Partially saturated soil

Fig. 10.2 Pattern of contaminant advance (water containing dye) in a partially saturated soil. Note the high degree of spatial variability in the pattern, both laterally and with depth. Reprinted from Ghodrati M, Jury WA (1992) A field study of the effects of soil structure and irrigation method on preferential flow of pesticides in unsaturated soil. J Contam Hydrol 11 101-125 Copyright 1992 with permission of Elsevier...

The field experiments from the Bet-Dagan site were used to test different theoretical models for field-scale chemical transport. One study expanded a simple column model for flow and transport in partially saturated soils (Bresler and Dagan 1983),... 

While electrokinetic treatment of soils looks promising, most of the work performed was bench-scale, under carefully controlled laboratory conditions. For electrokinetic remediation to be a viable alternative for in-situ cleanup of waste sites, a number of factors will have to be investigated. All of the work to date has dealt with uniformly contaminated soil samples. Studies performed on partially saturated soils will yield different results. Further studies on the removal of mixed metal contaminated soils, using different soil types, are needed. The presence of organic compounds in the soil will also influence successful treatment of real contaminated soils. The use of reagents which could increase desorption and/or solubilization (without further contaminating the soil matrix) may also be areas of future investigation. Finally, field tests need to be performed to substantiate studies accomplished on the bench scale. 

Jakubick, A. T., "Water Movement in Partially Saturated Soils - Tritium Tracer Method for Estimation of the Ground Water Recharge. Thesis, University of Heidelberg, Heidelberg (1972). 

Beven, K.J., D.E. Henderson, and A.D. Reeves. 1993. Dispersion parameters for undisturbed partially saturated soil. J. Hydrol. 143 19-43. ... 

The classical CDE equation has been intensively used to model solute transport in saturated porous media (1989). In partially saturated soils, the CDE model is usually considered to be more appropriate to describe transport in repacked or non-structured soil (Radcliffe, D.E. et al. (1998) Wilson, G.V. et al. (1998)), but has also successfully been used to describe transport in real structured soils (Bejat, L. et al. (2000) Comegna, V. et al. (1999)). 

Lloret A. Alonso E.E. 1985. State surfaces for partially saturated soils. Proc 11"" Int Conf Soil Mech Fdn Engng, San Francisco, Vol. 2, pp. 557-562. 

Important applications for coupled finite element strategies can be found in the theory of consolidation of partially saturated soils. Some test cases were performed, (see Kohlmeier et al. (2002)) using the Richards formulation. 

Alonso, E.E, Gens, A. and Josa, A. (1990). A constitutive model for partially saturated soils. Geotechnique, N 40,405-430. 

The actual implementation of the element in ANSYS was carried out as a plane rectangular element via the user element interface. The element allows the simulation of partially saturated soils under plane strain as well as under rotationally symmetric stress states. 

Hydraulic conductivity, which is one of the important soil parameters for seepage analysis, is estimated from the pore-size distributions measured by a mercury intrusion porosimeter. Two different relations are indispensable for analysis of partial saturation soil because hydraulic conductivity depends on the capillary potential, which is usually characterized by soil-water retention. 

There are various kinds of the conceptual models for estimating hydraulic conductivity, taking into account the pore size of soil. The models are clustered into the three types from the mathematical treatment of the pore structure of soil as shown in Table 1. All these are models requiring laboratory values on water retention. Therefore, they cannot predict hydraulic conductivity without knowledge of soil-water retention. The hydraulic conductivity of the partial saturation depends on water saturation, which also relates to the capillary potential. It is nessesary to establish two relations for the modelling of pore-water movement through partially saturated soil. 

TABLE 1. Models of hydraulic conductivity for partially saturated soil... 

The fifth factor is the water content of the soil. Electroosmotic flow is promoted at higher water contents. Therefore, high moisture content, and in particular saturated conditions are favored. However, the technique can be used in partially saturated deposits by supplying a pore fluid at... 

The zone between land surface and the water table, which forms the upper boundary of the groundwater region, is known as the vadose zone. This zone is mostly unsaturated— or more precisely, partially saturated— but it may contain a saturated fraction in the vicinity of the water table due to flucmations in water levels or capillary rise above the water table. The near-surface layer of this zone—the soil—is generally partially saturated, although it can exhibit periods of full saturation. Soil acts as a buffer that controls the flow of water among atmosphere, land, and sea and functions as a sink for anthropogenic contaminants. 

The volume of solution in the subsurface, under partially saturated conditions, varies with the physical properties of the medium. In the soil layer, the composition of the aqueous solution fluctuates as a result of evapotranspiration or addition by rain or irrigation water to the system. Changes in the solution concentration and composition, as well as the rate of change, are controlled by the buffer properties of the sohd phase. Because of the diversity in the physicochemical properties of the sohd phase, as well as changes in the amount of water in the subsurface as result of natural and human influences, it is difficult to make generalizations concerning the chemical composition of the subsurface aqueous solution. 

The COj concentration in the subsurface may be different in small and large pores and vary as a function of the aerobic or anaerobic activity of the microbial population. Paul and Clark (1989) showed that a change from aerobic to anaerobic metabolism occurs at an concentration of less than 1% (by volume). The overall aeration of the soil layer is not as important as that of individual aggregates. Calculations show that water-saturated aggregates larger than 3 mm in radius have no in their center (Harris 1981). This means that aerobic and anaerobic zones may coexist in a porous medium even under partially saturated conditions. 

