Environmetal Soil Properties and Behaviour
DisciplinaControle e Remediação da Poluição dos Solos5 materiais • 18 seguidores
60 80 100 0 0.1 0.3 0.2 0.4 Matric Suction, (cm) Vo lu m et ric W at er C on te nt , \u3b8 FIguRE 3.19 Hysteretic phenomena observed for water characteristic curves for sand. Arrows indicate wet- ting and drying processes. 114 Environmental Soil Properties and Behaviour saturated water content obtained in this wetting process is less than that of the fully saturated water content since the soil will entrap air bubbles in the wetting process. When drying of the apparently saturated soil (which includes the entrapped air bubbles) occurs, the measured water characteris- tic curve will differ from the primary boundary curves. When the wetting or drying occurs in the middle of the drying or wetting process, the developed characteristic curves, which are defined as the secondary wetting or secondary drying curves, will be different from the primary curves. The water charac- teristic curves are important pieces of information that reflect the changing water content processes, and are useful in analyses of water uptake, drain- age, and water flow in engineering projects. The hysteretic phenomena of water characteristic curves can be analyzed and simulated quantitatively using two kinds of pore models since it is a function of the characteristics of the soil pore structure: \u2022 Blown tube model: The pore geometry is simplified by assuming a shape similar to the pore structure model illustrated by the left-hand sketches in Figure 3.2 in Section 3.2.1 (Nakano, 1980). \u2022 Sphere packing model: The pore geometry is assumed to be the space formed in an ideal packing of spheres of similar radii (Zou, 2007). Hysteretic performance in clays is not easy to explain with a simple model. This is because of the ink-bottle effect (as shown in Figure 3.2) and also because of changes in the nature and sizes of the various microstructural units. Movement of particles in these microstructural units occurs in the wet- ting and drying processes, resulting in fabric distortion and \u201cplastic readjust- ment.\u201d When volume change occurs, the fabric changes accompanying volume changes will produce corresponding changes in pore size distributions and clay\u2013water interactions arising from physicochemical forces. Interparticle con- tacts and forces at the points of contacts differ on wetting and drying. 3.5 Water Uptake and Transfer The discussion in this section directs its attention to the principles govern- ing movement of moisture into partly saturated soils. The discussion on water movement in fully saturated soils will be found in the chapters and sections dealing with the hydraulic properties and performance of soils (Chapter 7). The term water is generally considered to mean liquid water, whereas the term mois- ture is used as a more inclusive term, that is, liquid water and vapour. Where it is necessary to avoid confusion, the term liquid water will be used in place of water. The term partly saturated soil is used in preference to unsaturated soil. 115Soil\u2013Water Systems The water content of a soil is a significant property of a soil because of its role in establishing the properties of soils. It is not a static quantity. Additions of water come naturally from rainfall, snow-melt, subsurface flow, and con- densation, and natural depletion of water content occurs as water losses due to evaporation, transpiration, and drainage. Additions and depletions of water content also occur due to anthropogenic activities such as irrigation in agricultural practices, alteration of surface hydrology features, land reclama- tion, and so forth. The terms water uptake and migration refer specifically to water entering a dry or relatively dry soil (water uptake), and water transfer in partly saturated soils after moisture uptake (water migration). 3.5.1 Moisture Transfer In natural circumstances, most of the water movement in soils is due to gra- dients of matric potential \u3c8m or capillary potential \u3c8c that arise from differ- ences in water content. Concentration gradients (differences in concentration of solutes in porewater in different parts of the soil) will also provoke water transfer. The osmometer experiment shown as a schematic in Figure 3.20 illustrates this phenomenon. In this example, the soil sample in the left-hand cell is fully saturated with ionic solutes in its porewater. The total potential in the soil is \u3c8 = \u3c8s. For a partly saturated, the potential will be, \u3c8 = \u3c8m + \u3c8s . De-ionized water Suction device Selective membrane permitting only diffusion of water molecules through membrane Saturated soil with ionic solutes in porewater Soil-water potential \u3c8 = \u3c8m + \u3c8s Mercury manometer for measurement of soil suction S S = SS + SM Osmometer system FIguRE 3.20 Osmometer-type cell showing development of suction required to counter flow of water into the left-hand side chambers because of the total potential \u3c8 in the saturated soil. S, SS, and SM refer to the total suction, solute suction, and matric suction, respectively. 116 Environmental Soil Properties and Behaviour The right-hand cell in the osmometer system contains deionized water which is separated from the left-hand cell by a fixed-position selective mem- brane that permits only diffusion of water molecules through the mem- brane. When the concentration of a solution differs from that at another point, there is a tendency for the more dilute liquid to diffuse into the region of higher concentration. This is the case for the left-hand and right-hand cells shown in Figure 3.20. The potentials in the soil in the left-hand cell pro- duce gradients that will induce the deionized water in the right-hand cell to diffuse into the soil to attain a more uniform ionic concentration. To restrain diffusion of water in the right-hand cell through the selective membrane to the left-hand cell, one needs to apply suction to the water in the right-hand cell. The suction required to restrain diffusion, S, can be considered to con- sist of SS and SM , where SS and SM are the solute suction and matric suction, respectively. 184.108.40.206 Water Transfer and Wetting Front Water movement in soil above the water table occurs when both water and air are present in the voids. This is the partly saturated soil zone of interest in many soil and geoenvironmental engineering projects. Water transfer in partly saturated soils is of considerable interest also to the agro industry. Commonly accepted terminology that describes water transfer in partly sat- urated soils as unsaturated flow will be used in this book. The characteristics of unsaturated flow in soils are demonstrated in the portrayal of wetting-front advance results obtained from a horizontal soil column permeation experi- ment (Figure 3.21). Beginning with a dry soil, a constant head source of water is supplied by the double Mariotte flask. The ability to locate the Mariotte flask air entry position (up or down) allows one to conduct the permeation experiment with various values of negative or positive heads. The profile depicted in the diagram, which is called a wetting front pro- file, consists of a wetting zone ahead of a transmission zone and behind the wetting front. The wetting zone and wetting front combine to form characteristic shapes that can inform one on the nature of water diffusion into the soil. In general, moisture transfer without convective flow is expressed using a Darcy-type equation as follows: v k x = \u2212 \u2202 \u2202( )\u3b8 \u3c8 (3.23) where v is the flux, k(\u3b8) is the Darcy coefficient, \u3b8 is the volumetric water con- tent, x is the spatial distance from the source of water, and \u3c8 is the soil\u2013water potential\u2014that is, the sum of the various potentials associated with the vari- ous forces operating in the soil\u2013water system (see Section 3.4.1).