 1984; Smith and Burgess 1998; Kettles et al. 1997; Environment Canada 1998; IPCC 1995, 2001) predict that the mean annual ambient air temperature could rise by 2 to 5 C in the region covering the Doris North Project over the next century. Historical ambient temperature data between 1895 and 1991 (Environment Canada 1992) from the MacKenzie District shows the highest overall warming in the country, with an increase of 1.7 C over that 96 year period.

The Intergovernmental Panel on Climate Change (IPCC) concluded that the temperature trends indicate that some global climate change has already occurred (IPCC 1995, 2001). They recognised that global climate change is very difficult to predict and contains considerable uncertainties. Their predictions for the year 2100 estimate a global mean temperature increase between 1.5 C and 4.5 C, with a best estimate of 2.5 C.

The influence of climate change will also be dependent on the latitude of the region. For instance, the Doris North Project, which is situated at a latitude of approximately 67 30, could see a more pronounced impact of climate change in the winter, (relative to the global prediction), and lesser but still noticeable effect in the summer. Assuming the best estimate global temperature increase of 2.5 C, the predictions made by IPCC for the latitude at the Doris North Project translate into a predicted increase of up to 5.8 C in the winter, 4.2 C in the spring and about 1 C in the summer and fall. These increases would raise the mean annual ambient temperature by 3.1 C. If the worst case scenario is assumed (global temperature increase of 4.5 C), the predicted temperature increase would reach 10.1 C during the winter, 7.2 C in the spring and about 2 C in the summer and fall. This scenario would see the mean annual ambient temperatures increase by 5.3 C. The predictions advanced by IPCC show that climate change would eventually modify the thermal regime that currently exists at Doris North. The warming trends described herein are consistent with Burn et al. (2004) which presents the anticipated climate change scenarios for the Mackenzie River Valley.

There is no clear trend based on the historical precipitation data, although most predictions show lower summer precipitations and larger winter precipitations. Smith (1988) indicated that precipitation would probably increase with global warming, thus increasing the probability of higher snow accumulation. The larger snow accumulation will increase the insulation provided by the snow cover, thus less heat losses during the winter months. Goodrich (1982) showed that a hypothetical doubling of snow accumulation from 0.25 to 0.50 m could increase the minimum ground surface temperature by about 7 C and the mean annual surface temperature by 3.5 C. This is consistent with the work presented by Nicholson and Thom (1973), and Nicholson and Granberg (1973). The larger snow accumulations could potentially accelerate the impact of climate change on the permafrost.



The following is an extract from Climate Change, Impact and Adaptation: A Canadian Perspective published by the Climate Change Impacts and Adaptation Directorate, Natural Resources Canada (NRCan 2004, page ix):

A recurring issue in the field of climate change impacts and adaptation is uncertainty. There is uncertainty in climate change projections (degree and rate of change in temperature, precipitation and other climate factors), imperfect understanding of how systems would respond, uncertainty concerning how people would adapt, and difficulties involved in predicting future changes in supply and demand. Given the complexity of these systems, uncertainty is unavoidable, and is especially pronounced at the local and regional levels where many adaptation decisions tend to be made. Nonetheless, there are ways to deal with uncertainty in a risk management context, and most experts agree that present uncertainties do not preclude our ability to initiate adaptation. In all sectors, adaptation has the potential to reduce the magnitude of negative impacts and take advantage of possible benefits. Researchers recommend focusing on actions that enhance our capacity to adapt and improve our understanding of key vulnerabilities. These strategies work best when climate change is integrated into larger decision-making frameworks.

It is therefore important the design incorporates a risk component for climate change, and as indicated above, it should also provide the ability to adapt to the variability introduced by the climate change.

2.13 Subsurface Investigations

Six geotechnical drilling programs have been undertaken at the Doris North Project during 2002, 2003, 2004 and 2005, all of which specifically targeted geotechnical and thermal information at potential dam locations and along the perimeter of Tail Lake. The winter 2002 investigation comprised nine drill holes along three section lines across Tail Lake. The fall 2002 program consisted of five holes at the proposed North Dam location. The winter 2003 and summer 2003 programs involved further drilling at the North (5 holes) and South Dam (6 holes) locations, as well as three deep holes around the Tail Lake perimeter to investigate potential talik development. The summer 2004 program consisted of drilling one hole at the North Dam for the spillway and three more along the perimeter of Tail Lake to assess the shallow thermal regime and slope stability. The 2004 field program also included the installation of a 200 m long thermistor string located in the vicinity of the mill area and Doris Lake. The winter 2005 investigation included 5 boreholes at or adjacent to the North Dam alignment and three additional ones were drilled near the shoreline of Tail Lake. The results from the 2002 and 2003 investigations are compiled in SRK (2003b, 2005a). The 2004 investigation is summarised in SRK (2005d) and the 2005 investigation in SRK (2005e). These two reports are included in Appendices D and E at the end of this report.