Soils, typically, are not fully saturated by water the soil layer and the region reaching to the water table contain water contents below full saturation. These regions usually are referred to as the vadose zone and said to be unsaturated, but they are more correctly considered partially saturated. The degree of saturation is the ratio of the volume of water to the pore volume within the porous medium. Saturation levels usually are a few percent at land surface (or even zero in perpemaUy dry arid zones) and increase slowly with depth until the region of the capillary fringe (water table), where it increases rapidly to 100%. 

In partially saturated media, the diffusion coefficient also is a function of the volumetric water content, 9. Calvet (1984) showed that the variation in soil water content influences the apparent diffusion coefficient for organic contaminants in two ways by changing the ratio of gas diffusion of volatilizable pollutants to liquid diffusion, because the air-filled porosity is affected, and by modifying pollutant... 

Finally, it should be noted that NAPL infiltration is accompanied by dissolution of some components into the aqueous phase, volatilization of other components into the air phase of the partially saturated zone, and possibly organic complexation with resident soil (Jury and Fluhler 1992). Mass flux of volatilized phases in the near surface can be included in transport equations, as discussed in Sect. 11.3. 

Russo D, Zaidel J, Laufer A (1994) Stochatic analysis of solute transport in partially saturated heterogeneous soils. 1. Numerical experiments. Water Resour Res 30 769-779... 

Despite some different results between the one- and three-dimensional models that might be due to the fact that simple models (such as the one-dimensional MRTM) provide averaged values, results from both models seem to be consistent. The concentrations estimated by the three-dimensional MRTM seem to better reflect the actual phenomena since this MRTM shows that immediate saturation of the top soil layers does not occur (this would occur only if flow and dispersion in the top horizontal planes would have unrealistic high values). Furthermore, even after ten time steps the topsoil layers are only partially saturated. 

A partially saturated granular soil weighs 115 pcf in its wet condition, and has a 10% water content. Estimate the void ratio and porosity. How much would the soil weigh if it were fully saturated What would be its submerged unit weight ... 

The Ca—Mg—HCO3- and Ca—Mg—SO4-type groundwater from the glacial drift aquifers reflects the dissolution of calcite and dolomite by carbonic acid formed in the soil zone, and the production and leaching of secondary gypsum through oxidation of sulfide in the presence of calcite or dolomite under conditions of partial saturation. In cases where the content of carbonates is low, silicate mineral weathering potentially occurs. 

Oil slug — A downward-moving oil mass that often results when oil is spilled on relatively porous soil. The slug-like shape results from the tendency of the descending oil mass to leave behind a funnel of soil that is partially saturated with oil. 

Soil Moisture Potential. Work must be performed to remove water from any saturated or partially saturated material. This work is, in effect, a negative potential energy, and provides a useful measure of the total force holding water in the material. 

This relationship applies equally well to both saturated and partially saturated flow. Equation 3 presumes that flow velocities are sufficiently low to allow equilibrium to occur between the solid and liquid species, but this condition may not always pertain. A further complication on the application of this relationship is that the distribution coefficient is not constant with varying moisture content. The magnitude of l< is strongly influenced by the macro-concentrations of other dissolved species in the soil solution, including calcium, sodium, and hydrogen (pH). Inclusion of these relationships in the solute transport process... 

The calculated transit time - which was under the conditions described taken as identical to the mean residence time - had a value of t = 5-6 years for a 5 cm thick layer of partially saturated natural loan y soil. 

ROCMAS is a finite-element code for analysis of coupled THM processes in partially saturated geological media (Noorishad and Tsang, 1996, Rutqvist et al., 2001). In ROCMAS, the Biot (1941) formulation is extended to partially saturated media through Philip and de Vries (1957) theory for heat- and moisture-flow in soil. This results in a comprehensive coupled THM formulation for partially saturated geological media that includes the coupled processes shown in Figure 2. 

For the analysis of the FEB EX in situ test, a soil mechanics state-surface model was implemented. This state-surface approach provides a better representation of bentonite behavior under partially saturated conditions than a single effective stress approach. The logarithmic state surface model proposed by Lloret and Alonso (1985) was adopted in this analysis. In their model, void ratio (e) is a function of both net mean stress, (Ora" = Om - Pg. where = total mean stress and Pg is gas pressure) and suction (s = Pg -Pi, where Pg and pi is gas and liquid pressures, repectively). 

We present now the extension of the constitutive equation (7) to partially saturated porous media. The material is assumed to be saturated by a liquid phase (noted by index w) and a gas mixture (noted by index g ). The gas mixture is a perfect mixture of dry air (noted by index da) and vapour (noted by index va). Based on most experimental data of unsaturated rocks and soils (Fredlund and Rahardjo 1993), and on the theoretical background of micromechanical analysis (Chateau and Dormieux 1998), the poroelastic behaviour of unsaturated material should be non-linear and depends on the water saturation degree. We consider here the particular case of spherical pores which are dried or wetted under a capillary pressure equal to the superficial tension on the air-solid interface. By adapting the macroscopic non-linear poroelastic model proposed by Coussy al. (1998) to unsaturated damaged porous media, the incremental constitutive equations in isothermal conditions are expressed as follows ... 

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