Fig. 3.3. (a) Distribution of volumetric stresses by depth for various durations of freezing of a sample of kaolinite clay; (b) development of the stresses and depth of freezing as a function of time r. 1 - freezing front; 2 - visual boundary of segregated ice formation).

(0.08-0.2 MPa and more) and the smallest in sandy-silty materials (not more than 0.04 MPa).

Freezing conditions have the greatest effect on shrinkage in freezing ground. For example, a higher freezing rate produces lesser dehydration of the soil, with a thinner dehydration zone and, consequently, less shrinkage deformation. Moreover, not only is the general shrinkage small, but the 'prevented' shrinkage is correspondingly smaller, thus causing lesser Pshr values. In rapidly freezing ground, however, greater grad Pshr appears (Fig. 3.4a) as a result of greater grad W and a thinner dehydration zone. All these processes finally produce a larger number of ice layers (Fig. 3.4b). Three factors are active in the frozen part of the soil characterized by the gradual fall of negative temperature: freezing of part of the water, migration of unfrozen water, and intensive ice segregation. These three factors play an important part in the creation of stresses and heaving deformations: 1) crystallization of water on transition to ice and the increase of its volume by 9%, which cause stresses and heaving deformations, 2) the wedging effect of thin films of migrating unfrozen water causing stresses and swelling deformations, and 3) consolidation of the mineral part of frozen soils and dehydration of soil particles with lower negative temperature and freezing of part of the unfrozen water causing stresses and shrinkage deformations.

The shrinkage process in the frozen part of the soil is, apparently, most important only in the zone of intensive phase transitions, i.e. near the freezing front. This is probably the reason why maximum stresses and a

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