where oa/dx is the stress gradient. Moisture transfer and ice formation in frozen soils affected by a shear stress gradient depend on soil composition. The thickness of the zone of ice formation and desiccation diminishes in transition from clays to silty-sands and from clays of kaolinite composition to polymineral and montmorillonite clays.
Moisture migration and ice formation in frozen ground affected by the gradient of electrical field
The moisture migration mechanism appears as follows. The electric field applied to the frozen soil sample disturbs the dynamic equilibrium of the liquid and solid phases of the water and results in the migration of hydrated cations of the electric double layer to the cathode. The bound water layers, which surround the cations, move in the same direction carrying along all other liquid.
These forces cause the migration first of the less bound water from anode to cathode. The equilibrium between water solid and liquid phases is disturbed but is reestablished by some ice melting and replenishing the water which has flowed from the anode region. The unfrozen water that enters the cathode region proves excessive, i.e. above the equilibrium value of water content Wunf at the given temperature. As a result it freezes out, enriching ice content and forming ice layers. An equation for the moisture flow in frozen soils due to the electrical field gradient is:
where dU/dx is the electrical field gradient.
The moisture redistribution in frozen ground under the influence of the electric field is rather important (see Fig. 2.8b). In an experiment with a clay sample, after six days under a voltage of 2-3 V cm ~1 and at a mean temperature of about — 2°C, heaving had occurred around the cathode and many thin ice layers (schlieren) had formed. These experiments have verified the linear dependence of the velocity of electrokinetic migration of unfrozen water on the electric field intensity and shown the existence of a threshold gradient below which moisture transfer is practically absent.
Moisture transfer and ice formation in frozen soil under the effect of osmotic forces (in the absence of temperature gradients) This process is inseparably associated with salt diffusion in frozen strata and the migration of chemical elements. In fact, frozen soil interacting with salt solutions results in the simultaneous development of two interdependent processes: migration of salt ions and migration of unfrozen water. Both normal and reverse osmosis can take place. Under normal osmosis the moisture migration in the direction opposite that of ion flow results in osmosis involving the shrinkage of the soil sample. Yet this process occurs only in the interaction of frozen soil with highly concentrated solutions. More often than not frozen ground shows reverse osmosis of water, i.e. its migration in the same direction as the flow of the ions. This is due to the total thermodynamic potential of the solution being higher than that of the unfrozen water of the sample. The resulting driving force for the migration gives rise to the transfer of water molecules which are more mobile in the solution than in the film water bound by mineral particles. Water molecule migration from solution into frozen soil results in the formation of ice macro- and micro-segregations (Fig. 2.10). The sample of frozen soil is then somewhat strained, with its total moisture and salt content increased. As experiments performed at a temperature of — 4°C show, the contact of a saturated solution or of salt crystals with frozen samples of kaolinite clay results in osmotic transfer of unfrozen water from soil to the salt. Salt
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