Geomorphological, geotechnical and geothermal conditions at Diavik Mines

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Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Geomorphological, geotechnical and geothermal conditions at Diavik Mines X. Hu & I.

1 Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN 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, icewedges, 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 1.1 Location The Diavik Diamond Mines Project is situated just north of the tree line, approximately 320 km northeast of Yellowknife, Northwest Territories, Canada. The site is located in the continuous permafrost zone (Brown 1970, Johnston 1981), 200 km south of the Arctic circle. Figure 1 shows the site location. The Diavik site is situated on East Island in Lac de Gras, which covers an area of about 20 km 2 and has an undulating topography with the highest point being about 35 m above the Lac de Gras mean water level. Three water retention dams were constructed (Holubec et al. 2003) in complicated terrain and two large processed ore storage retention dams are currently under construction. The foundations of these dams involve YUKON TERRITORY Figure 1. Horman Wells Deline Tulita MACKENZIE RIVER Wrigley Wha Ti Edzo Rae N'dho Fort Jean Marie River Simpson Nahanni Butte Fort Liard GREAT BEAR LAKE Enterprise Trout Lake Kugluktuk Umingmaktok Bathurst Inlet ARCTIC CIRCLE NUNAVUT Lupin Mine TERRITORY Diavik Diamonds Project BHP Gameti Fort Providence Hay River Wekweti Dettah Fort Resolution Fort Smith Lac de Gras NORTHWEST TERRITORIES Lutselk'e Location of Diavik Diamond Mines kilometres complex ground conditions. This paper discusses the general geomorphologic, geotechnical and geothermal conditions that are involved with the design and construction of these dams. 1.2 Climate The project site lies within the Arctic Climatic Region, where summers are generally short and cool, and winters are long and extremely cold. The mean annual air temperature is about 12 C, with a maximum monthly temperature of about 10 C in July and a minimum monthly temperature of 35 Cin January. The estimated monthly air temperatures are summarized in Table 1. The daily air temperatures measured between January 1999 and December 2000 are presented in Figure 2. The mean annual thawing index is 1100 degree-days, with a mean annual freezing index of 5000 degree-days. The mean annual precipitation is 374 mm, about 40% falls as rain during summer months. Snow may occur in any month of the year, however, the snow cover exists for about seven months, between October and April. During the winter, most of the snowfall is blown into hollows and depressions, leaving much of the higher ground exposed. Even though the summer is short, the evaporation is high, due to the low relative humidity of the air and windy conditions. For small ponds on the island, the estimated evaporation rate averages about 315 mm per year and 275 mm per year for Lac de Gras. Extreme winds, which for the 1:100 year one-hour duration exceed 90 km/hr, can occur in most directions. The northerly location results in daylight hours ranging from a minimum of 4 hours/day in winter to 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.

