soils (see Fig. 3.11a) higher tjsh and lower tjb plastic viscosities can be distinguished. The larger viscosity (Shvedov) is determined from the equation tjsh = (a — <rcr)/e, while the lower one (Bingham) from rjb = [a — <r'cr)/e. Fig. 3.11a shows that plastic viscosity is equal to the cotangent of the angle of inclination of the corresponding straight part of the rheologic curve to the stress axis. Critical stresses <rcr and <r'cr are called, respectively, conditional static and conditional dynamic yield points. The ratio of the plastic viscosities is considered to be an important characteristic of the soil, reflecting the degree of its structural disturbance in the course of flow caused by variations of stresses. The greatest plastic viscosity is associated with the flow of soil having practically undisturbed structure, while the lowest one corresponds to the deformation of soil with effectively broken structure (4). The structure is assumed to be disturbed most intensively within the limits of transition (between Shvedov and Bingham) in the curved part of the rheologic curve. Wide variations of soil viscosity occur with lowering of temperature, especially in the range of the basic phase transformations of moisture. This is caused by general strengthening of soil as a result of the development of ice-cement bonds. Absolute values of viscosity accordingly increase by 100 to 1000 times and more. Simultaneously, there are variations of location and configuration of rheologic curves: they shift and stretch along the abscissa towards higher stress. Stretching of curves is associated with the widened range of stresses within the limits of which Shvedov and transitional sites are located. In many cases the interval of stresses for the Bingham part is reduced at the same time. These variations are induced by stronger ice-cement cohesion in the soil, diminished amount of unfrozen water and structural transformations of the mineral skeleton. The expansion of the stress interval for the Shvedov part of the curve, i.e. at low values of stress e 104, hr 1

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