The influence of pore fluids on shear strength and stress-strain behavior of granular soils

When a dry granular soil is subjected to an external load, the stresses are transferred through mineral to mineral contacts. The shear strength of such a soil is governed by its contact properties. Presence of a thin layer of fluid between mineral to mineral contacts in a saturated soil will alter its contact properties. Therefore any change in the physical properties of pore fluid can logically be reasoned to influence the deformation characteristics of granular soils.; To investigate the influence of pore fluid viscosity on shear strength and stress-strain behavior of a soil, a series of consolidated drained triaxial tests were performed on a silty sand. The samples were prepared with four different pore fluids of varying viscosities and tested subjected to different consolidation pressures. Tests were performed with the aid of an automated triaxial testing apparatus.; Since the stress-strain behavior of this silty sand can not be normalized, the variation of residual shear strength with critical void ratio was used for the analysis of results. When residual shear strength values were plotted against its critical void ratios, a family of curves that could be identified by their pore fluid viscosities were obtained. For a given critical void ratio, as pore fluid viscosity increased the residual shear strength was found to decrease.; To provide a mechanistic explanation to the changes in the strength characteristics due to pore fluid viscosity, a microscopic model was used to study the changes in the contact properties at particle level. The microscopic numerical model TRUBAL, based on the DEM was used in this research to simulate the mechanical behavior of a granular soil subjected to a triaxial stress condition. Since this numerical model was developed to simulate the behavior of a dry granular assembly of spheres, it was modified to incorporate the effects of pore fluid viscosity by using Reynolds equation for hydrodynamic lubrication. The modified TRUBAL program was used to simulate deformation characteristics of saturated granular soils.; The modified program was then used in a qualitative study to simulate stress-strain behavior of a granular assembly having different pore-fluid viscosities, subjected to consolidated drained triaxial test conditions. Numerical simulations showed a similar trend as observed in the physical tests.