2 Table 1. Average monthly air temperature at Lac de Gras. Air temperature ( C) Month Minimum Mean Maximum Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average /10/99 03/10/99 05/10/99 07/10/99 2 GEOMORPHOLOGICAL CONDITIONS 2.1 Vegetation The area is located north of the tree line. The vegetation found in the study area is typical for arctic tundra (Bliss 1978). In the upland areas, the soil surface is covered with a thin layer of lichen and mosses, due to the well-drained soil conditions. Occasionally, small shrubs can be found in local depressions. The soil is poorly drained in valley and low ground areas. The vegetation is therefore of typical arctic wetland type, consisting of coarse grasses, mosses and peat (Fig. 3). Vegetation can be quite lush along drainage courses, where willows can be as tall as 2 m in height. Taliks can exist in such areas, and bedrock is badly weathered and broken. Seepage rates through this broken bedrock zone could be very high should it remain in an unfrozen condition. 2.2 Hydrology 09/10/99 11/10/99 01/10/00 03/10/00 05/10/00 07/10/00 Date (MM/DD/YY) 09/10/00 11/10/00 Figure 2. Daily air temperature variation ( ). Hydrological processes are similar to other Arctic regions (Dingman 1975, Woo 1986). The dominating characteristics include: (a) the permafrost acting as an aquiclude due to the negligible permeability; Figure 3. Surface conditions in the valley bottom with palsas and ice-wedge polygons. (b) most hydrological activities being confined within the active zone; and (c) most of the hydrological processes becoming dormant during the winter. The main hydrological process starts with snowmelt. At the Diavik site, melt can occur as early as the last week of April. However, the melted water is usually refrozen at the base of snow pack, therefore little runoff occurs at that time. Snowmelt runoff usually starts around early to mid May, depending upon the temperature. However, as soon as the melt becomes the dominating 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 flows over the frozen ground. The ground surface in the valley bottoms is usually wet in the summer because drainage is impeded by permafrost and low relief. Generally, surface drainage systems are poorly developed and water movements occur only above and within the active zone. Seepage through the active zone occurs seasonally under low gradients and is retarded by the relatively low permeability materials. After the snowmelt season, very little surface runoff occurs for the remainder of the summer, as rainfall is lost to infiltration, storage in the tundra vegetation and in local depressions and high evapotranspiration. Virtually the entire runoff flow in the creeks occurs between May and October. The runoff runs through many inland lakes and discharges into Lac de Gras. The water level in Lac de Gras has a very small variation and generally is between 415 and 416 m above sea level. During the winter, the small inland lakes can freeze to 2 m deep or more and the very small ones are completely frozen. However, Lac de Gras usually freezes to a depth of between 1.0 and 1.5 m due to the heat contribution from the large water body. 3 GEOTECHNICAL CONDITIONS 3.1 General Lac de Gras lies within the Bear Slave Upland physiographic region of the Canadian Shield, characterized 438

3 by a treeless landscape with low relief. There are innumerable water-filled hollows in the bedrock, surrounded 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 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 Island, the metasedimentary rocks are located within an east-west running central zone of the island, and the diorites are situated in the southern area of East Island. The metasedimentary rocks belong to the Yellowknife Supergroup (Kjarsgaard & Wyllie 1994) and are comprised of thinly-bedded metagreywacke to locally thick-bedded porphyroblastic schists. The porphyroblasts are composed of biotite, cordierite and andalusite with a variable percentage of garnet. At the surface, this unit is typically weathered with a dark grey to green-brown or rusty brown colour. The direction of the foliation trends east-west on East Island and is steeply dipping to sub-vertical. Continuous, well-developed, widely spaced jointing within the metasedimentary rock units can result in distinctive concentrations of frost-jacking and shattered boulders on top of a frost-dilated and jacked bedrock surface. Only minor occurrences of frost jacking were observed at the Diavik project site. The granitic rocks are light grey, fine to pegmatitic and are comprised of biotite, muscovite (5 to 10%) and equal portions of quartz, plagioclase and potassium feldspar. Accessory minerals include apatite, tourmaline and garnet. The diorites are generally massive and homogeneous, with only weak foliation present. The diorites are comprised of equal portions of biotite and hornblende (approximately 35%) with up to 10% quartz. Pegmatite dykes form an important part of the diorite suite of rocks. It is estimated that pegmatite dykes comprise 20 25% of the diorites. Several faults have been identified on East Island. The major fault of interest is the Double Bay fault which is located within the metasedimentary rocks and runs generally east-west through the central portion of the island. This fault is associated with a depression that is being used for the processed ore storage facility. 3.3 Surficial geology The surficial soils of the island are glacially derived and consist predominantly of ablation till or glaciofluvial Figure 4. Boulder field and stone circles. till. The till particle sizes vary from silts with some claysized particles, to well-graded silts, sands and gravels with cobbles and boulders. The surface of the till has been greatly reworked by solifluction, annual freeze thaw cycles and cold temperatures. This has produced typical periglacial features including concentrated boulder fields, solifluction areas, frozen mounds and patterned ground (Figs 3 & 4). These processes have also produced a wide range of material types on the island. Generally, the variable distribution of the periglacial deposits and soils with high ice contents (ice-rich soils) are of concern for dam design and construction. Large areas of East Island have rock outcrops and local deposits of ablation till. An intermittent esker deposit spans the northern portion of East Island in a general east-west direction. 3.4 Soil stratigraphy To ensure that the dams are designed for this extreme climate and soil conditions, extensive geotechnical drilling programs were carried out during the winter and summer. Chilled brine was used to obtain continuous frozen cores. Core samples were selected from varying depths to analyze soil gradations, moisture contents, bedrock quality and fracture distribution. For the Diavik site, the field experiences have shown that when the soil moisture content (W w /W s ) is higher than 26%, the soil contains a significant amount of ice, and is thus classified as ice-rich soil. Ice-rich soil is usually correlated with a large percentage of fine soil particles, between 60 and 98%. On the upland areas, the surface is either exposed bedrock or covered with a thin layer of tundra soil comprised of thin vegetation and silty sand with some gravel and boulders. This soil is normally ice-poor. Table 2 illustrates an example of the relatively icepoor soil stratigraphy. The total overburden thickness was 7.6 m and ice-rich soil only occurred around the depth of 3.0 m for this borehole. In the valley bottoms, the soil is usually ice-rich and rests on top of a layer of ice-poor basal till. Extensive 439

