Cryogenic structure of a glacio-lacustrine deposit

Transcription

1 Cryogenic structure of a glacio-lacustrine deposit Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN Y. Shur University of Alaska Fairbanks, Fairbanks, Alaska, USA T. Zhestkova Fairbanks, Alaska, USA ABSTRACT: The glacio-lacustrine deposits of the Late Pleistocene are some of the most ice-rich deposits that have been developed during the last glaciation. Their properties create great challenges to engineers because these soils are both highly frost-susceptible in the active layer, and highly thaw-susceptible if temperatures rise above the freezing point. The glacio-lacustrine deposits are widely spread in many areas around the permafrost region. The properties are closely related to the laminated nature of the varved sediments and the prominent cryogenic structures. The authors have studied the cryogenic structure (patterns of distribution of ice inclusions in soil) and properties of glacio-lacustrine deposits in Russia (European Northeast and Middle Siberia) and in Alaska (the Copper River Basin). A literature search revealed data for several other regions. Data on glacio-lacustrine permafrost in Canada are also presented. 1 INTRODUCTION Cryogenic structures (patterns formed by ice inclusions in the soil) are the signature of the frozen ground and can be directly correlated with soil properties in both frozen and thawing states. Studies of soil cryogenic structures have traditionally concentrated in two main areas. The first one is focused on laboratory freezing of soils of different compositions and water contents. Tests are carried out under different thermal and moisture conditions in order to explore the impact of these factors. The second one includes morphogenetic studies of soil cryogenic structures in the field, which relate the structure to the geological origin of the soil component in the permafrost (glacial, marine, etc.) and aim to find further correlations between cryogenic structures, type of permafrost formation and subsequent modification of the thermal state of the soil. If strong correlations exist between the origin of soil and its cryogenic structures, they could apply to soils of similar origin in different parts of the world s permafrost regions. Relationships were established by Shur & Jorgenson (1998) for cryogenic structures in the floodplain deposits of arctic rivers in Russia by comparing this data with data obtained from the Colville River Delta, Alaska. The last glaciation had a great impact on permafrost formation, often leading to unique properties that are not repeated in the permafrost formed during the Holocene. Such permafrost is relic in the contemporary environment and, if thawed, its properties are not restored after refreezing. The existence of a periglacial environment during the Upper Pleistocene was extremely favorable to formation of thick bodies of ice-rich syngenetic permafrost, with enormous ice wedges penetrating the entire thickness. Buried glacial ice, and ice-rich glacio-lacustrine deposits in areas associated with glaciation, are the most prominent permafrost features remaining from that time. Permafrost of glacio-lacustrine origin occurs widely and occupies large areas. Many of these areas are located at the southern fringe of the discontinuous permafrost zone where contemporary climate alone cannot protect permafrost from thawing. Development of such areas and destruction of the organic layer protecting the permafrost lead to degradation of ice-rich permafrost, and this requires great effort to be made to keep the ground frozen to protect structures. The bestknown example is the Trans-Alaska Pipeline, which crosses an extended area with ice-rich glacio-lacustrine deposits in the Copper River Basin. To protect permafrost from thawing, the pipeline is elevated, and numerous thermo-siphons chill the ground. This paper focuses on the morphogenetic characterization of permafrost of glacio-lacustrine origin. Descriptions of cryogenic structures of glaciolacustrine deposits are based mainly on the authors studies in two areas of Russia and one in Alaska. Literature and data from Canadian studies are also presented and discussed. 2 RUSSIAN PERMAFROST REGIONS 2.1 Bezymianka material site The site is located in the vicinity of Vorkuta City on the second terrace of the Vorkuta River. Clays have been quarried from this site for use by a local factory to make bricks. 1051

