Geomorphological, Geotechnical and Geothermal Conditions at Diavik Mines

Geomorphological, geotechnical and geothermal conditions at Diavik Mines

X. Hu & I. Holubec SNC-LAVALIN Engineers & Constructors Inc., Toronto Ontario, Canada J. Wonnacott, R. Lock & R. Olive Diavik Diamond Mines Inc., Yellowknife, Northwest Territories, Canada

ABSTRACT: Three water retention dams were constructed and two dams for storage of processed ore are under construction on difficult and variable permafrost terrain to store dredged sediments/water from an adjacent lake and processed ore from the mine process plant. Dam construction in the Arctic requires detailed geotechnical and geothermal site conditions, which were determined by geotechnical drilling, surficial terrain mapping and close field inspection during construction. Soil conditions varied from very ice-rich sandy silt and massive ground ice to esker deposition, ice-poor silty sand and boulder zones. The terrain features included sorted and non-sorted polygons, ice- wedges, poorly drained hummocky wetlands, well and poorly drained tundra lands, palsas and massive boulder fields. The entire project site is on permafrost, except below several small on-land lakes and streams, where taliks exist. Thermistor cables were installed at numerous locations to determine the ground thermal regime and it was found that these varied with the location, vegetation and soil conditions and distance relative to water bodies.

1 INTRODUCTION complex ground conditions. This paper discusses the general geomorphologic, geotechnical and geothermal 1.1 Location conditions that are involved with the design and construction of these dams. The Diavik Diamond Mines Project is situated just north of the tree line, approximately 320 km northeast 1.2 Climate of Yellowknife, Northwest Territories, Canada. The site is located in the continuous permafrost zone (Brown The project site lies within the Arctic Climatic Region, 1970, Johnston 1981), 200 km south of the Arctic circle. where summers are generally short and cool, and Figure 1 shows the site location. winters are long and extremely cold. The Diavik site is situated on East Island in Lac de 2 The mean annual air temperature is about 12°C, Gras, which covers an area of about 20 km and has an with a maximum monthly temperature of about 10°C in undulating topography with the highest point being July and a minimum monthly temperature of 35°Cin about 35 m above the Lac de Gras mean water level. January. The estimated monthly air temperatures are Three water retention dams were constructed (Holubec summarized in Table 1. The daily air temperatures et al. 2003) in complicated terrain and two large pro- measured between January 1999 and December 2000 cessed ore storage retention dams are currently under are presented in Figure 2. The mean annual thawing construction. The foundations of these dams involve index is ϳ1100 degree-days, with a mean annual freezing index of ϳ5000 degree-days. The mean annual precipitation is ϳ374 mm, about

Umingmaktok 40% falls as rain during summer months. Snow may Kugluktuk occur in any month of the year, however, the snow cover

Bathurst Inlet ARCTIC CIRCLE exists for about seven months, between October and NUNAVUT Lupin Mine Horman TERRITORY April. During the winter, most of the snowfall is blown Wells Deline GREAT BEAR LAKE Diavik Diamonds Project into hollows and depressions, leaving much of the higher BHP Tulita MACKENZIE RI ground exposed. Even though the summer is short, the Lac de Gras Wekweti Gameti evaporation is high, due to the low relative humidity of VER NORTHWEST the air and windy conditions. For small ponds on the Wrigley TERRITORIES Wha Ti Rae island, the estimated evaporation rate averages about Edzo Dettah N'dho Lutselk'e 315 mm per year and 275 mm per year for Lac de Gras. Fort Jean Marie River Simpson Extreme winds, which for the 1:100 year one-hour Fort Providence Fort YUKON Nahanni Butte Resolution 0 100 200 duration exceed 90 km/hr, can occur in most direc- TERRITORY Hay kilometres Fort Enterprise River Liard Trout Lake Fort Smith tions. The northerly location results in daylight hours ranging from a minimum of ϳ4 hours/day in winter to Figure 1. Location of Diavik Diamond Mines. a maximum of 20 hours/day in summer.

