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. 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 geot- hermal 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 con- struction 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 Ϫ 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 Ϫ35°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 Ϫ34.8 Ϫ31.2 Ϫ27.5 Feb Ϫ33.8 Ϫ30.1 Ϫ26.4 Mar Ϫ31.1 Ϫ26.7 Ϫ22.2 Apr Ϫ21.9 Ϫ17.0 Ϫ12.0 May Ϫ9.9 Ϫ5.8 Ϫ1.6 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 Ϫ1.0 1.7 4.4 palsas and ice-wedge polygons. Oct Ϫ10.8 Ϫ8.1 Ϫ5.5 Nov Ϫ24.7 Ϫ21.1 Ϫ17.4 Dec Ϫ30.9 Ϫ27.3 Ϫ23.7 (b) most hydrological activities being confined within Average Ϫ15.7 Ϫ12.3 Ϫ8.8 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 C) April. However, the melted water is usually refrozen º 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 ( -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 granodi- orite 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.