2 (a) Varved clay is exposed at the site from 1.5 m to 15 m in depth. It is represented by the repetition of numerous layers (couplets) of light gray fine sand, gray silt and dark gray clay. The thickness of sand layers varies from less than one centimeter to 20 cm in the upper part of the exposure. Layers of silt and clay are a few centimeters thick. Cryogenic structure is reticulated to a depth of 2 to 3 m, with very thin ice lenses forming. Varved clay below 3 m in depth has clearly layered cryogenic structures, with continuous horizontal layers of ice of thickness varying from about 0.5 to 20 cm, with spacings from 5 to 40 cm. Several layers of soil may form between ice layers and may include many couplets. The most prominent ice layers were identified at the contacts of sand layers with silt and clay layers. The ice is transparent, with some air bubbles and thin inclusions of clay. The visual ice content varies from 15 to 40% by volume, with the ice content of soil decreasing abruptly at a depth of about 10 m. In the layer of soil with a depth from 3 to 10 m, the total thickness of ice is about 2 m. Water content of the soil between 3 to 5 m reaches 60% by volume (Fig. 1a), which is illustrated by a sketch made from one of the photographs (Fig. 1b) and shows the cryogenic structure of the varved clay. Post-cryogenic structures at places previously affected by thawing, are marked by voids and reflect the original cryogenic structure. Kritsuk (1962) also described formation of similar voids in refrozen glacio-lacustrine deposits that had been previously affected by thawing. 2.2 Lower Enisey River Varved clay occurs widely in the Lower Enisey River. It has been well studied in the vicinities of the Ice Water content Figure 1. (a) Water content (as a function of the solid phase) and (b) cryogenic structures of varved clay at the Bezymianka material site. Scale in meters. Ice is black. (b) Igarka City, especially in the underground laboratory of the Permafrost Institute. Cryogenic structures and related properties of these varved clays were studied by Pchelintsev (1964), Zhestkova (1978, 1982), Kusnetsova et al. (1985) and Kasansky (1996). Cryogenic structures of glacio-lacustrine varved clay were examined in an underground laboratory in exposures that were 17 m thick, with sequences occurring from 2.6 to 19.5 m. Different cryogenic structures were divided into several horizons. From 2.6 to 5.4 m, the varved clay contains the remains of vegetation. Ice inclusions are generally horizontal, hair like to 1 mm thin, and they are cross-folded with laminations of clay. From 5.4 to 14.5 m, undisturbed varved clay contains repetitions of the coupled layers of light gray silt, with a thickness of a few millimeters, and dark gray clay with a thickness of 5 to 10 millimeters. Continuous layers of sand have not been found, but some sand occurs at the contacts between the layers. The soil is icerich, with a volumetric ice content between 30 and 50%. The cryogenic structure of the soil is layeredreticulate. Continuous horizontal and sub-horizontal ice layers have thicknesses generally from 3 to 30 cm. They are spaced by layers of soil with thicknesses from 5 to 40 cm. The cumulative thickness of ice layers at the site of these studies varied from 0.5 to 2.5 m. According to Kasansky (1996), the lateral extent of ice layers exceeded 8 m in places. Boundaries between ice layers and layers of clay are sharp. Ice is transparent with air bubbles and soil inclusions, which occupy from 10 to 20% of the ice volume. Vertical and subvertical ice lenses are not so prominent as the horizontal lenses, but their existence results in practically all ice lenses being interconnected. Water content of soil between ice layers varies from 30 to 36% of solids, which is in accordance with a consolidation curve of soil in thawed state (Kasansky 1996). Total water content varies from 60 to 85% by weight. From 14.5 to 19.5 m, the same varved clay was found with a few ice layers of thicknesses between 0.3 to 2 cm. Visible ice content was less than 2%. Gravelly silty (moraine) clay underlies glacio-lacustrine varved clay at a depth of 19.5 m. Figure 2 shows the cryogenic structure of a glaciolacustrine deposit at the Igarka site. It is based on the photograph taken by Karpov (1986) in an under-ground cold storage, which is located in conditions similar to the underground laboratory. The cryogenic structure shows continuous horizontal ice layers, with thicknesses from a few centimeters to 20 cm, and spacings from 10 to 25 cm. The height of exposure in Figure 2 is about 3.5 m. Pchelintsev (1964) presented averaged data for the composition of a glacio-lacustrine deposit in the area (sand 7%, silt 52% and clay 42%). According to 1052