437 Table 1. Average monthly air temperature at Lac de Gras. Air temperature (°C) Month Minimum Mean Maximum Jan 34.8 31.2 27.5 Feb 33.8 30.1 26.4 Mar 31.1 26.7 22.2 Apr 21.9 17.0 12.0 May 9.9 5.8 1.6 Jun 0.6 3.2 6.8 Jul 5.2 8.2 10.1 Aug 4.9 6.8 9.5 Figure 3. Surface conditions in the valley bottom with Sep 1.0 1.7 4.4 palsas and ice-wedge polygons. Oct 10.8 8.1 5.5 Nov 24.7 21.1 17.4 Dec 30.9 27.3 23.7 (b) most hydrological activities being confined within Average 15.7 12.3 8.8 the active zone; and (c) most of the hydrological processes becoming dormant during the winter. The main hydrological process starts with snowmelt. At the 25 Diavik site, melt can occur as early as the last week of 15 April. However, the melted water is usually refrozen 5 at the base of snow pack, therefore little runoff occurs -5 -15 at that time. Snowmelt runoff usually starts around -25 early to mid May, depending upon the temperature.

Temperature ( º C) -35 However, as soon as the melt becomes the dominating -45 process, surface runoff is relatively rapid. About 85% of snowmelt water can runoff within two weeks and usually peaks around the 7–10 days, resulting in large 01/10/99 03/10/99 05/10/99 07/10/99 09/10/99 11/10/99 01/10/00 03/10/00 05/10/00 07/10/00 09/10/00 11/10/00 Date (MM/DD/YY) flows over the frozen ground. The ground surface in the valley bottoms is usually Figure 2. Daily air temperature variation (1999–2000). wet in the summer because drainage is impeded by permafrost and low relief. Generally, surface drainage 2 GEOMORPHOLOGICAL CONDITIONS systems are poorly developed and water movements occur only above and within the active zone. Seepage 2.1 Vegetation through the active zone occurs seasonally under low gradients and is retarded by the relatively low per- The area is located north of the tree line. The vegeta- meability materials. After the snowmelt season, very tion found in the study area is typical for arctic tundra little surface runoff occurs for the remainder of the (Bliss 1978). In the upland areas, the soil surface is summer, as rainfall is lost to infiltration, storage in the covered with a thin layer of lichen and mosses, due to tundra vegetation and in local depressions and high the well-drained soil conditions. Occasionally, small evapotranspiration. Virtually the entire runoff flow in shrubs can be found in local depressions. The soil is the creeks occurs between May and October. poorly drained in valley and low ground areas. The The runoff runs through many inland lakes and dis- vegetation is therefore of typical arctic wetland type, charges into Lac de Gras. The water level in Lac de Gras consisting of coarse grasses, mosses and peat (Fig. 3). has a very small variation and generally is between Vegetation can be quite lush along drainage courses, 415 and 416 m above sea level. where willows can be as tall as 2 m in height. Taliks During the winter, the small inland lakes can freeze can exist in such areas, and bedrock is badly weath- to 2 m deep or more and the very small ones are com- ered and broken. Seepage rates through this broken pletely frozen. However, Lac de Gras usually freezes bedrock zone could be very high should it remain in to a depth of between 1.0 and 1.5 m due to the heat an unfrozen condition. contribution from the large water body.

2.2 Hydrology 3 GEOTECHNICAL CONDITIONS

Hydrological processes are similar to other Arctic 3.1 General regions (Dingman 1975, Woo 1986). The dominating characteristics include: (a) the permafrost acting as Lac de Gras lies within the Bear Slave Upland physio- an aquiclude due to the negligible permeability; graphic region of the Canadian Shield, characterized

438 by a treeless landscape with low relief. There are innumerable water-filled hollows in the bedrock, sur- rounded by low hills. The northern half of the island is covered predominantly by silty sand till deposits and the southern half is mostly exposed granitic bedrock with minor till deposits.

3.2 Bedrock geology East Island is underlain by three main lithological units, namely: 1) greywacke-mudstone metaturbidites (metasedimentary rocks), 2) biotite-hornblende tonalite Figure 4. Boulder field and stone circles. to quartz diorite (diorite), and 3) 2-mica granite and suite (granite to granodiorite). The granite and granodiorite rocks are concentrated in the northern area of East till. The till particle sizes vary from silts with some clay- Island, the metasedimentary rocks are located within an sized particles, to well-graded silts, sands and gravels east-west running central zone of the island, and the with cobbles and boulders. diorites are situated in the southern area of East Island. The surface of the till has been greatly reworked by The metasedimentary rocks belong to the solifluction, annual freeze thaw cycles and cold tem- Yellowknife Supergroup (Kjarsgaard & Wyllie 1994) peratures. This has produced typical periglacial features and are comprised of thinly-bedded metagreywacke including concentrated boulder fields, solifluction to locally thick-bedded porphyroblastic schists. The areas, frozen mounds and patterned ground (Figs 3 & 4). porphyroblasts are composed of biotite, cordierite and These processes have also produced a wide range of andalusite with a variable percentage of garnet. At the material types on the island. Generally, the variable surface, this unit is typically weathered with a dark distribution of the periglacial deposits and soils with grey to green-brown or rusty brown colour. high ice contents (ice-rich soils) are of concern for The direction of the foliation trends east-west on dam design and construction. East Island and is steeply dipping to sub-vertical. Large areas of East Island have rock outcrops and Continuous, well-developed, widely spaced jointing local deposits of ablation till. An intermittent esker within the metasedimentary rock units can result in deposit spans the northern portion of East Island in a distinctive concentrations of frost-jacking and shat- general east-west direction. tered boulders on top of a frost-dilated and jacked bedrock surface. Only minor occurrences of frost jack- 3.4 Soil stratigraphy ing were observed at the Diavik project site. To ensure that the dams are designed for this extreme The granitic rocks are light grey, fine to pegmatitic climate and soil conditions, extensive geotechnical and are comprised of biotite, muscovite (5 to 10%) and drilling programs were carried out during the winter equal portions of quartz, plagioclase and potassium and summer. Chilled brine was used to obtain contin- feldspar. Accessory minerals include apatite, tourma- uous frozen cores. Core samples were selected from line and garnet. varying depths to analyze soil gradations, moisture The diorites are generally massive and homogeneous, contents, bedrock quality and fracture distribution. with only weak foliation present. The diorites are com- For the Diavik site, the field experiences have prised of equal portions of biotite and hornblende shown that when the soil moisture content (W /W ) is (approximately 35%) with up to 10% quartz. Pegmatite w s higher than 26%, the soil contains a significant dykes form an important part of the diorite suite of amount of ice, and is thus classified as ice-rich soil. rocks. It is estimated that pegmatite dykes comprise Ice-rich soil is usually correlated with a large percent- 20–25% of the diorites. age of fine soil particles, between 60 and 98%. Several faults have been identified on East Island. On the upland areas, the surface is either exposed The major fault of interest is the Double Bay fault bedrock or covered with a thin layer of tundra soil which is located within the metasedimentary rocks and comprised of thin vegetation and silty sand with some runs generally east-west through the central portion of gravel and boulders. This soil is normally ice-poor. the island. This fault is associated with a depression Table 2 illustrates an example of the relatively ice- that is being used for the processed ore storage facility. poor soil stratigraphy. The total overburden thickness 3.3 Surficial geology was 7.6 m and ice-rich soil only occurred around the depth of 3.0 m for this borehole. The surficial soils of the island are glacially derived and In the valley bottoms, the soil is usually ice-rich and consist predominantly of ablation till or glaciofluvial rests on top of a layer of ice-poor basal till. Extensive

