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Fig. 18.3. The main types of foundations used in the course of construction on the permafrost: I - shallow surface (a - brick and concrete, b - wooden); II - posts; III - piles (a - lowered, b - bored-lowered, c - bored-driven); d^, d2 - diameters of the thawing zone and of the bore hole, respectively; 1 - the upper surface of the permafrost.

Fig. 18.3. The main types of foundations used in the course of construction on the permafrost: I - shallow surface (a - brick and concrete, b - wooden); II - posts; III - piles (a - lowered, b - bored-lowered, c - bored-driven); d^, d2 - diameters of the thawing zone and of the bore hole, respectively; 1 - the upper surface of the permafrost.

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Fig. 18.4. Foundation structure following the second principle of using the ground as a base: a - with permissible thawing of the permafrost during the course of operation; b - with prethawing of the permafrost; c - using the method of stabilizing the permafrost table at its original position; 1-4 - boundaries (1 - of the permafrost in natural conditions, 2 - after the prethawing, 3 - formed in the course of operation, 4 - of the seasonally freezing layer).

under the centre and edges of a building. This method can only be used if the consequences of thawing are taken into account completely and if measures are undertaken for ensuring the adaptation of foundations and structures to the deformations which will take place in the course of thawing. Therefore when construction on thawing ground is carried out, there is a need to maintain as slow and uniform thawing of the ground as possible, this being a factor in the uniformity of settlement. If rapid settlement is allowed, the building construction will, probably, have no time to adapt resulting in unacceptable deformations in the structure causing an emergency.

Foundation designs and calculations with permissible thawing of perennially frozen ground must include: 1) determination of the depth of thawing of the perennially frozen ground for various times during operations, 2) determination of the bearing capacity of the thawing ground, 3) determination of the differential settlements of the thawing ground, 4) determination of size and type of foundation, 5) testing of foundation for heaving.

Two differentfoundation structures are mainly used on thawing ground: the rigid and the flexible. The former is not sensitive to differential settlements because of its rigidity. Raft slab foundations, a system of rigid concrete strip foundations and waffle foundations able to withstand nonuniform thaw settlements of the base are used for greater rigidity. To gain rigidity in structures, perimeter beams of reinforced concrete or steel are used when bracing building walls, load-carrying structures and pile foundations. Buildings covering an extensive area and their foundations are often divided into separate rigid sections (with settlement joints) each of which is reinforced by a grade beam, i.e. the building is constructed on the basis of a rigid structural model for separate blocks able to move independently of one another. The flexible structure model ensures the structure accommodating differential settlement. It is based on the principle of possible realignment of deformed elements of the construction. Articulated joints of elements and building sections are used to achieve this. Stresses due to warping are ruled out when this principle is applied so the buildings in practice follow the settlements. The flexible structure model of construction presents a way of treating displacements in the buildings. In this case preliminary provision is made for jacks to be installed in a cellar with the help of which constructions are realigned.

Following up the state of buildings and structures constructed by the method of allowing for differential settlements shows that many of them are being deformed in the course of operation. Evidently this method can only be used on ground that is practically incompressible in the course of thawing with a settlement value of not more than 0.5 cm m"1.

A method for guaranteeing the structure stability by permafrost prethawing

This method (see Fig. 18.4b) has been known from the 1930s but justification for using construction with prethawing was given only in 1958 by V.F. Zhukov. From this time, widespread use of this method occurred. It is most profitable to use the prethawing method in the following cases: 1) when ice-rich ground occurs in a fairly thin layer (to 7-10 m) on little-compressible monolithic hard rocks; 2) when heaved rudaceous soils have compacted rapidly in the course of prethawing and have sufficient bearing capacity; 3) on sandy soils with the rate of contraction being practically in line with the rate of thawing. Prethawing in clay soils also works well, however, only when they alternate with well-filtered sandy and rudaceous interlayers favourable for rapid drainage, or when their consolidation, stabilization and strengthening are accelerated artificially, simultaneously with the thawing.

Thawing to the depth of the whole calculated thaw basin is the best way for preparing the foundation base. However it is very expensive and not used now. Prethawing is carried out usually to a lesser depth (some 60% of the steady-state thaw basin depth) thus representing 60-80% of the settlement possible with thawing of the whole basin. Settlement during further thawing (in the period of construction and operation) proceeds very slowly and does not cause important deformations. Ground prethawing is carried out using thermal methods, or water and thermal amelioration.

After prethawing the soils have as a rule high macroporosity, excess water saturation and are in a poorly consolidated state and unsuitable for construction of foundations. Such soil requires preliminary preparation such as ameliorative drainage, consolidation and strengthening before construction starts. Design and construction on prethawed and stabilized soils are carried out according to their condition without considering their frozen state. The types of foundations are similar to those used in the course of construction on unfrozen weak soils.

A method for ensuring the stability of structures by stabilization of the permafrost table at its original position

This method was developed by L.N. Khrustalev and others in 1967 for developing the areas with permafrost of the unconnected type. The method implies that the upper permafrost boundary is maintained throughout the period of operation at the initially determined level. Construction involves using ventilated cellars with the foundations situated between the permafrost table and the base of the seasonally thawing layer (see Fig. 18.4c). Stability of the permafrost table can be attained only when there is no temperature gradient in the ground above the permafrost table. This condition is achieved when the mean annual ground temperature is 0°C at the depth reached by seasonal freezing. Thus, maintaining a mean annual temperature near 0°C in the ventilated cellar is required for this method. This is achieved with the help of a temperature regulating system in the cellar using air holes that open and close, by using seasonal self-cooling installations, etc. This method of stabilization has been used in civil and industrial engineering in the city of Vorkuta for many years.

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