Application of Unsaturated Soil Theory in the Investigation of Slopes Stability

In this research the stability of slopes, made of unsaturated soils, constructed by compacted layers has been studied. The properties of unsaturated soils such as the stability of slopes made of them and shear strength have been investigated, and the priory theoretical backgrounds have been cited from the related literature.In this study the program “UDAM” (which have been already developed by Gatmiri and Nanda for the unsaturated soils) has been specially applied.As the development of pore water pressure and also pore air pressure developed during (and after that) the embankment construction and the major effective factors on the stability, so the computations of the quantities of these pore pressures were one of the main purposes of this research.In order to evaluate the stability of unsaturated slopes, it was necessary to a computer program which be suitable for analyzing the unsaturated soils. Because of this, the program CASSAP which has already been developed by Syrous Aryani, has been modified to be able to catch the pore water pressure and the suction of the soil and compute the stability safety factor.The results of these two mentioned programs (UDAM & MCASSAP) have illustrated in different diagrams and figures the final conclusion revealed some new and interesting results.Following is a brief description of UDAM program:The main concepts in the computations are based on the differential equations governing the equilibrium equations for unsaturated porous media. These equations which represent the relationships between the different affecting parameters of soil, air and water in equilibrium conditions are as follows:where is the porosity; the degree of water saturation; the degree of air saturation; are the densities of soil, water and air respectively; and are the velocities of water and air respectively; and is the acceleration.Furthermore, since the variations of void ratio and saturation degree are functions of some physico-mechanical properties of the medium, these variations should be expressed inside a three dimensional space, which is named as the state surface. In UDAM program, the equations of state surface for void ratio and saturation degree are as follows: (Gatmiri and Delage, 1995; Gatmiri et al., 1993)where and are the air and water pressure respectively; and are the net normal stress and matric suction respectively; is the atmospheric pressure; are parameters. Then the permeability coefficients of water and air as functions of parameters and are calculated by the following relationships, respectively: