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Fig. 18.8. Diagram of electro-osmotic consolidation: 1 - electric conductor;

2 - electrodes; 3 - filters; 4 - connecting hose; 5 - spillway.

Fig. 18.8. Diagram of electro-osmotic consolidation: 1 - electric conductor;

2 - electrodes; 3 - filters; 4 - connecting hose; 5 - spillway.

regime for the supporting ground is set up. In addition, changes in the temperature regime of the permafrost can cause development or activation of unfavourable cryogenic geological processes such as heaving, ther-mokarst or icing formation, frost fracturing, thermal erosion, etc. And finally, the heat losses from the structure depending on the ground thermal state, affect the operational expenditures. Thus selection of the type of foundations, calculation of their area and the depth to which they are laid begins with determination of the ground temperature field.

The thermal physics of frozen ground is closely associated with its mechanics; therefore the combined consideration of three fields (temperature, moisture and stress), that is, of the so-called interconnected problem, is most appropriate. However as these problems are very complex, they may only be defined qualitatively. In addition, in the majority of cases the temperature field depends only slightly on the stressed state of the ground under the pressures that occur in practice. This makes possible consideration of the thermal physics problems without regard to mechanics, i.e. consideration of the problems as not interconnected. For example, first of all the ground temperature pattern for the particular point in time is determined and then the strength parameters are estimated.

Analytical determination of the ground temperature field is also associated with a number of complexities. Within the locality under construction it is necessary to take into account the thermal effects of buildings, structures and heating systems on each other; to estimate the change with time of the ground temperature field, and to take account of conductive as well as of convective heat transfer by water movement. Such problems belong in the class of non-steady-state, multidimensional problems of the Stefan type. Their analytical solution is obtained only for particular cases.

As a rule, the main simplifications of the problem of the ground temperature field in the locality under construction amount to the following: 1) water moving in the ground and, consequently, heat transfer by the water, is assumed to be absent; 2) phase transitions are assumed to proceed at the boundary of the thawing ground under buildings, at a temperature t of — 0°C; 3) heat transfer in the ground (frozen and thawed) is assumed governed by the Fourier transfer equation. Given these prerequisites, the quantitative assessment of the thermal effect of buildings and structures on the permafrost temperature regime has been obtained by many authors and presented in the works of G.V. Porkhaev, L.N. Khrustalev and others. In a general form the non-steady-state two- and three-dimensional heat conduction problems are solved by numerical methods using computers. When designing structures according to the first principle one carries out the heat engineering calculations with the aim of: (a) justification of the measures selected to guarantee retention of the frozen state of the ground (calculations for ventilated cellars, cold ground storages, cooling pipes and ducts, cooling devices, etc.); (b) determination of the design seasonal thawing depth of the ground; (c) calculation of the design permafrost temperatures at various depths for determining the strength characteristics of the frozen ground. When designing according to the second principle one determines with the thermal engineering calculations (a) the depth of seasonal freezing; (b) the depth to perennially frozen ground under buildings and structures.

After determination of the thermal state of the ground, strength and deformation characteristics of the soils are calculated in order to ensure the operational reliability of buildings and structures.

Whether the first or second principle of ground use as a foundation is followed, when the structure is erected the calculation for the foundations are carried out from two limiting states: 1) from bearing capacity (the first group of limiting states); 2) from strains (the second group of limiting states). In addition, foundations are tested for stability with respect to frost heaving. Calculations from the first group of limiting states are performed to determine the capacity of the ground to carry the structural load. The purpose of the calculations from the second group of limiting states is to determine the permissible ground strains without causing a loss of structural stability.

The procedure for calculating the stability of foundations is outlined in Building Norms and Regulations II-18-76 'Bases and foundations on permafrost'. The fundamental tenets for assuring ground support for foundations designed on the principle of keeping the ground frozen are of the greater interest, because when design is according to the second principle, foundation design is the same as for unfrozen ground.

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