B) Multiphase behavior of soil

            Conventional or classical soil mechanics assumes soil media to be completely water or air saturated. This is a typical example of a two phase media consisting of soil solids and water/air. The assumption of two phases considerably simplifies the mathematical quantification of the complex phenomena that take place in porous media. Of late, geotechnical and geoenvironmental engineering problems require the concept of three or multiphase behaviour of soil for realistic solution of several field situations. For example: a partially saturated soil is a three phase porous media consisting of air, water and soil. The three phases result in transient and complex behaviour of unsaturated soil. Such cases are encountered while designing waste containment facility where flow characteristics of unsaturated soil need to be determined. When it comes to soil-water-contaminant interaction there are multi-phase interactions involved. The migration of non-aqueous phase liquid (denoted as NAPL) through porous media is a typical example. Fluidized bed, debris flow, slurry flow, gas permeation through unsaturated soil media are some problems where multiphase behaviour becomes important. Such studies are handy while designing remediation scheme for contaminated soil and groundwater, which are very important issues for the geoenvironmental engineer to solve. Understanding the complex interaction of different phases is challenging and has paved way for the study of multiphase behaviour of porous media. Such a realization has generated a lot of interest in the research fraternity for developing experimental and mathematical procedures for clearly delineating the phenomena in multiphase porous media.

C) Role of soil in geoenvironmental applications

          All civil engineering structures are ultimately founded on soil and hence its stability depends on the geotechnical properties of soil. Conventional geotechnology is more concerned about rendering soil as an efficient load bearing stratum and designing foundations that can transfer load efficiently to subsurface. Apart from this, soil is directly related to a number of environmental problems, where the approach should be a bit different. Consider the case of groundwater recharge as shown in Fig. 1.1. The infiltration and permeation property of homogenous or layered soil mass above water table decides the rate of recharge. In this case, a geotechnical engineer has to work closely with hydrogeologists for deciding different schemes of artificial groundwater recharge.

Fig. 1.1 Artificial groundwater recharge

          Consider the case of waste dumped on ground surface. During precipitation, water interacts with these wastes and flow out as leachate. When the leachate flows down, soil act as buffer in retaining or delaying several harmful contaminants from reaching groundwater. Such a buffering action obviously depends on the texture and constituents of soil mass. While designing a waste containment facility, the role of soil in such projects is enormous. A coarse grained soil with filter property is required for leachate collection where as a fine grained soil is required for minimizing flow of leachate. These are two entirely different functions expected from soil in the same project. The cap provided for waste dumps also necessitate the use of specific type of soils with the required properties. The amount of water that infiltrates into the waste below is minimized by soil used in such caps. Special type of high swelling soils is used as backfills for storing high level radioactive waste in deep geological repositories. Another important geoenvironmental problem, namely, carbon sequestration uses the geological storage capacity for disposal of anthropogenic CO₂ to mitigate the global warming. Therefore, soil plays a very vital role in geoenvironmental projects and the property by which it becomes important is problem-specific.