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Physical Geography of Northern Eurasia

Permafrost

<<< Climate and Permafrost | Physical Geography Index | Permafrost: Summary >>>

Engineering Aspects and Protection of Permafrost

Prospecting and mining of mineral deposits, construction works, and exploitation of industrial and transport facilities in the cryolithozone have significant effects on permafrost. Thermokarst, thermal erosion, frost heaving, solifluction, and icing are the most common human-induced phenomena that have negative effects on the natural environment. On the other hand, many engineering problems result from occurrence of permafrost and the above-mentioned activities require specific approaches. Since the 1930s, economic activities in the permafrost zone have been on an extremely large scale and at present a geotechnical assessment of the permafrost environment is much needed.

Permafrost environments are extremely sensitive to thermal disturbances and the development of thermokarst, which causes subsidence of soils, is among the most common and damaging anthropogenic impacts. The construction of oil and gas pipelines causes intensive subsidence, which in turn is one of the major factors in pipeline damage. Studies have shown that in the West Siberian cryolithozone the rate of soil subsidence under pipelines reaches 0.8-1 m in five to seven years. Pipeline ruptures are much more common here than in permafrost-free areas (Geocryology of the USSR: Western Siberia, 1989). It is estimated that about 70 per cent of pipeline ruptures in the areas underlain by permafrost occur due to the thaw subsidence of soils (Zotova et al., 1995). Another common cause of thermokarst is the clearance of surface vegetation and peat. Thawing of ice-rich sediments causes the development of the distinctive forms of thermokarst microrelief such as baydjarakhs or 'graveyard mounds'. These form troughs and depressions which are between 0.5 m and 8 m deep with a diameter ranging between 10m and 300 m (Soloviev, 1973; Geocryology of the USSR: Western Siberia, 1989). With regard to construction, thermal erosion produces significant impact on permafrost through the intensive and rapid development of gullies (Geocryology of the USSR: Western Siberia, 1989). At the initial stage, the rate of thermoerosion gullying can be as high as 100 m a-1; it subsequently decreases to 5-25 m a-1. In the 1960s and 1970s, the construction of settlements of Salemal, Tazovskoye, and Soleonoye in Western Siberia triggered the development of thermoerosion gullies which reached 200-300 m in length, were 6-10 m wide and 3-4 m deep. A gully of these dimensions was formed as a result of construction works at the town of Labytnangy in just two months. Construction of hydroelectric dams also produces strong impacts on the permafrost environment. The Vilyuy hydroelectric dam was built in the 1960s in the continuous permafrost zone. In the Vilyuy valley, the thickness of perennially frozen ground varies between 50 m and 200 m and its temperature ranges between -3 and -7°C (Geocryology of the USSR: Central Siberia, 1989). After twenty years of operation, the thawing of frozen rocks under the reservoir bed (flooded area) has reached 15 m. At the shore, the ground temperature has increased by 3-4°C and the thickness of the active layer has increased by a factor of 2 as a result of the disturbance of vegetation (Obolin and Kamensky, 1993).

Frost heaving and icings are two specific problems related to construction and maintenance. Frost heaving causes the displacement of buildings, foundations, road surfaces, and pipelines; in Western Siberia, it occurs at a rate of 3-5 cm a-1 (Geocryology of the USSR: Western Siberia, 1989). Icings are of great practical concern for the construction and maintenance of railways and roads. They develop if excavations intersect the supra-permafrost groundwater table; they also form when seepage water freezes (Brown and Kupsch, 1974; French, 1996). Sixty-four icings covering in all approximately 1 million m2, have been observed along a 17 km stretch of the Amur-Yakutsk highway; and over a hundred icings formed along the highway in the first year of its operation (Geocryology of the USSR: Central Siberia, 1989). By far the largest engineering project carried out in the permafrost zone was the construction of the Baikal-Amur railway in northern Transbaikalia between the 1960s and 1980s. The railway crosses the discontinuous permafrost zone where about 30 per cent of the area is permafrost-free (Geocryology of the USSR: East Siberia and the Far East, 1989). Poor drainage of the frozen ground under and near the railway is a major cause of roadbed deformations (Dydysko et al., 1993). Icings, thaw subsidence, frost heaving of the track, thermoerosion gullying, rapid solifluction, and sliding of the embankment hillsides cause expensive maintenance problems.

A number of approaches and techniques are available to minimize the impact of construction works on permafrost. The main aim of these techniques is to maintain the thermal equilibrium of permafrost. Temperature and ice content of frozen deposits are the main criteria used to assess permafrost resistance to anthropogenic impacts. If the temperature of frozen ground is lower than -1°C, pile foundations are constructed (Zotova and Tumel, 1996). An air space left between the surface and the building allows free circulation of cold air which dissipates heat emitted by the building and minimizes its impact on permafrost (Kudryavtsev et al., 1978a; French, 1996). This is the most common technique used in Russia, and the high-rise buildings in Yakutsk, Norilsk, Vorkuta, and other northern towns have been constructed using this approach.

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