Shear strength of cohesionless soils at low stress

Abstract

Mobilisation of shear strength in shallow translational slips occurs at very low effective stress. Accordingly, a series of direct shear box tests was performed to characterise the strength of colluvium, under drained conditions, at vertical effective stresses between 5 and 20 kPa. Results from in situ tests on undisturbed block samples of moist soil are compared with laboratory tests on dry reconstituted specimens of the soil matrix only. The in situ tests determine a mean value of peak or maximum angle of shearing resistance between 58° and 64°, and a mean value at large displacement between 46° and 52°, with 46° believed representative of shearing at constant volume. Mobilisation of similar angles of shearing resistance, between 46° and 48°, at large displacement in laboratory tests on the 25 mm minus fraction indicates that the soil matrix is controlling strength. The values at large displacement are attributed to mineralogy, grain shape and angularity, and grain size distribution of the soils. Observations of the angle of repose corroborate the measured values at large displacement. The peak or maximum values are consistent with the phenomenon of stress dilatancy and, although very high, compare well with limited data reported for coarse granular soils at very low effective stress.