The subsurface investigations included the installation of temperature measuring devices (thermistors) installed in selected holes. Nine thermistor strings were installed along the North Dam alignment, four at the South Dam and six along the perimeter of Tail Lake.

2.14 Foundation Conditions

2.14.1 North Dam

The North Dam is located about 200 m downstream of north extremity of Tail Lake as shown in Figure 3. The proposed dam alignment, shown in more detail in Figure 4, is within a relatively narrow valley and is essentially perpendicular to the valley. The valley bottom is about elevation 26 m and consists of a narrow marshy area that drains the flow from Tail Lake. This surface flow then discharges into Doris Lake.

The ground surface has generally a vegetative cover, although some areas are clear of vegetation and show the underlying overburden deposits. Rock outcrops are also visible within the valley but are commonly at higher elevations.

Ground temperature measurements along this dam alignment indicate that permafrost is present over the entire length of the dam, with mean annual ground temperatures ranging between -9 C and -7 C. No talik was encountered under the shallow stream of water that traverses the valley. The geothermal gradient is generally isothermal in the upper 100 m. The ground temperatures measured in the 200 m drillhole near Doris Lake (SRK 2005d) indicate a geothermal gradient of about 11 C km-1. Based on measured surface conditions, the permafrost depth is estimated to be 550 m.

The interpreted stratigraphy at the North Dam that is shown in Figure 6 is characterised by two relatively distinct zones. About two thirds of the dam longitudinal section, on the southwest side, is dominated by a sand deposit with a thickness of 10 to 15 m, which is ice-saturated. The ice-saturated condition was confirmed during winter 2005 investigation. The sand deposit is overlain by a silt and clay deposit that does not exceed about 3 m on the southwest side of the valley. Peat was encountered over a short distance in the middle portion of the dam. The remaining one third portion on the northeast side is dominated by marine clayey silt with a thickness reaching a maximum of about 15 m. This deposit is ice-saturated and contains excess ice. A relatively thin layer of granular soils, primarily sand and gravel was encountered overlying the bedrock surface in the upper portions on both sides of the valley.

The bedrock consists primarily of basalt and is at a maximum depth of about 20 m in the middle of the valley and become shallower towards the crest of the valley. The bedrock condition is considered good at depth based on the recovery and the rock quality designation (RQD) as measured from recovered rock core. There were some zones at the overburden interface where the bedrock quality was poor, but it is reasonable to assume that this weathered zone was saturated with pore ice.

The abutments at the proposed dam will be founded on overburden. Frozen sand forms the foundation material to the southwest, while overburden at the northeast side of the alignment is silt and clay.



Hydraulic conductivity tests conducted within the bedrock formation along this dam alignment show very low values or no flow intake, indicating that the bedrock can be considered impervious, for practical purposes, when frozen. The RQD values were typically high, indicating that the bedrock would remain relatively impervious in the unfrozen state. Regional geology suggests that the Tail Lake Shear Zone intersects immediately south of the proposed dam alignment. While minor fractures were encountered in the recovered drill core, no evidence of the Tail Lake Shear Zone or any major faulting was observed.

Salinity measurements on pore water extracted from soil samples indicate that the frozen pore water in the marine deposit (silt and clay) is saline, with salinity similar or slightly higher to that of sea water (salinity of about 30 to 50 ppt).

2.14.2 South Dam

The South Dam is located about 400 m south of Tail Lake and is located at the watershed boundary that separates Tail Lake and Ogama Lake to the south (see Figure 3). The proposed dam alignment is along a flat valley section that remains above elevation 33 m (see Figure 5). The ground surface along the dam alignment is covered with hummocky vegetation and is well drained. Bedrock outcrops are present on both sides of the valley.

