DISCRETE ELEMENT METHOD MODELING OF BEHAVIOR OF A GRANULAR SOIL AND THE EFFECT OF UNSATURATED CONDITIONS ON SHEAR STRENGTH PARAMETERS

Soils consist of solid particles, pores, and pore uids (air, water, other uids), sometimes with interparticle bonding, to form a complex fabric. The behavior of natural soils is complex and dicult to model adequately by conventional non-linear elastic models or elasto-plastic ones. Micromechanics may prove helpful as an alternative method to overcome this complexity and physically describe its behavior. Micromechanics is used in this aim by two approaches. The rst is an experimental approach, which tries to understand the granular behavior by the laboratory experiments on sands, glass spheres and rods to observe the fabric changes, contact distribution, and shear bands. The second approach uses Discrete Element Method (DEM) to numerically simulate the soil behavior. DEM considers a granular medium as an assemblage of particles interacting with each other in contact points. DEM is able to monitor the evolution of micro variables as force in contacts and their orientations. In DEM, the contact between the particles is detected and then contact force is calculated. These contact forces are used to calculate the displacements and rotations of each parti- cle and their new coordinates. Literature review shows that DEM is a useful tool, particularly in soil mechanics. To predict the shear strength of the soils, the Coulomb theory revised by the eective stress theorem is used, which is suitable for dry or saturated conditions. However, unsaturated condition is the main state of soils in arid or semi-arid regions. From micromechanical point of view, the main dierence between the dry or saturated condition and unsaturated condition is in the interparticle force due to menisci formed by pore water and air. In this paper, after formulating the geometry of the menisci and attraction force, they are programmed in a DEM code, PFC2D. The eect of saturation ratio on capillary force and its eect on global behavior of the medium is investigated. The simulations show that attraction forces, due to capillarity, are re ected as the cohesion on macro scale, but the internal friction is not aected by capillary eect.