439 Table 2. Density, moisture content and material gradations for an ice-poor soil. Sample location Gradation From (m) To (m) Dry density (kg/m3) Moisture content (%) Clay & silt (%) Sand (%) Gravel (%) 0.35 0.56 1502.0 22.3 43.0 41.0 16.0 0.88 1.02 14.3 2.22 2.39 1916.0 13.7 16.0 25.0 59.0 3.07 3.18 29.5 4.14 4.42 1920.0 13.6 14.0 79.0 7.0 5.16 5.33 18.4 6.30 6.53 1944.0 21.9 21.0 77.0 2.0 7.28 7.46 9.9

Table 3. Density, moisture content and material gradations for an ice-rich soil. Sample location Gradation From (m) To (m) Dry density (kg/m3) Moisture content (%) Clay & silt (%) Sand (%) Gravel (%) 0.30 0.55 1174.0 40.5 92.0 7.0 1.0 1.42 1.86 202.3 3.35 3.55 141.8 4.21 4.50 818.0 76.2 96.0 4.0 0.0 5.44 5.77 114.6 6.40 6.60 60.8 7.15 7.37 1340.0 34.0 93.0 6.0 1.0 7.72 7.92 38.3 8.33 8.58 14.4 9.15 9.40 1734.0 17.8 33.0 64.0 3.0 9.77 10.1 13.7 segregation ice formation can be identified and palsas and ice-wedges are common phenomena. Table 3 provides an example of the soil stratigraphy in a very ice-rich area. The total overburden thickness was 10.1 m. The surface was covered with a layer of peat, followed by 5.6 m of silts. A 0.9 m thick layer of stratified ice and silt occurred between depths of 5.7 m and 6.6 m. It was followed by another layer of silt and eventually, at depth of 8.1 m, the soil became ice-poor sandy till. The high ice content is usually associated with a high content of clay and silt sized soil particles, as illustrated in Tables 2 & 3. When sand and gravel con- Figure 5. Buried glacier ice resting on bedrock and buried tents increase, the ice content decreases. by sand and gravels in an esker.

The ice had a maximum thickness of about 2.0 m and 3.5 Massive ground ice spanned a length of about 30 m. This ice was white in color with numerous randomly distributed air bubbles. Massive ground ice and ice-rich soils are major con- It rested immediately above the bedrock surface and cerns in dam designs in permafrost areas. Com- was buried beneath 8–12 m of gravelly sand. This ice paratively, the occurrence of massive ground ice in dam was identified as buried glacier ice that was covered by foundations represents a more risky situation. Ground sands during the development of the esker and pre- ice can develop in several forms (Mackay 1989). Three served from subsequent thaw by the cover (Fig. 5). types of ground ice were found on the Diavik site, Buried glacier ice was usually located at the base of including: (a) buried glacier ice, (b) thick segregation upper or middle sections of the esker deposit. At the ice lenses and ice cored frost mounds (palsas), (c) ice- end of an esker, the chance of finding buried glacier wedge ice. ice becomes slim. However, because of the fine nature Massive ground ice was found in an esker deposit of the deposited material, segregation ice lense forma- during the foundation construction of one of the dams. tion is inevitable.

440 and monitoring of thermistor cables at depths varying from 0 to 150 m. These thermistor cables were installed at locations that included high ground, valley bottom, in-land lakes, small islands within Lac de Gras and the shoreline of Lac de Gras. Generally, the mean annual ground temperatures at a depth of about 20 m vary from 3°C to 6°C. The active layer is about 1.5 to 2.5 m deep in till deposits, 2 to 3 m in well-drained granular deposits and about 5 m in bedrock. In poorly-drained areas, including bogs, with thicker vegetation cover, the active zone Figure 6. Segregation ice formation in fine soils. is less than 1 m in depth. Taliks can be found under the drainage channels and underneath the inland lakes. Ground temperature measurements in a small lake on site, with an average depth of 3 m, indicated that a talik was 70 m in depth.