3 Depth, m Water content (WC) and Visible ice, % Total WC WC Mineral aggregates Visible ice Figure 2. Cryogenic structure of varved clay in Igarka. The height of sequence is 3.5 m. Ice is black (From photograph by Karpov 1986). this data, the liquid limit of the soil varies from 26 to 66% and the plasticity index varies from 6 to 42%. Figure 3 shows total water content of soil and visible ice content in the underground laboratory. The first two sets of data are based on Pchelintsev (1964). The volumetric content of visible ice is based on thickness of ice and soil layers in the section with the highest amount of visible ice. Data for total water content and visible ice, obtained from different locations in the underground laboratory, show similar patterns of distribution with depth (Fig. 3). Boreholes drilled in Igarka and neighboring areas, which contained glacio-lacustrine deposits, also revealed sequences with very thin, layered cryogenic structures and low ice content. Such profiles represent previously thawed and refrozen deposits. Cryogenic structure and properties of soil help to interpret the type of cryogenic genesis for the glaciolacustrine permafrost. Properties of this soil as well as the cryogenic structures are typical for epigenetic permafrost, but there is disagreement on the condition under which the ground froze. Pchelintsev (1964) thought that soil was frozen in a closed system and the ice layers were formed as a result of redistribution of the initial moisture in soil. Zhestkova (1978) found that freezing had to take place in an open system because the initial water content of soil cannot be much greater than the liquid limit of soil before freezing. The source of water and the manner of transport to the freezing front has remained disputed (Zhestkova 1978, Kusnetsova et al. 1985, Kasansky 1996). It was speculated that the lowering of the water level in the Figure 3. Water content and volumetric content of visible ice in varved clay exposed in the underground laboratory of Permafrost Institute in Igarka. lake provided conditions for soil freezing and water supply in the horizontal direction through layers of high permeability. Horizontal movement would be facilitated by the presence of thin sandy layers in the horizontal varved sediments. 3 COPPER RIVER, ALASKA Varved clay occupies a significant portion of the Copper River Basin in south central Alaska. The region was partly covered by an extensive glacial lake during the last glaciation. The known age of sediment varies from 10,000 to 38,000 years (Ferrians 1971, Péwé 1975). Permafrost in the area is widespread, in spite of its location at the southern boundary of the discontinuous permafrost zone. This occurrence is attributed to the properties of the glacio-lacustrine deposit, and particularly to the low permeability of this soil. Permafrost degradation is common in areas where construction is underway or there are related surface disturbances. Thermokarst lakes are abundant in some areas (Wallace 1948, Osterkamp et al. 2000). The Copper River Basin is well known for its geotechnical problems related to settling of thawing permafrost and heaving of highly frost-susceptible soil in the active layer (Nichols 1956). Investigations by the US Army Corps of Engineers (1954) described varved clay in the area to a depth of 30 m, as ice-rich with stratified ice lenses up to 5 cm thick and with the occasional occurrence of buried massive ice as thick as 4 m below the varved clay. Nichols (1956) reported that ice distribution was homogeneous and a few ice layers, up to 60 cm thick were recognized, based on samples from a chipping bit 1053

