Cohesionless Soil Fabric and Shear Strength at Low Confining Pressures

Mohr-Coulomb strength is significantly influenced by vertical effective confining pressures. However, traditional Mohr-Coulomb mechanics do not adequately describe soil behavior under low confining pressures. Herein we present results of total stress laboratory testing to investigate the influence of soil fabric and deviance from Mohr-Coulomb strength for an engineered cohesionless silty sand (SM) under low to zero vertical confining pressure environments. Specimens were prepared and tested under drained simple shear, consolidated-drained triaxial, and unconsolidated-undrained triaxial conditions with total stress confining pressures between 0 kPa and 100 kPa. The data show that the internal granular friction increasingly dominates the total shear resistance of the material for confining pressures below 100kPa, and the soil fabric has a greater influence on the shear resistance for a cohesionless soil continuum. Therefore, for confining pressures less than 100kPa, a better representation of the relationship between shear stress and normal stress can be found by curving the failure envelope and rejoining the 33 degree line observed at high pressures (MPa range) at a confining pressure between 50 kPa and 100 kPa.