4 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 (%) Table 3. Density, moisture content and material gradations for an ice-rich soil. Sample location Gradation From (m) To (m) Dry density (kg/m 3 ) Moisture content (%) Clay & silt (%) Sand (%) Gravel (%) 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 contents increase, the ice content decreases. 3.5 Massive ground ice Massive ground ice and ice-rich soils are major concerns in dam designs in permafrost areas. Comparatively, the occurrence of massive ground ice in dam foundations represents a more risky situation. Ground ice can develop in several forms (Mackay 1989). Three types of ground ice were found on the Diavik site, including: (a) buried glacier ice, (b) thick segregation ice lenses and ice cored frost mounds (palsas), (c) icewedge ice. Massive ground ice was found in an esker deposit during the foundation construction of one of the dams. Figure 5. Buried glacier ice resting on bedrock and buried by sand and gravels in an esker. The ice had a maximum thickness of about 2.0 m and spanned a length of about 30 m. This ice was white in color with numerous randomly distributed air bubbles. It rested immediately above the bedrock surface and was buried beneath 8 12 m of gravelly sand. This ice was identified as buried glacier ice that was covered by sands during the development of the esker and preserved from subsequent thaw by the cover (Fig. 5). Buried glacier ice was usually located at the base of upper or middle sections of the esker deposit. At the end of an esker, the chance of finding buried glacier ice becomes slim. However, because of the fine nature of the deposited material, segregation ice lense formation is inevitable. 440

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 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 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. Figure 6. Segregation ice formation in fine soils. 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 properties of soils and relative distance to the large water bodies on the Diavik site. The surface is covered with a thin layer of mosses overlying gravel and boulders. This is followed by gravelly sand till to a depth of 4.6 m. The distance between the borehole and Lac de Gras is 150 m. Three examples are presented in this section for different locations, surface cover and soil conditions. These thermistor cables were installed to a depth of 30 m. Thermistor beads were located at depths of 0.25, 0.75, 1.25, 1.75, 2.25, 3.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.5, 17.0, 20.0, 25.0 and 30.0 m. Figure 8 provides an example of the ground temperature variation in a wetland area. The area was flat and snow cover was thin, less than 30 cm. The borehole was about 470 m from Lac de Gras. The soil stratigraphy was as follows: 0.3 m of peat overlying a 1.2 m thick layer of silt, and followed by a 1.6 m thick layer of fine grained sand and silt to a depth of 3.1 m. Bedrock consisted mainly of good quality granite. Segregation ice lenses with a maximum thickness of 66 cm, were observed. The temperature data shows that the active layer is less than 1.0 m. The ground temperatures fluctuate 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 depth of 30 m. Figure 9 provides the ground temperature variation 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 temperatures fluctuate annually over a range of about 37 C at a depth of 0.25 m. This fluctuation Figure 7. Ice-wedges found in dam foundation. Segregation ice formation was found in all the peat covered fine soils. Ice lenses varied in thickness from 0.1 to 2.0 m, with thickness dependent on uniformity of the soil conditions (Fig. 6). Palsas are found in valley bottoms where dams were constructed (Fig. 3). These palsas were less than 2 m in height. However, they were distributed over a wide area and were often bordered with extensive ice-wedges, about 2 4 m in width. The ice-wedges usually reach about 4 5 m in depth (Fig. 7). With the exception of buried glacier ice, the other types of ground ice are typical of those found in other areas (French & Pollard 1986). These ices display a wide range of colour, from cloudy to grey and brown, due to the inclusion of sediment and organics. Bands of silt and organic inclusions were found in horizontal, inclined and vertical directions. Numerous drill holes confirmed that the ice-rich soil was usually located in the top 5 m of the soil on the Diavik site. A layer of basal till below the ice-rich soil contained a large amount of sand, cobbles and boulders. The clay and silt sized soil particle component reduced to less than 50% of total weight. 4 GEOTHERMAL CONDITIONS 4.1 Permafrost Ground temperature conditions and permafrost depths have been investigated at the site through the installation 441

