Fig. 17.9. Design of plain earth and rock-fill dams on the first (a) and second (b) principles: 1 - body of dam; 2-3 - bases (2 - natural, permeable; 3 - impermeable, hard); 4 - cooling installations; 5 - anti-seepage screen; 6 - permafrost surface.

The location of the water storage is a major factor in the design of the hydrotechnical structure. When the engineering-geocryological assessment of the site of the proposed water storage is made it is as well to bear in mind that permafrost degradation will take place in the course of its filling with water and future existence. If the ground is hard, faulted rocks, the fissures of which are cemented by ice, or carbonate rocks with karst cavities filled with ice, or rudaceous materials which are a water confining stratum when in a frozen state can, on thawing, cause water leakage from the reservoir. In addition it is necessary to predict the possibility of thermal abrasion modifying the reservoir banks and to justify the measures for protecting the area from flooding.

Ice and permafrost as building materials

Climatic conditions within most of the former USSR, especially its northern regions, are favourable to wide use of ice, snow and permafrost as building materials. Thus it is possible to use them for constructing seasonal and perennial storehouses, shelters, ice roads and temporary bridges, dams, temporary piers, etc. Underground structures such as storages, coolers, laboratories, etc. are constructed in permafrost. The advantage over the normal underground structure lies in the fact that it can be used without supporting walls and a roof of rock or with light, protective support. Ice lining is also often used.

When designing the ice, snow and permafrost structures it is necessary to select constructional methods providing stability for the structures as a whole if deformations of elements of these structures take place. At the same time, it is not permissible that elements experience bending stresses (beams, plane ceilings) and tensile stresses. It is necessary to follow such constructional principles that all the elements of a structure are subject to compression. In such cases repeated deformations cause only bulging of vaults, height decrease and increase of thickness of columns and walls, without the structure being destroyed. The value of allowable deformations for the period of operation of a structure which will not cause any emergency is fed into the design calculations.

The most widespread constructions made from ice and snow are ice storages. All the basic elements of these constructions are just ice; ceilings are made in the form of adjacent semicircular arches joined at the top with continuous ice plates, with the thickness of the vaults reaching 2 m. The ice storage is protected from thawing by an insulation layer of sawdust, slag and other thermally insulating materials on the outside. During the period of use a temperature varying from 0 to — 10°C is maintained inside the storage. In winter latent heat of water freezing is used to prevent too much chilling inside the storage, by spraying the floor and walls with water. In summer a temperature varying from 0 to — 2°C is maintained. Ice-salt cooling may be used. It is possible to construct heated rooms inside the ice and snow structures with the help of light thermal insulational screens and cold air ventilation between these screens and the main structures.

Experiments on ice and ice-ground islands and platforms constructed for placing of bore holes within the shelf area of Arctic seas, have been carried out in Russia recently. Experience with such construction in the USA and Canada has shown great promise for their use in prospecting and extraction of oil and gas from the shelf mineral deposits. Various methods are used for platform build-up: layer-by-layer build-up, build-up by water spraying, cooling with the help of self-regulating installations; and forced cooling with the help of various cold-carriers.

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