Undrained behaviour of two sands in direct simple shear loading conditions

Shear strength and stress3strain behaviour of soil can be investigated using different types of laboratory tests. Among them, triaxial compression and direct shear tests are commonly performed for practical engineering. However, in many field problems, the stress state during shearing is similar to the simple shear conditions. The response of soil during shearing depends on several factors, including the size and shape of the particles, relative density, consolidation pressure and drainage conditions. For sand, drained response governs many geotechnical problems as it is highly permeable; however, the undrained behaviour is equally important for other cases where rapid loading occurs (e.g., static liquefaction and earthquake loading). Also, undrained tests give some soil parameters that could be related to drained behaviour. In the present study, the behaviour of two poorly graded sands, namely silica sand and Ottawa sand, is investigated by conducting a series of constant height (equivalent undrained) direct simple shear tests. The particle shape (angularity) of these sands is different. Monotonic undrained tests are conducted on dry sand specimens of varying densities and consolidation pressures. Steady state, quasi steady state, and ultimate steady state develop in the specimens due to strain-softening and strain-hardening, depending upon the density and consolidation pressure. Both sands initially show contractive behaviour during shearing, even for very dense conditions, due to the rotation of principal stresses. Particle shape plays a major role in dilation, normal stress change (equivalent pore pressure) and overall stress3strain behaviour. The friction angle mobilized at the phase transformation for these sands in direct simple shear (DSS) tests is not independent of soil density and consolidation pressure. However, the mobilized friction angle at 10% shear strain is independent of density but decreases with an increase in consolidation pressure. This mobilized friction angle from constant height (undrained) tests is similar to the friction angle mobilized at large shear strains (~20%) in constant stress (drained) DSS tests.