The ground temperature measurements at the South Dam indicate that permafrost is present over the entire dam alignment. The temperature measurements indicate a similar thermal regime to the one present at the North Dam, with mean annual ground temperatures ranging between -9 to -7 C with similar surface temperatures and being isothermal within the top 100 m. It is also reasonable to assume the same geothermal gradient profile exists as for the North Dam.

The stratigraphy along the South Dam alignment is illustrated in Figure 7. It consists of a marine deposit comprised of silt and clay overlying a till deposit. The surficial marine deposit reaches a thickness of about 20 m in the middle of the valley and gradually become thinner towards the valley sides. Zones with silty fine sands were encountered within this upper zone, which may be attributed to lacustrine deposit from the evolution of Tail Lake. As for the North Dam, the fine grained marine deposit is ice-saturated and contains excess ice. The till deposit consists of a fine sand matrix with variable amounts of silts, gravel, cobbles and boulders. Some of the boreholes that encountered this till were limited to recovered coarse particles and did not recover the finer grained matrix. Borehole SRK-43 was the only one that used a polymer based drilling mud and managed to recover intact samples within the till. The recovered samples showed the typical nature of tills, which consisted of a fine grained matrix with unsorted gravel and cobbles. The till deposit, which generally overlays bedrock, has a maximum thickness of about 15 m and is limited to the middle portion of the dam alignment. A shallow pocket of sand and gravel was encountered above the bedrock surface at the east abutment.

The bedrock formation beneath the dam alignment consists primarily of basalt along the west and central portions of the valley and argillite near the east abutment. The surrounding bedrock outcrops suggest that the argillite is probably underlain by basalt. The rock core yielded high RQD values,



thus indicating good rock quality. The hydraulic conductivity tests in the bedrock resulted in very low values, thus supporting the good rock quality and the expectation of ice-saturated pore space.

The abutments at the proposed dam consist of a thin overburden cover or exposed bedrock. Bedrock is expected to contain minor discontinuities, some of which will be contiguous; nevertheless, the bedrock appears competent overall.

Pore water recovered from the marine deposit at the South Dam (borehole SRK-43) had salinities ranging from 30 to 46 ppt. These values encompass or slightly exceed the range for seawater.

2.15 Ground Temperature and Permafrost

The Doris North Project site is underlain by continuous permafrost that has been estimated to extend to depths in order of 550 m (SRK 2005e). This permafrost depth was estimated from a 200 m deep drillhole (SRK-50) where the mean surface temperature is about -6.3 C and the geothermal gradient is 11.4 C km-1. The geothermal gradient in the upper 100 m appears to be isothermal or slightly negative. For comparison, the deep ground temperature profile measured at the Boston Camp also suggested a similar permafrost depth, about 560 m (EBA 1996; Golder 2001). The mean annual surface temperature is however colder at 10 C and the geothermal gradient is higher at 18 C km-1. The difference in the ground temperature profiles at those two sites can be attributed to different surface conditions and the thermal conductivity of the ground at depth. The geothermal gradient measured at the Doris North site is probably representative of the conditions in the vicinity of Tail Lake.

Temperature data collected around Tail Lake indicates that the active layer in the marine clay/silt soils appears to be about 0.5 m, while the sand deposit has an active zone no greater than 2 m. The depth of zero annual amplitude varies between 11 and 17 m. The ground temperatures at the depth of zero annual amplitude are generally in the range of -9 to -7 C. Figure 8 shows the ground temperature and permafrost characteristics present at the site. SRK (2005e), which is included in Appendix E, contains the most recent compilation of the ground temperature data collected by SRK since 2002.

2.16 Freezing Temperature

The foundation material is composed in large part with a marine deposit that is saline. Pore water salinity measurements indicated that the pore water in the marine deposit is generally similar to seawater, with a few measurements that are slightly higher. Seawater has a freezing temperature of about -2 C while the highest salinity measurement (54 ppt) corresponds to a freezing temperature of about -3.1 C. Given the range of values measured, a freezing temperature of -2.5 C probably includes most of the marine soils.

Salinity levels of 4 ppt or less were measured in the sand deposit that was encountered at various locations around Tail Lake (SRK 2005e). The low salinity values indicate that the sand deposit is not



saline. Given the relatively high salinity of the fine grained soils, the sand deposit is treated as a saline soil as a precautionary measure for the purpose of design and thermal modelling.