4 from the cable-tool drilling rig. He also indicated that massive ice up to 5 m in thickness extended horizontally in river exposures over 30 m. Massive ice was also found beneath the glacio-lacustrine deposit in Canada (Wolf 1998a). At the Gulkana Airport, it was reported that Considerable settlement has occurred along the East-West runway. Settlement is due to the thawing of ice in the subgrade soils and, in places, amount to a foot or more. (US Army Corps of Engineers 1954). This runway was later closed due to this excessive thaw settlement (Ferrians & Nichols 1965). It was expected that the icerich glacio-lacustrine sediments in the Copper River Basin would present serious problems in the construction of the Trans Alaska Pipeline (Lachenbruch 1970). Shur (unpubl.) logged soil data from three boreholes in the area of the Gulkana Airport in The area has been developed for the last 50 years, and natural vegetation has been removed. It was expected that the permafrost table was lowered and that the permafrost temperature would be close to 0 C. The permafrost table was encountered at a depth from 7 to 8 m during drilling, which took place without refrigerated fluid so that the auger and coring bits caused the permafrost to thaw. Several techniques were tried in order to protect the permafrost during drilling. Continuous sampling by a split-spoon sampler was found to be the best way to preserve cores from thawing and crushing. Blows for one-foot penetration intervals above the permafrost table varied from 12 to 15 in three boreholes. The number of blows varied from 16 to 28 in the permafrost, with an average of 21. This difference between unfrozen soil and permafrost was consistent, but not as striking as could be expected. It was explained by the specific thermal state of the varved clay at temperatures close to 0 C. At this temperature, components of varved clay may be found in three different thermal states frozen, partly frozen and unfrozen. The first one is presented by ice, which is the frozen state. The second one is presented by silt, which is partly frozen at 0 C. The third is clay, which is unfrozen at the temperature of 0 C. The soil is either classified as a silty clay with traces of sand, or as a sandy clayey silt. The amount of sand varied from 0.4 to 15% in five samples, the amount of silt varied from 7 to 50% and the amount of clay varied from 49 to 80%. Between the depths from 7 to 11 m, the permafrost had a layered cryogenic structure, with ice layers from 2 to 15 cm thick and spacings from 10 to 30 cm (Fig. 4). The water content of unfrozen soil above the permafrost varies from 30 to 35%, whereas the water content of soil between the ice layers is about 35%, which is similar to the water content of soil above the contemporary permafrost table. In samples that included ice and mineral soil between ice layers, total water content of Figure 4. Cores taken by a split spoon sampler at Gulkana Airport at depth from 8 [or 7 see text] to 11 m. Scales in inches (1 inch 2.5 cm). Depth, m Water content,% Figure 5. Water content of soil in two boreholes (G1 and G2) at Gulkana Airport. permafrost varies from 60 to 85%. Mud brought to the surface from a depth of 9 to 10 m by auger sampling at borehole G1 had a water content of 86%. The Alaska Department of Transportation drilled more than 30 holes in the Gulkana Airport area in The holes drilled in the existing runway did not encounter permafrost within the upper 8 m. At the site for the new equipment building, the permafrost table was located at a depth from 3.5 to 5.5 m. Based on auger cuttings, the soil was characterized as muddy, and broken ice lenses were noticed in places. Groundwater levels were documented from closely spaced boreholes at various depths ranging from 8 to 10.5 m. These levels were attributed to water accumulating in the holes from melting during drilling of the ice-rich, warm permafrost. The geologist on a site agreed with this interpretation (Brazo, pers. comm. 1996). A soil exploration hole of 6.5 m depth was drilled by continuous coring by CRREL at the Gulkana Airport in 1955, about a hundred meters from the boreholes described above. The permafrost table was encountered at a depth of 3.5 m. The permafrost did not contain visible ice. The water content of permafrost and unfrozen soil above permafrost table was practically uniform at about 25% (CRREL 1964). G1 G2 1054

