W

Fig. 17.6. Diagrams of heat interaction between underground pipelines and soils with respect to relations between mean annual temperature tmean, minimum tmin and maximum tmax gas temperatures: a - tmean > 0°C, tmin > 0°C ;b- tmean > 0°C; tmin < 0°C; c - tmean < 0°C, tmax > 0°C; d-tmean < 0°C, tmax < 0°C; 1 - 3 - soil (1 - perennially frozen; 2 - seasonally frozen; 3 - thawed); I, II - regions of permafrost and non-permafrost ground respectively.

ity of the embankment itself. Experience gained from operation of the Western Siberian pipelines has shown that native soils are not suitable in practice for construction because within two to three years after the beginning of operation the embankment is disturbed and soil is washed out exposing the pipeline. In addition the extended embankment changes the conditions of surface runoff, contributes to paludification of the right-of-way and of the adjacent terrain and to thermokarst development. When the pipe is laid above ground the main problem is ensuring the stability of the pile trestles against the effect of frost heaving processes developing in the seasonally frozen layer. If pipeline weight is insignificant it is necessary to put piles deeper in order to 'spread out' the tangential forces of pile heaving.

Water pipelines and conduits

These are rather expensive constructions within the permafrost zone. At the same time it is necessary to heat the water for the 6-8 winter months to keep it unfrozen. To select the requirements for laying of these pipes, thermal calculations are made resulting in determination of such

Fig. 17.7. Diagram of underground service lines laid in inaccessible (a) and accessible (b) ventilated ducts: 1 - heating system; 2 - water pipeline; 3 - sewer; 4 - reinforced concrete section; - electric cables; 6-7 - soils (6 - natural base; 7 - fill, non-frost susceptible); 8 - sandy padding; 9 - sand-gravel-clay building mixture; 10 - permafrost surface.

Fig. 17.7. Diagram of underground service lines laid in inaccessible (a) and accessible (b) ventilated ducts: 1 - heating system; 2 - water pipeline; 3 - sewer; 4 - reinforced concrete section; - electric cables; 6-7 - soils (6 - natural base; 7 - fill, non-frost susceptible); 8 - sandy padding; 9 - sand-gravel-clay building mixture; 10 - permafrost surface.

characteristics as temperature variations along the water pipeline, its heat losses, the zone of thawing around pipes, etc.

Construction practice has established the following means of laying service lines: a) above-ground laying; b) laying inside an embankment; c) underground laying in trenches; d) underground laying in ducts with or without ventilation, in which all the service lines can be laid together (Fig. 17.7). Whatever means is selected, thermal insulation is required. All the means of laying call for use of thermal insulation. Inside the ventilated ducts the air temperature is close to the outside air temperature in summer time. Slight thawing of frozen parts takes place in this period in the base of the duct. In winter the negative air temperature is maintained because of the natural ventilation inside the duct resulting in freezing of that soil which was thawed in summer. In such a manner the perennially frozen ground occurring below the prepared base will be in the frozen state at all times.

Power lines

Having no heat release, these change geocryological conditions to a smaller degree than the other linear structures (motor- and rail roads and pipelines). Importance is attached to calculations of stability of power line supports in power line design. The supports are placed on pile, post and bedplate foundations designed as a rule on the principle of preserving the frozen ground. Stability and strain characteristics of frozen ground deter mined with regard to possible change of soil temperature regime within the line right-of-way are used in the course of calculations for mechanical stability of the supports. When determining the stability characteristics of frozen soils it is necessary to take into account not only static loads (the trestle and foundation weight) but also dynamic loads caused by the effect of gusts on the supports. In addition, calculations are made of the effect of heaving forces on the foundations of the supports.

Airfield pavements

These consist of artificial pavement and an artificial base overlying the natural ground. The artificial pavement is the uppermost relatively thin and strong layer taking the main impact of loads. The artificial base is composed of a layer or of a few layers of crushed rock, gravel, and sand pretreated with chemical binding materials. It serves to redistribute the wheel stresses over a larger area and their transmission to the natural base, i.e. to the upper layers of ground levelled and compacted artificially (Fig. 17.8). Airfield pavements should correspond to the following specifications: 1) strength and durability; 2) plane surface and wear resistance; 3) roughness of surface necessary for wheels to grip the pavement well; 4) absence of dust; 5) water resistance and tolerance for climatic effects.

To select the pavement in the design of the airfield, the results of strength and stability calculations for constructions and their foundations are used. To do this requires also knowing the strength and deformation characteristics of the ground in the natural base. Usually airfield construction within the permafrost regions is carried out variously, with protection of frozen ground in the base from thawing; with thawing of frozen ground in the course of construction and operation; or with prethawing and improvements of ground properties in the base. The principles for selection of one or another method of airfield design and construction and the procedure for stability calculations for the base of the airfield pavements, are similar to those in roadway construction. The specific character of airfield pavement design compared to that of roadbeds lies in the heavy demand on their stability.

Hydrotechnical structures

The experience in construction of such structures in the permafrost regions is not very great, but includes Vilyuy, Khantayka, Mamakan, Kolyma and Zeya water-power stations, Arkagala National Regional Electric Power Station, and dams in the Irelyakh, Pevek and other rivers. Within the permafrost regions, the most widespread are plain earth and rock-fill

25 m or more

50-60m

Fig. 17.8. Airfield pavement structure: 1 - pavement; 2-3 - foundations (2 - constructed; 3 - natural); 4 - vegetation layer; 5 - direction and degree of slope of surface.

dams, more rarely concrete dams are used. Plain earth dams represent an embankment of trapezoidal cross-section. Their stability results from great weight. The greater part of rock-fill dams consists of rock fill, i.e. highly moisture-permeable material. To reduce water head special-purpose anti-seepage devices such as shields, diaphragms and cores are used.

Design and construction of hydrotechnical structures are performed on either of two principles: 1) protection of perennially frozen ground in the base from thawing during the whole period of construction and operation; 2) progressive thawing of the permafrost in the course of the construction and operation of the structure and its use as a base after thawing (Fig. 17.9). Construction according to the first principle is performed usually in the conditions when highly ice-rich soils with considerable thaw consolidation are used as a base. When keeping the ground in the frozen state the frozen curtain (the core of a dam) is arranged to guarantee the stability and to prevent water seepage through the dam body. Until recently the freezing systems in which cold atmospheric air was used as a heat-transfer medium were used widely to freeze the core of a dam. Cold air circulation was forced through coaxial columns placed in holes drilled from the crest of the dam or through horizontal air offtakes inside the body of a dam. However, experience tells us that the cost of such systems is high and the reliability is not sufficient because the columns and the air offtakes are gradually filled with ice and clearing them is a tedious procedure. Freezing systems with natural circulation of a heat-transfer agent (self-regulated cooling installation) are used with increasing frequency today.

Dams designed for progressive thawing of permafrost are allowed when it is impossible to keep a base in the frozen state or when it is not economically profitable. When designing dams on the principle of thawing of the base soils, provision should be made for smooth, gradual thaw settlement of soils, not exceeding the allowable values. At the same time the soils in the base must not have high water permeability after thawing.

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