The salinity of the pore water from rock core was not measured during the investigation. It was considered as saline like the fine grained soils. Both dams will be constructed using crushed rock that will be amended with fresh water. The pore water in this material will therefore freeze near 0 C.

2.17 Unfrozen Water Content

The unfrozen water content is important for fine grained soils, such as the marine deposit. Such soils will have amounts of unfrozen pore water well below the freezing point, usually characterised by the relationship between the unfrozen water content and the temperature. The unfrozen water content was measured on one sample that was recovered at the surface near the North Dam (SRK 2005d). This sample consisted of shallow sandy clayey silts with some organics and had probably low salinity values due to its shallow depths. The results show that about 20 to 30% of the pore water remained unfrozen at temperatures of -12 to -8 C (Figure 9). The unfrozen water content was recently measured on three additional samples following the winter 2005 investigation. Two of these three samples showed much higher unfrozen water contents than the previous measurements. They are however similar to data reported by Hivon and Sego (1995) for saline soils (salinity 30 ppt). The results shows 50% of the pore water will remain unfrozen at -8 C and about 15% will be unfrozen at -20 C. Although a portion of pore water will remain unfrozen at subzero temperatures, the frozen fraction will still be sufficient to reduce the hydraulic conductivity by probably a few orders of magnitude (Newman and Wilson 1997).

The unfrozen water content of the sandy soils has not been measured. Given that sandy soils generally exhibit relatively low unfrozen water content below the freezing temperature, it is reasonable to use data from literature, such as Hivon and Sego (1995).

The unfrozen water content of bedrock was also not measured. Bedrock has a similar behaviour to sand in relation to the unfrozen water content, normally exhibiting low unfrozen water content below the freezing point of the pore fluid. Bedrock often has low to very low porosity values, which further reduce the influence of the unfrozen water content. The unfrozen water content is therefore not critical in bedrock and a relationship similar to sand is considered adequate.

2.18 Strength

Soil strength was not measured during the investigations for this project. Permafrost soils normally have three states for which the strength will vary considerably, which depends whether the soil is frozen or not. The strength will obviously be greater for frozen soils, due to the inter-particle bonding provided by the frozen pore water. The thawing of permafrost soils often induce very low strengths because of the excess pore pressure caused by the thawing of the pore ice and the often unconsolidated nature of permafrost soils. Finally, thawed frozen soils will eventually consolidate over time, thus gradually gaining strength.



The presence of brine within the soil matrix of saline soils generally decreases the strength of the frozen soil, in particular its resistance against creep deformation (long term deformation). The unfrozen water content will also play a role in reducing the strength (Hivon and Sego 1995). Although fine grained saline soils will have lower strengths amongst frozen soils, they still have greater strengths than their unfrozen counterparts. It can be expected that creep deformation will occur during the life of the dams, but the magnitude of the deformation will likely be small and therefore, have minimal impact on the deformation of the dam.

Thawing will occur along the upstream side of the dams, thus creating a talik. This will introduce zones of very low strength during the thawing process, followed by a gain in strength as the excess pore water dissipates and the soils consolidate. The weak zones from the thawing will not sustain the load from the dam and will result in settlements that are generally proportional to the amount of pore ice present in the soil. The rate of settlement will be dependent on the rate at which the talik develops. The talik will likely begin at the toe and gradually progress towards the core of the dam. The design of the dams will therefore have to accommodate possible settlements and longitudinal cracks by using flat slopes. This would however only occur when the talik develops.

2.19 Seismicity

A site specific seismic hazard assessment was done by the Geological Survey of Canada, according to the procedures documented in Adams and Halchuck (2003). Peak ground accelerations and velocities for various annual probabilities of exceedence were determined and are listed in Table 2.

Table 2: Probabilistic seismic ground motion analysis

Annual Probability of Exceedence

Return Period (Years)

Peak Ground Acceleration (g)

Peak Ground Velocity (cm/sec)

0.01 100 0.014 0.033 0.005 200 0.018 0.039

0.0021 475 0.023 0.049 0.0010 1,000 0.028 0.060 0.0004* 2,475 0.059 -

*The 1:2,475 return period data is not site specific to the Doris North Project area, but are for Kugluktuk (Coppermine).