5 4 MANITOBA AND NORTHWEST TERRITORIES IN CANADA Perennially frozen varved clay was described for two sites in Manitoba, Canada (Johnston et al. 1963, Johnston 1965) and a third site at Yellowknife, NWT (Wolf 1998b). The first site was at the new Thompson townsite. The second site was at the Kelsey Hydro Electric Generation Station. Permafrost at both sites has an irregular distribution in both horizontal and vertical directions. Cryogenic structures of permafrost were observed in trenches and were logged from boreholes. Publications present several extremely well described borehole logs (Johnston et al. 1963, Johnston 1965). At both sites, permafrost has layered cryogenic structures formed mainly by horizontal ice layers. Two types of cryogenic structure profiles are recognizable; very thin (practically hair like) either horizontal ice lenses throughout the studied section, or a two-layer structure. The upper layer contained very thin lenses and the lower layer had ice lenses up to 20 cm thick. Most of the thick ice layers occur at depths below 5 m. The extent of some horizontal ice layers exceeded 10 m. Photographs of permafrost in trenches show cryogenic structures similar to those presented in Figures 1 and 2. There is no apparent relationship between the location of ice lenses and the composition of the varved layers. Volumetric content of visible ice varies from 15 to 25% in horizons with thick ice lenses. Water content was estimated mainly for mineral soil between ice layers. Average water content of the silt is about 25% and for clay of 40%. Moisture content over 45% coincided with ice inclusions in the samples. The liquid limit of the silt varied from 20 to 30% and the plasticity index from 10 to 20%. The liquid limit of the clay varied from 60 to 80% and the plasticity index varied from 35 to 50%. Thaw of permafrost and the resulting settlement beneath dykes at the Manitoba Kelsey Hydro Electric Generation Station were monitored (Johnston 1969). Maximum vertical strain due to thawing reached between 30 to 40%. At the third site at Yellowknife (Wolf 1998b), glacio-lacustrine deposits occurred at scattered locations in town. They had the characteristic sequence of thick ice lenses up to 20 cm thick. Photographs of the varved sequences are very similar to those observed at other locations. The materials have been particularly problematic for engineering because of the large thaw settlement characteristics. Several buildings have had to have remedial designs to stabilize the foundations. 5 DISCUSSION An analysis of cryogenic structures of the glaciolacustrine deposit in four areas from different parts of the world s permafrost regions reveals three typical sequences. The first one is presented by layered cryogenic structures formed by thick layers of ice, practically from the permafrost table. The thickness of continuous ice layers varies from several to dozens of centimeters and the ice content reaches as high as 85%. It makes soil extremely thaw-susceptible, with vertical strain reaching 40% in some cases. The second typical sequence is presented by soil with uniformly low water content and with very thin ice lenses and unfilled voids. The third sequence is the combination of the two previous sequences with ice-poor soil in the upper and ice-rich soil in the lower part of the sequence. All three sequences were found in some areas to be just a short distance apart. Most known areas of such glacio-lacustrine deposits are located in the discontinuous permafrost zone. The permafrost there has been affected for a long time by the Holocene climate changes and wildfires. These areas have experienced human impact during the last 100 years. As a result, permafrost has thawed partially in some places, with later refreezing in some areas, or it has completely degraded in others. Refreezing after thawing and consolidation of the glacio-lacustrine permafrost does not lead to the original cryogenic structures being replicated. The first typical sequence of the cryogenic structure was considered to be the original one. The second sequence was formed by refreezing previously thawed soil, which is reflected by measurements of low soil water and visible ice contents. The combination of these two sequences often occurs with the ice-poor part overlying the ice-rich one. Degrading permafrost, with a lowered permafrost table, also