4.2 Ground temperature regime

Ground temperature regimes are controlled by many factors (Gold & Lachenbruch 1973). However, the most important ones, beside climatic conditions, are found to be surface vegetation, snow cover, thermal Figure 7. Ice-wedges found in dam foundation. properties of soils and relative distance to the large water bodies on the Diavik site. The surface is covered Segregation ice formation was found in all the peat with a thin layer of mosses overlying gravel and boul- covered fine soils. Ice lenses varied in thickness from ders. This is followed by gravelly sand till to a depth 0.1 to 2.0 m, with thickness dependent on uniformity of 4.6 m. The distance between the borehole and Lac of the soil conditions (Fig. 6). de Gras is 150 m. Palsas are found in valley bottoms where dams Three examples are presented in this section for were constructed (Fig. 3). These palsas were less than different locations, surface cover and soil conditions. 2 m in height. However, they were distributed over These thermistor cables were installed to a depth of a wide area and were often bordered with extensive 30 m. Thermistor beads were located at depths of 0.25, ice-wedges, about 2–4 m in width. The ice-wedges 0.75, 1.25, 1.75, 2.25, 3.0, 4.0, 6.0, 8.0, 10.0, 12.0, usually reach about 4–5 m in depth (Fig. 7). 14.5, 17.0, 20.0, 25.0 and 30.0 m. With the exception of buried glacier ice, the other Figure 8 provides an example of the ground tem- types of ground ice are typical of those found in other perature variation in a wetland area. The area was flat areas (French & Pollard 1986). These ices display a and snow cover was thin, less than 30 cm. The bore- wide range of colour, from cloudy to grey and brown, hole was about 470 m from Lac de Gras. The soil due to the inclusion of sediment and organics. Bands stratigraphy was as follows: 0.3 m of peat overlying a of silt and organic inclusions were found in horizon- 1.2 m thick layer of silt, and followed by a 1.6 m thick tal, inclined and vertical directions. layer of fine grained sand and silt to a depth of 3.1 m. Numerous drill holes confirmed that the ice-rich Bedrock consisted mainly of good quality granite. soil was usually located in the top 5 m of the soil on Segregation ice lenses with a maximum thickness of the Diavik site. A layer of basal till below the ice-rich 66 cm, were observed. soil contained a large amount of sand, cobbles and The temperature data shows that the active layer is boulders. The clay and silt sized soil particle compo- less than 1.0 m. The ground temperatures fluctuate nent reduced to less than 50% of total weight. annually over a range of about 26°C at a 0.25 m depth, and the fluctuation reduces to 0.3°C at a depth of 20 m. The temperature becomes constant at 5.6°C at 4 GEOTHERMAL CONDITIONS depth of 30 m. Figure 9 provides the ground temperature variation 4.1 Permafrost in an ice-poor moraine to a depth of 30 m. The temperature data shows that the active layer is 3.0 m deep. Ground temperature conditions and permafrost depths Ground temperatures fluctuate annually over a range have been investigated at the site through the installation of about 37°C at a depth of 0.25 m. This fluctuation

441 0 5 CONCLUSIONS

-5 99-04-15 99-06-15 -10 99-08-15 Water retention and processed ore storage dams were 99-10-15 constructed for the Diavik Project. Extensive pre- -15 99-12-15 00-02-15 construction investigations were carried out for the

Depth (m) -20 00-04-15 00-06-15 design and planning of the construction. The soil condi- -25 00-08-15 00-10-15 tions were determined by geotechnical drilling, ground -30 penetration radar survey, surficial terrain mapping and -11 -9 -7 -5 -3 -1 1 3 5 7 9 11 close field inspection during construction. The soil con- Temperature (ºC) ditions varied from very ice-rich sandy silt and massive Figure 8. Ground temperature variations in a very ice- ground ice to esker deposition, ice-poor silty sand and rich area. boulder zones. The terrain features included sorted and non-sorted polygons, ice-wedges, poorly drained hum- 0 mocky wetlands, well and poorly drained tundra lands, -5 99-04-15 99-06-15 palsas and massive boulder fields. The entire project -10 99-08-15 99-10-15 site is permafrost except below several small on-land