The Doris North Project falls within the stable zone of Canada. This region has too few earthquakes to define reliable seismic source zones. However, international experience suggests that large earthquakes can occur anywhere in Canada, although the probability is very low.

Within this stable zone, the project area falls in acceleration zone 1 (Za = 1) and experiences zonal accelerations of 0.05 g. The velocity zone in which the area falls is zone 0 (Zv = 0) which corresponds to zonal velocities of 0.05 m/s. These zonal classifications are the lowest zones classified on the seismic hazard maps of Canada (Adams and Halchuck 2003).

For design purposes, the 1:475 year earthquake for the site is calculated as having a peak horizontal ground acceleration of 0.023 g, with a peak ground velocity of 0.049 m/s. In conjunction with



proposed changes to the National Building Code of Canada, it is indicated that it would be prudent to evaluate the performance of the structures during an earthquake with a 2,475-year return period. Since the site specific seismic hazard calculation did not have a peak ground acceleration for this return period, we have used the closest available data which is at Kugluktuk (Coppermine), and has a reported peak ground acceleration of 0.059 g.



3 Preliminary Dam Design

3.1 General Layout

The general layout of the tailings impoundment is illustrated in Figure 3. The tailings impoundment consists of two dams to be constructed at both ends of Tail Lake (see Figure 4 for the North Dam layout and Figure 5 for the South Dam layout). A spillway will be constructed immediately east of the North Dam. The invert of the spillway will be constructed at elevation 33.5 m, which correspond to the FSL of the tailings pond. Both dams will rely on permafrost to restrict seepage through the dams.

3.2 Design Criteria

The design criteria for the tailings dams that are presented in the subsequent sections follow the guidelines provided in Dam Safety Guidelines (Canadian Dam Association 1999).

3.2.1 Dam Classification

The dam classification system recommended in the Canadian Dam Association (1999) guidelines is shown in Table 3. For the dams proposed herein, the potential incremental life safety consequence of failure is no fatalities, due to the remote nature of the site and the topography and conditions downstream of the dam. No fatalities corresponds to a very low consequence category.

Table 3: CDA dam classification in terms of consequences of failure Potential Incremental Consequences of Failure[a] Consequence

Category Life Safety[b] Socioeconomic, Financial & Environmental[c] Very High Large number of fatalities Extreme damages

High Some fatalities Large damages Low No fatalities anticipated Moderate damages

Very Low No fatalities Minor damages beyond owners property a) Incremental to the impacts which would occur under the same natural conditions (flood, earthquake or other event)

but without the failure of the dam. The consequence (i.e. loss of life or economic loses) with the higher rating determines which category is assigned to the structure. In the case of tailings dams, consequence categories should be assigned for each stage in the life cycle of the dam.

b) The criteria which define the Consequence Categories should be established between the Owner and the regulatory authorities, consistent with societal expectations. Where regulatory authorities do not exist, or do not provide guidance, the criteria should be set by the owner to be consistent with societal expectations. The criteria may be based on levels of risk which are acceptable or tolerable to society.

c) The Owner may wish to establish separate corporate financial criteria which reflect their ability to absorb or otherwise manage the direct financial loss to their business and their ability to pay for damages to others.



The potential incremental socioeconomic, financial and environmental consequences of failure include environmental impacts associated with the release of contaminated water to the downstream environment and financial impacts related to the cost of mitigation. As indicated by note b, stakeholders need to define these consequence categories to be consistent with societal expectations. For purposes of this assessment, we have assumed that the potential incremental consequences of failure of the new dam would be moderate damages. Our reasoning is that failure of the new dam would release only contaminated water, not tailings. The uncontrolled release of contaminated water would result in limited adverse environmental effects, but more importantly would discourage the use of the area for subsistence hunting and fishing, which would represent an impact on the people that may want to use the area at the time of the uncontrolled release. For instance, the failure of the dam during extreme flood conditions would likely have very low consequences because of the dilution involved. Additionally, if the dams are exposed to an unforeseen earthquake, the consequence will likely be limited to the release of potentially contaminated water.

Based upon the life safety factor and the socioeconomic, financial and environmental aspects, the tailings dams are classified as low in terms of consequences of failure. This classification is based on SRKs best judgement and may differ from the local stakeholders. This needs to be assessed and endorse by the local stakeholders prior to the final design.

