Moisture exchange properties of soils

Analysis of the state of moisture in the soil and its capacity to move is the first priority in the assessment of mass-exchange characteristics of hydraulic conductivity, mass capacity, coefficient of diffusion and moisture potential. It was shown by experiment that with higher moisture content the values of ground water potential and differential water capacity increased. Low absolute values of the chemical potential of the moisture of high moisture content materials indicate small amounts of bound capillary moisture. Being situated in the largest capillaries of the soil the water does not differ substantially from free water with respect to energy potential. Then, as moisture comes out of finer capillaries and pores, falls greatly, i.e. moisture becomes more cohesive to the soil. The value Cm (the volumetric differential moisture capacity) diminishes insignificantly. The transition from macro-capillary moisture to micro-capillary, for soils, is more distinctly revealed in inflections of the curves of the soil volumetric differential moisture capacity.

Grain-size variation (see Fig. 2.1) brings about changes in the energy of cohesion to the soil mineral fraction for a given moisture content - as much as one or two orders and more. Changes of granulometry giving smaller sizes of mineral particles enlarge the specific active surface of the soil and lead to ultra-porosity. As a result, the same amount of water is, energetically, more bound by the mineral surfaces of the soil, as it occurs in finer films and pores and is characterized by a lower value of moisture potential. Finer-grained soils contain more fine pores so that water-filled capillaries and ultrapores have menisci of greater curvature. The latter serve as an additional factor leading to the growth of cohesion energy of pore moisture in the case of small size of particles.

Chemical-mineral composition exerts influence over and Cw first of all through the differing surface energies of the mineral skeleton as well as through differences determined by mineral composition, in grain size, speci fic active surface, differential and total porosity of soils and other characteristics. Thus, hydromica-montmorillonite clay have much smaller values of moisture potential and differential water capacity than kaolinite clay (see Fig. 2.1).

Consolidation of unfrozen soil according to experimental data, leads to a substantial increase (at the same moisture content) of chemical potential of the moisture (see Fig. 2.1). Total porosity of soils is reduced in the course of consolidation at the expense of larger pores (those less strong), small pores being only slightly disturbed which leads to partial transfer of film moisture into capillary moisture, thus increasing the maximum radius of water-filled capillaries. With respect to frozen soils, lowering of temperature below freezing point is accompanied by gradual crystallization of water over a range of temperature with reduction of its amount and, accordingly, potential.

Coefficients of water transfer for soils are dependent on moisture content, density, granulometry and chemical-mineral composition as well as structure and temperature.

These coefficients increase when fine-grained unfrozen soils grade into coarse-grained (see Figs. 2.1 and 8.6). Thus, saturated sands which are characterized by the presence of poorly bound capillary moisture, have water-transfer coefficients exceeding those of clay rich soils by 100 times and more. While in clay-rich soils the Km values are lower than those of coarser (at the same porosity and moisture content) this is due to greater values of ultraporosity and larger specific active surface of mineral particles which is typical of fine-grained soils in general with more fine water-conveying pores. At low moisture content this leads to the situation where the same amount of film moisture in clay-rich soils (Fig. 8.7, curve 5, section III) is energetically more bound and, therefore, less mobile, characterized by less intense translational motion of molecules. At high moisture content when a capillary mechanism of water transfer prevails (section I) the energy of cohesion and mobility of pore moisture are in direct proportion to the square of the radius of the active water-filled capillary.

Water-conveying properties of a soil are much dependent on mineral composition. Thus, the diffusion coefficient in montmorillonite clay at Wvol = 0.46 g cm ~3 (see Fig. 8.6) was as low as one tenth that of hydromica-montmorillonite clay and one fifteenth that of kaolinite. The above is associated with the fact that moisture in pores of montmorillonite clay is energetically more bound and, accordingly, has a lower mobility thereby reducing the coefficient of moisture transfer. In general, with higher content of silty, clayey particles and peat in the soils as well as minerals of the

Fig. 8.6. Dependence of moisture diffusion coefficient Kw on granulometry and mineral composition: 1 - medium-grained sand with porosity n = 33-38%, 2 - sandy silty material, 3 - silty-clay material, 4 - clay with porosity n = 42%; 5, 6, 7 - kaolinite, hydromica, montmorillonite clays respectively, with n = 46%, 8 - peat - macellanicum, pd = 0.075 g cm-3 (degree of degradation 25%, mineralization 3%).

Fig. 8.6. Dependence of moisture diffusion coefficient Kw on granulometry and mineral composition: 1 - medium-grained sand with porosity n = 33-38%, 2 - sandy silty material, 3 - silty-clay material, 4 - clay with porosity n = 42%; 5, 6, 7 - kaolinite, hydromica, montmorillonite clays respectively, with n = 46%, 8 - peat - macellanicum, pd = 0.075 g cm-3 (degree of degradation 25%, mineralization 3%).

Fig. 8.7. Dependence of moisture diffusion coefficient on moisture content of soils of different granulometry and density: 1-4 - sandy silty material, pd = 1.98, 1.93, 1.82,1.77g cm"3 respectively; 5-10-clay, pd = 1.69,1.65,1.55,1.41,1.35,1.25g cm-3 respectively.

montmorillonite group, the potentials of moisture and coefficients of moisture transfer diminish by one to two orders and more.

In considering the variation law of mass exchange properties one should bear in mind porosity of the material. Reduced porosity of the soil due to its compaction leads (at a constant moisture content) to higher moisture potential (see Fig. 2.1) and, consequently, higher coefficients of moisture transfer (see Fig. 8.6). As shown by experiment, the curves are similar for soils having different densities and they are shifted on the abscissa in relation to each other in proportion to the change of density p. The effect of temperature on moisture mobility diminishes with greater dispersion of soils (13). Thus, with a temperature rise of 10°C Km increases, in sandy silty materials by 15%, in more clay-rich materials by 10% and in clay by only 5%. This can be explained in that the same amount of pore moisture in clayey soils is subject to greater energetic influence of mineral surface than in more sandy materials and is therefore to a lesser degree subject to the effect of temperature.

The effect of temperature on the water-transmitting properties of frozen soils has much significance as the temperature determines the amount of unfrozen water in them, the thickness of liquid films and extent of water saturated capillaries and, correspondingly, mobility of the liquid phase. Fig. 8.8 shows the dependence of the moisture diffusion coefficient on volumetric moisture content, obtained experimentally for soils of various granulometry and mineral composition. The left part of these curves located within the range of highest moisture content refers to positive temperatures (10 to 20 °C), i.e. to the unfrozen water-saturated materials. The right part was obtained for negative temperatures, that is, for the frozen soils with a degree of saturation, ice and unfrozen water, of one. The gently sloping pattern of the curve of Km against PFvol allows us to assume that Km is gradually falling on freezing. This is explained by a uniform film mechanism of moisture transfer in unfrozen and frozen soils. An extreme pattern of the Kv) versus Wvoi curve for the frozen soils is associated with ice segregation in the frozen ground.

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