-15 99-12-15 00-02-15 lakes and streams where taliks exist. Thermistor cables

Depth (m) -20 00-04-15 00-06-15 were installed at numerous locations to determine the -25 00-08-15 00-10-15 ground thermal regime. These regimes were found to -30 vary with the location, vegetation, soil conditions and -26 -21 -16 -11 -6 -1 4 9 14 distance relative to water bodies. Temperature (ºC)

Figure 9. Ground temperature variations in an ice-poor soil. REFERENCES

0 Bliss, L.C. 1978. Vegetation and revegetation within permafrost terrain. In Proc. 3rd International Conference -5 99-04-15 99-06-15 on Permafrost. Edmonton, Alberta 2: 31–50. Ottawa: -10 99-08-15 99-10-15 National Research Council of Canada. 99-12-15 00-02-15 -15 Brown, R.J.E. 1970. Permafrost in Canada, its influence on 00-04-15 00-06-15 Northern Development. NRC of Canada, Canadian Depth (m) -20 00-08-15 00-10-15 Building Series 4. University of Toronto Press. -25 Dingman, S.L. 1975. Hydrologic effects of frozen ground.

-30 US Army CRREL Special Report 218. -5 -3 -1 1 3 5 7 9 11 13 15 French, H.M. & Pollard, W.H. 1986. Ground-ice investiga- Temperature (ºC) tions, Klondike District, Yukon Territory. Canadian Figure 10. Ground temperature variations in a talik area. Journal of Earth Sciences 23(4): 550–560. Gold, L.W. & Lachenbruch, A.H. 1973. Thermal conditions in permafrost – a review of North American literature. reduces to 0.3°C at a depth of 20 m, with a constant In Proc. 2nd International Conference on Permafrost, temperature of 3.3°C at a depth of 30 m. North American Contribution: 3–23. Washington Figure 10 shows the ground temperature variations D.C.: Science Press. in a talik below a small creek. The borehole was Holubec, I., Hu, X., Wonnacott, J., Olive, R. & Delarosbil, D. 2003. Design, construction and performance of dams located about 165 m away from the lake. The ground in continuous permafrost. In Proc. 8th International was covered with thick vegetation, mainly willows, up Conference on Permafrost. Zurich, Switzerland. to 2 m in height. The soil conditions were sand and Rotterdam: Balkema. sandy gravel with some cobbles and boulders. The Johnston, G.H. (ed.) 1981. Permafrost engineering design and area traps snow during the winter to more than 2 m in construction. Association Committee on Geotechnical thickness. Research, NRC of Canada. Toronto: John Wiley & Sons. Due to thick snow cover, the ground temperatures at Kjarsgaard, B.A. & Wyllie, R.J.S. 1994. Geology of the the depth of 0.25 m reach a minimum of about 2.0°C Paul Lake Area, Lac de Gras, Lac de Sauvage Region and the frost penetrates only to a depth of 1.25 m in of the Central Slave Province, District of Keewatin, the winter. The soil between depths of 1.25 m and NWT. In Current Research, 1994-C, Geological Survey of Canada. 23–32. 10 m is in a thawed condition all year long. The annual Mackay, J.R. 1989. Massive ice: some field criteria for temperature fluctuation is about 14°C at a depth of identification of ice types. In Current Research, part G, 0.25 m and the fluctuation reduces to 0.3°C at a depth GST Paper 89-IG. 5–11. of 10 m. The temperature becomes constant at a depth Woo, M.K. 1986. Permafrost hydrology in North America. of 15 m and remains at 2.3°C at a depth of 30 m. Atmosphere-Ocean 24(3): 201–234.

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