3.2.2 Design Earthquake

The CDA guidelines indicate that the minimum criterion for the design earthquake for a dam in the low consequence category would be an earthquake with an annual exceedence probability of 0.01 to 0.001. These probabilities represent return periods of 100 and 1,000 years, respectively. The Geological Survey of Canada indicated that the 1,000 year event has a peak ground acceleration (PGA) of 0.028 g. However, in relation to upcoming changes to the National Building Code of Canada, Natural Resources Canada has been suggesting that it would be prudent for designers to evaluate the performance of the structures during an earthquake with a 2,475-year return period. Although no estimates are available for the Doris North site for that return period, a peak ground acceleration of 0.06 g was estimated for a 2,475 year return period earthquake based on Kugluktuk data.

3.2.3 Design Capacity

A detailed water balance for Tail Lake is documented in SRK (2005c). This water balance was used in conjunction with the water quality model (SRK 2005c) to determine an appropriate design capacity for Tail Lake. An iterative procedure was followed, where storage capacity was balanced with decant requirements, such that a robust water management design could be implemented that would account for all foreseeable uncertainties and upset conditions, whilst keeping the dams as small as practical for reasons of minimising potential shoreline erosion concerns as well as keeping the construction costs down. Based on this evaluation a FSL of 33.5 m was selected as the design capacity. At this capacity, all proposed water management strategies would be able to function



effectively, whilst still allowing a significant but reasonable margin of safety as deemed appropriate by MHBL.

3.2.4 Design Freeboard

The tailings dams have been designed with both North and South Dams at a final crest elevation of 37.0 m. The frozen core of the dams, including the geosynthetic clay liners (GCL) will terminate at an elevation of 34.5 m. The FSL in Tail Lake and the operational spillway will be at 33.5 m. This allows for 3.5 m of freeboard, which constitutes 1.0 m for the core against potential spillway blockages, as well as 2.5 m of thermal protection on top of the frozen core.

The design freeboard requirement for wind induced waves was calculated as 0.3 m. Complete details of this calculation are presented in Appendix F. Considering the peak flood depth in the spillway will be about 0.2 m (see next section), this means a total hydraulic freeboard requirement of 0.5 m, which half of the 1.0 m that has been allowed for.

3.2.5 Design Flood

The CDA guidelines indicates that the minimum criterion for the inflow design flood (IDF) for a dam which coincides with the low consequence category would be a flood with an annual exceedence probability of 0.01 (100-year return period) to 0.001 (1,000-year return period).

The freeboard requirement for the Dams is 3.5 m, and far exceeds the requirements based on any possible flood estimates. Therefore, it was deemed appropriate to size the spillway for a 24-hour storm event with a return period of 1:500 years. For the preliminary design calculations, no attenuation of the flood peak within Tail Lake has been accounted for, and it was assumed that 100% of the precipitation event in the catchments will flow over the spillway. Even under this scenario, it is inconceivable that the dams could overtop, even if a complete blockage of the spillway was to occur. The design depth of the water over the spillway crest when it is passing the 1:500 year, 24-hour duration flood would be approximately 0.17 m.

3.2.6 Stability

The current stability requirements for earth and rock fill dams, advocated by the International Committee on Large Dams (ICOLD) and the Canadian Dam Association (1999), were adopted for preliminary design of the North and South Dams. These requirements are summarized in Table 4. As indicated in this table, the case of rapid drawdown conditions was not examined. Rapid drawdown conditions were considered unlikely for the facility because the dam materials on the upstream face of the dams are coarse; the upstream slope is limited to 6H:1V and the core is frozen.



Table 4: Minimum factors of safety

Loading Condition Minimum Factor of Safety Slope

Steady state seepage with maximum storage pool 1.5 Downstream

Full or partial rapid drawdown Not applicable Upstream End of construction before reservoir filling 1.3 Downstream and Upstream Earthquake (pseudo-static) 1.1 Downstream

3.2.7 Seepage

Seepage through the dam or foundation will not be present because both dams are designed as frozen core dams constructed on frozen foundations. The seepage of properly constructed frozen core dams will be at the lower bound for all types of dams and may, in some cases, approach zero.

3.2.8 Freezing Temperature

As a measure to introduce a margin of safety in the design, the interpretation of the frozen conditions was reduced by 2 C. Such reduction corresponds to a freezing temperature of -2 C for the dam material and -5 C for the natural soils and the bedrock. The freezing temperature of the marine deposit was further reduced to -6 C to account for its salinity and unfrozen water content.

