Unsaturated materials soils

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 ... 

Capillary barrier cover systems consist of a finer-grained soil layer (like that of a monolithic cover system) overlying a coarser-grained material layer, usually sand or gravel, as shown conceptually in Figure 25.3. The differences in the unsaturated hydraulic properties between the two layers minimize percolation into the coarser-grained (lower) layer under unsaturated conditions. 

Finer-grained materials such as silts and clayey silts are typically used for monolithic ET cover systems and the top layer of a capillary barrier ET cover system because they contain finer particles and provide a greater storage capacity than sandy soils. Sandy soils are typically used for the bottom layer of the capillary barrier cover system to provide a contrast in unsaturated hydraulic properties between the two layers. Many ET covers are constructed of soils that include clay loam, silty loam, silty sand, clays, and sandy loam. 

A second approach is to obtain (extract) water as it naturally occurs in soil pores. Typically, a porous ceramic cup (other materials are available) is placed in an unsaturated soil, either in the field or laboratory. A vacuum is applied (slightly more negative pressure than the water in the soil pores see also Chapter 5) to the ceramic cup via tubing to move water into a receiving container. This water can be analyzed for all its constituents. A reason for obtaining this type of soil water sample is to analyze it for one specific constituent such as a herbicide, insecticide, or pollutant. Water extracted in this way may also better represent the concentration of the analyte of interest to which plant roots are exposed. 

The viscosity of separate LNAPL products varies significantly, ranging from far less to many times that of water. Flow of LNAPL in the unsaturated zone is largely dependent upon viscosity and soil grain size. Finer-grained materials have a higher residual saturation of water, which restricts the number of pores available for LNAPL entry in this region. 

The squeezing process involves the expulsion of pore water from the material being compressed. In general, the material consists of solid particles (mineral phase), and spaces (voids), which in an unsaturated environment such as a soil, contain both air and water. When a squeezing stress is applied to a water-saturated material, its volume decreases by three main mechanisms ... 

The texture of the porous material forming the saturated and unsaturated zones may range from coarse sands, through finer-textured silt, to extremely fine textured clay. Loams, consisting of roughly equal parts of sand, silt, and clay, are generally considered optimum for agricultural soils (Fig. 3-2). 

The reversal of relative hydraulic conductivities of unsaturated fine and coarse textured porous media is an interesting consequence of the greater suction required to remove water from fine-textured material. As shown in Fig. 3-24, a coarse-textured sandy soil has a higher hydraulic conductivity than a fine-textured clayey soil at full saturation, when i)j is zero. As pore water pressure decreases, however, the soil water content decreases to a lesser extent for the fine-textured soil a point is reached at which the fine-textured soil holds appreciably more water than the coarse-textured soil and has a higher hydraulic conductivity because many of its pore spaces, despite their smaller size, remain filled with water. 

The bathtub effect occurs, in part, because most wastes have much higher hydraulic conductivities than the natural material into which they are placed they may also have very different unsaturated soil—moisture characteristics. The hydraulic conductivity of some wastes can be reduced by compaction. The bathtub effect also occurs because more infiltration enters the disposal excavation than would under normal undisturbed conditions. Trench covers may be constructed to achieve the desired hydraulic conductivity and to limit infiltration for the required period of containment or until compaction of the wastes occurs however, it is difficult to maintain the trench covers. The covers must withstand attack by plants, weather (freeze—thaw, wet—dry), erosion, and strains caused by consolidation within the trench. Most trench covers are not capable of meeting these demanding requirements without costly long-term maintenance programs. The cover should be designed to allow for expected consolidation and to utilize hydro-geological concepts of saturated and unsaturated flow systems present at the site. 

Sorption. The interaction of a pesticide with the soil solids or organic matter, termed sorption, retards the pesticide movement through the unsaturated zone. This interaction is being increasingly associated with the soil organic matter (18), with the extent of the interaction dependent upon the type of organic materials and the molecular characteristics of the pesticide (19). Linear and Freundlich isotherms have been used to describe sorption, where... 

For stationary flow conditions, D and v are independent parameters describing the transport process. In transient conditions, however, the relationship between D and v must be taken into account. Experimental evidences show that for transport in homogeneous saturated porous media, D is a monotoneous function of v. In unsaturated media, this relation becomes extremely complicated since the transport volume 6<,u changes with the water flux. Therefore, the structure of the water fdled pore-space and, hence, the flow field depends on the saturation degree (Flury, M. et al. (1995)) so that the variance of local velocities and the mixing time cannot be simply related to the mean advection velocity. As a consequence, no validated theoretical models exist to calculate the relationship between D and v for unsaturated soils and the dispersivity X cannot be considered to be a material constant, i.e. independent of 0. 

Abstract This paper introduces the current state of research on the coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes in geo-materials and applications for geotechnical and geo-environmental engineering in China. The paper puts equal emphasis on both the achievements of experimental results for fundamental studies, and the applications in rock mechanics and foundation engineering, tunnel engineering, geothermal mining, unsaturated soil and dynamic consolidation. At the end, it presents a model for chemical mining. 

We briefly recall a model for hydromechanical coupling in unsaturated soils that is implemented in the used computer code. The relations and their application in the present context is extensively discussed in Klubertanz (1999) or Klubertanz et al. (1999), a broad discussion of the employed material laws for saturation can be found in Seker (1983). 

Hutter, K., Laloui, L. Vulliet, L. 1999. Mixture models of saturated und unsaturated soils - A clarification, Int. J. of Cohesive - Frictional Materials 4 295 - 338... 

Soil is a complex matrix consisting of air pockets, water, mineral matter and organic matter. It can vary enormously in its composition and texture and consists of various layers with different properties. The upper layer, which is more directly involved in the input of chemical substances and where distribution and fate processes are more complex, is the so-called vadose or unsaturated zone. This means that the pore spaces in the soil materials are not fully filled with water. Below the vadose zone, there is the saturated zone, where pore spaces are completely filled with water. The top of the saturated zone is the water table, corresponding to the level to which water will rise at atmospheric pressure in a hole dug in the earth. 

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