occurs very frequently and tends to overly icerich permafrost. Completely degraded permafrost is the last type. All these types occur simultaneously over a short distance at many of these sites. The formation, degradation and subsequent refreezing of glacio-lacustrine permafrost results in characteristic ice lense formation with highly variable distributions. Ferrians (1971) described rapid vertical and horizontal changes in properties of the glaciolacustrine deposit and unpredictable distribution of ice-rich facies. This distribution makes geotechnical characterization of such sites very complicated. It also hinders effective prediction of permafrost behavior, and design. As stated by Johnston et al. (1963): Great care must be taken therefore in locating structures under these conditions. ACKNOWLEDGEMENTS The University of Alaska EPSCoR program supported the study. We express our thanks to Dr. Jerry Brown and Torre Jorgenson for suggestions improving the original 1055

6 text in content and language. We appreciate helpful comments and suggestions made by the reviewers. REFERENCES CRREL Ground temperature observations Gulkana, Alaska. USA CRREL Technical Report 106. Ferrians, O.J Preliminary engineering geologic maps of the proposed Trans-Alaska pipeline route, Gulkana Quadrangle: US Geological Survey Open File Report , 2 sheets, scale 1: Ferrians, O. J. & Nichols, D.R Copper River Basin. In: T.L. Péwé (ed.) Guidebook to the Quaternary Geology. South and South-Central Alaska, INQUA VIIth Congress: Johnston, G.H Permafrost studies at the Kelsey hydro-electric generation station. Research and instrumentation. National Research Council of Canada, Division of Building Research. Tech. Paper 78. Johnston, G.H Dikes on Permafrost, Kesey generation station. Canadian Geotechnical Journal 6: Johnston, G.H., Brown, R.J.E. & Pickersgill, D.N Permafrost investigations at Thompson, Manitoba. Terrain studies. National Research Council of Canada, Division of Building Research. Tech. Paper 158. Karpov, E.G Underground ice of Enisey North. Novosibirsk: Nauka (In Russian). Kasansky, O.A Cryostructure method of reconstruction of paleo-permafrost conditions. Yakutsk: Melnikov Permafrost Institute. (In Russian). Kritsuk, L.N On genesis of voids in permafrost. In Publications of Permafrost Institute 19: Moscow, USSR Academy of Sciences (In Russian). Kusnetsova, N.P., Rogov, V.V. & Shpolianskaia, N.A Upper Pleistocene stage of cryolithogenesis at eastern boundary of northern part of West Siberia. In A.I. Popov (ed.) Development of the permafrost region of Eurasia in Upper Cenozoik: Moscow: Nauka. Lachenbruch, A.H Some estimates of thermal effect of a heated pipeline in permafrost. US Geological Survey, Circular 632. Nichols, D.R Permafrost and ground water conditions in the Glenallen area, Alaska. US Geological Survey Professional Paper 392. Osterkamp, T.E., Viereck, L., Shur, Y., Jorgenson, M.T., Racine, C., Doyle, A. & Boone, R.D Observation of thermokarst and its impact on boreal forest in Alaska, U.S.A. Arctic, Antarctic, and Alpine Research 32(3): Péwé, T.L Quaternary geology of Alaska. Geological Survey Professional Paper 835. Washington, United States: Government Printing Office. Pchelintsev, A.M Structure and properties of frozen soils. Moscow: Nauka (In Russian). Shur, Y. & Jorgenson, M.T Cryostructure development on the floodplain of the Colville River Delta, Northern Alaska. In A. Lewkowicz (ed.) Proceedings of 7th International Conference on Permafrost. Yellowknife, Canada: US Army Corps of Engineers Report on foundation investigations, Project F-23. Gulkana, Alaska. Wallace, R.E Cave-in lakes in the Nabesna, Chisna, and Tanana River valleys, Eastern Alaska. Journal of Geology 56: Wolf, S.A. 1998a. Massive ice associated with glaciolacustrine delta sediments, Slave Geological Province, N.W.T. Canada. In A. Lewkowicz (ed.) Proceedings of 7th International Conference on Permafrost. Yellowknife, Canada: Wolf, S.A. (ed.) 1998b. Living with frozen ground: a field guide to permafrost in Yellowknife, Northwest Territories. Geological Survey of Canada, Misc. Report 64. Zhestkova, T.N On cryogenic structure of varved clay. In: Permafrost Investigations 17: Moscow: Moscow State University. (In Russian). Zhestkova, T.N Formation of soil cryogenic structure. Moscow: Nauka (In Russian). 1056