3.2.9 Construction Temperature

The dams will be constructed during the winter months to enable the freezing of the core and to introduce cold temperatures into the dam materials as they are being placed. The placement of the dam materials will therefore require that the ambient air temperature is colder than -15 C. It is important that the snow accumulation be cleared from the dam working surfaces as it being constructed to maximise heat loss.

3.2.10 Climatic Data and Climate Change

This preliminary dam design is based on the 30 year average climate (MAAT of -12.0 C with an amplitude of -20.2 C) as initial condition. The 30 year average climate is then adjusted over time to reflect the impact of climate change. The daily temperatures were increased at a rate of 0.1 C per year over half of the year to reflect the winter increase, and by 0.03 C per year for the remaining half during the warmer period (see Section 2.12). This warming trend equates to increase the MAAT by 6.5 C over the next century, which is slightly higher than 5.3 C mentioned in Section 2.12. The difference is due to a simplification in the calculation: the above value is calculated using two six months periods instead of quarterly periods based on seasons. This simplification, which introduces slightly warmer conditions, was selected essentially for the thermal modelling presented later.



3.3 Upset Conditions

Upset events are incorporated in this preliminary design, essentially to assess the robustness of the design that is proposed. These upset events are described in the following sections.

3.3.1 Warm Climate

Climate change could potentially initiate exceptionally warm years which could impact the performance of the frozen core dams. An exceptional warm climate scenario was incorporated for this design. The warm condition listed in Table 1, which is based on the warmest three years over the last 30 years, was considered as being the average climate when the dam is constructed. The initial warm climate then incorporates a gradual temperature rise to reflect the impact of climate change. The same approach as described in the previous section was applied for this warm climate scenario: an increase of 0.1 C per year during the cold half of the year and 0.03 C for the remaining part of the year.

A statistical analysis performed on the 30 year climate data indicate that the above warm climate combined with the climate change effects would exceed the 1/100 Annual Exceedence Probability (AEP) value in about 12 years, and the 1/1000 AEP in about 25 years.

3.3.2 Over-Topping

Over-topping of the dam could introduce heat to the dam, especially if this event occurs during the late part of the summer. Overtopping will also expose the dam to erosion, which is addressed with proper material selection and transition zones within the dam.

The storage volume in Tail Lake between 33.5 m and 34.5 m is in excess of 1 million cubic meters. Assuming 100% of the 62.8 mm, 1:500 year, 24-hour precipitation event fall on the 450 ha Tail Lake catchment, that only accounts for approximately 300,000 cubic meters, or less than 0.3 m rise in the water level. Conversely, a 24-hour precipitation event in excess of 220 mm would have to occur to allow the water level in Tail Lake to rise 1.0 m, which is more than 3.5 times the magnitude of the design event. Therefore, given the storage capacity of Tail Lake and the magnitude of the precipitation at the site, it is inconceivable that the water level in Tail Lake can rise from 33.5 m to 34.5 m during a single peak storm event, even if a complete blockage of the spillway occurred. Additionally, elevation 34.5 m corresponds to the crest of the frozen core inside the dam, while the dam crest is at elevation 37.0 m. A rise of 1 m above the FSL would still remain within the overall freeboard of the dam. This situation was therefore considered unrealistic and overtopping was not included as a possible upset condition.

3.3.3 Extended Duration of Storage

The proposed water balance for the tailings impoundment indicates that, in the worst case scenario, the dams may have to retain water for a period of 22 years. The adopted design criteria were therefore to design the dam to function for at least 25 years. Although it is very unlikely, the storage period was extended to 40 years for the assessment of upset conditions.



3.4 Settlement

The dams will be subject to settlement, primarily from the thawing of the frozen ground below the upstream and downstream shells of the dams. The central portion of the dam, where the impervious core is located, will not settle since the foundation and the core are to remain frozen during the entire operating life of the dams. Because the core and the foundation will perform as a rigid block, the potential for crest settlement is practically nil. Therefore, there is no requirement to include a higher height of the core to compensate for settlement. The height of the core is therefore controlled by the hydraulic and thermal freeboard components of the dams.

