Evaluation of the intergranular coefficient of friction using distinct element modelling

Abstract

The shear strength of soil is generally expressed using macro-scale parameters, namely internal friction angle and cohesion. However this macro behavior is the resultant of dauntingly high number of intergranular interactions and the granular properties that control the mechanism of these interactions in the microscopic scale, such as granular size, shape, and surface texture. The aim of this study is to understand the relationship between the friction angle of the continuum and the intergranular coefficient of friction. The behavior of the granular materials under shear is dependent on the properties of individual grains that are interacting with each other. Distinct element method considers these interactions and the governing laws are defined for the contacts of these individual grains. Therefore, computer simulations of the soil mechanics laboratory experiments using distinct element programs such as PFC2D will give a better understanding of phenomena at particle level. The microproperties of granules can be identified in two scales; surface texture and surface shape. Surface texture is expressed by roughness of the grain and surface shape is expressed by roundness and sphericity. The coefficient of friction between interacting particles is mainly caused by the roughness of the grains. Physical measurement of roughness is not possible with known laboratory experiments, unless using specially developed measurement techniques. Therefore, intergranular coefficient of friction is an unknown parameter that can be investigated by numerical modeling. For this purpose, numerical models of angle of repose test are created and coefficient of friction value is varied until the angle of repose in the DEM model matches the physical one. Different dry sand specimens according to their shape parameters and gradation are used in the laboratory experiments. Generating an initially compacted assembly at the desired porosity is problematic in numerical models when clumps are used to reflect particle shapes. At this point, angle of repose test has an important advantage over other shear strength tests, such that there is no need to control porosity at the generation of initial assembly of particles. Therefore, porosity is not an initial condition for the angle of repose test. As a result, experimental and numerical data are compared and conclusions are drawn about the value of interparticle coefficient of friction.