6 Depth (m) Depth (m) Ground temperature variations in a very ice- Figure 8. rich area CONCLUSIONS Water retention and processed ore storage dams were constructed for the Diavik Project. Extensive preconstruction investigations were carried out for the design and planning of the construction. The soil conditions were determined by geotechnical drilling, ground penetration radar survey, surficial terrain mapping and close field inspection during construction. The 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 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. These regimes were found to vary with the location, vegetation, soil conditions and distance relative to water bodies. Figure 9. Ground temperature variations in an ice-poor soil. REFERENCES Depth (m) Figure 10. Ground temperature variations in a talik area. reduces to 0.3 C at a depth of 20 m, with a constant temperature of 3.3 C at a depth of 30 m. Figure 10 shows the ground temperature variations in a talik below a small creek. The borehole was located about 165 m away from the lake. The ground was covered with thick vegetation, mainly willows, up to 2 m in height. The soil conditions were sand and sandy gravel with some cobbles and boulders. The area traps snow during the winter to more than 2 m in thickness. Due to thick snow cover, the ground temperatures at the depth of 0.25 m reach a minimum of about 2.0 C and the frost penetrates only to a depth of 1.25 m in the winter. The soil between depths of 1.25 m and 10 m is in a thawed condition all year long. The annual temperature fluctuation is about 14 C at a depth of 0.25 m and the fluctuation reduces to 0.3 C at a depth of 10 m. The temperature becomes constant at a depth of 15 m and remains at 2.3 C at a depth of 30 m. Bliss, L.C Vegetation and revegetation within permafrost terrain. In Proc. 3rd International Conference on Permafrost. Edmonton, Alberta 2: Ottawa: National Research Council of Canada. Brown, R.J.E Permafrost in Canada, its influence on Northern Development. NRC of Canada, Canadian Building Series 4. University of Toronto Press. Dingman, S.L Hydrologic effects of frozen ground. US Army CRREL Special Report 218. French, H.M. & Pollard, W.H Ground-ice investigations, Klondike District, Yukon Territory. Canadian Journal of Earth Sciences 23(4): Gold, L.W. & Lachenbruch, A.H Thermal conditions in permafrost a review of North American literature. In Proc. 2nd International Conference on Permafrost, North American Contribution: Washington D.C.: Science Press. Holubec, I., Hu, X., Wonnacott, J., Olive, R. & Delarosbil, D Design, construction and performance of dams in continuous permafrost. In Proc. 8th International Conference on Permafrost. Zurich, Switzerland. Rotterdam: Balkema. Johnston, G.H. (ed.) Permafrost engineering design and construction. Association Committee on Geotechnical Research, NRC of Canada. Toronto: John Wiley & Sons. Kjarsgaard, B.A. & Wyllie, R.J.S Geology of the Paul Lake Area, Lac de Gras, Lac de Sauvage Region of the Central Slave Province, District of Keewatin, NWT. In Current Research, 1994-C, Geological Survey of Canada Mackay, J.R Massive ice: some field criteria for identification of ice types. In Current Research, part G, GST Paper 89-IG Woo, M.K Permafrost hydrology in North America. Atmosphere-Ocean 24(3):

The elevations on the interior plateau generally vary between 300 and 650 meters with

The elevations on the interior plateau generally vary between 300 and 650 meters with 11 2. HYDROLOGICAL SETTING 2.1 Physical Features and Relief Labrador is bounded in the east by the Labrador Sea (Atlantic Ocean), in the west by the watershed divide, and in the south, for the most part,