Settlement at the dam toe (upstream and downstream) will be dependent on the state of consolidation of the underlying soils but will be essentially controlled by the amount of excess ice present in the soil formation. The soil will subside by the amount of excess ice once the talik is fully developed. Excess ice mentioned herein corresponds to the volumetric portion of ice that is in excess to the pore volume of a soil when unfrozen.

The subsurface investigations encountered soils with excess ice. It was confirmed with the fine-grained marine deposit, although the coarser sand deposit is also expected to contain excess ice. Based on visual inspections of the soil samples at the time of recovery, the silty soils contain most of the visible ice. The clayey and sandy soils contain visible ice, but to a lesser extent and mostly intermittently. The gravimetric water content measured in the marine soils ranged from 30.4% to 146% (the second highest value is 82.2%) over 24 samples (see Table 5.1 of SRK 2003b), for an average of 52% (excluding the highest value). The marine soils without excess ice will have lower water contents and a value of 33.5% (second lowest value) was assumed for settlement estimations. Although these average values may represent average conditions over the entire length of the dam, it is important to note the variability in the water content, which suggests that there is a potential of having localised zones of higher ice content.

The maximum thickness of the marine deposit varies from about 20 m at the North Dam to 30 m at the South Dam. It is therefore reasonable to assume that 50% of the marine deposit contains excess ice based on field observations. The thickness of soil with excess ice would therefore be in the order of 10 to 15 m.

Using the method based on the ratio of bulk densities of the frozen and unfrozen soils, the conditions at the North and South Dams suggest that the talik could potentially induce settlements up to 2.5 m at the North Dam and to 3.7 m at the South Dam. As indicated above, there is also a possibility of having localised pockets where the settlement could exceed these values. These potential settlements will only affect the upstream portion of the dam because the core and foundation will be designed to remain frozen for the entire design life of the dam. It is therefore important that the upstream slopes of the dams be flat enough to compensate for the settlement that will eventually develop over time. However, in reality the magnitude of the talik will be limited since the dam will be operated for a finite time (i.e. 25 years).



In addition to the above potential differential settlement on the upstream side, the North Dam also has the potential of being subject to differential settlement along the longitudinal axis of the core if the foundation is allowed to thaw. This is due to the discontinuity in the overburden between the sand dominated foundation and the ice-rich clay-silt foundation.

The development of the talik along the upstream side of the dams will introduce weak zones that will potentially induce large deformation. Deformation analysis using models based on visco-elastic constitutive relationships will be undertaken at final design to quantify settlement. A model such as the FLAC model will likely be used because of its capacity to accommodate large displacements and strains, non-linear material behaviour, thermally induced deformation and creep deformation.

The settlement can be mitigated by selecting sufficiently flat slopes for the dam sections and by providing proper maintenance and care to the dams during their operations. Differential settlement would still occur along the upstream face of the dams caused by the retained water, but the central frozen zone would maintain the integrity of the dam against leakage and deformation. Depressions caused by settlement will require backfill and proper grading. The layout of the dams will therefore have to accommodate for potential large deformation caused by the ingress of the talik against the dam. As mentioned earlier, since it is a design requirement to keep the core and foundation fozen, the crest core of the dam will be subject to practically no deformation.

Creep deformation is another possible source of settlement, given that the foundation contains saline pore water. The rate of deformation will however be low to very low and mitigation measures can easily be implemented following annual dam inspections. Also, given the duration for which the dam will be required to retain water (maximum of 25 years), the magnitude of the creep settlement will be much less than the potential settlement caused by the talik along the upstream face of the dams.

3.5 Dam Section

3.5.1 Justification

The core of the dams will be processed crushed rock, and a synthetic liner will placed against the upstream face of the core as a secondary containment measure. The entire dam section will be constructed with crushed rock of various gradations. This type of dam configuration eliminates the uncertainties and concerns raised during the review process with the potential use of the natural fine grained material as the frozen core material (SRK 2003b). Those uncertainties and concerns are eliminated because there is no need to develop a borrow source, there is no risk of suspended solids that could potentially originate from the borrow source area, it eliminates the potential variability associated with natural borrow sources, and finally, there are precedents of frozen core dams using crushed rock similar to the configuration proposed herein (EBA 1998; 2003).

SRK Consulting (Canada) Inc. Preliminary Tailings Dam Design, Doris Nor

Revised Dam Design

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