Experiments and modelling of silty sands susceptible to static liquefaction

Most historic cases of liquefaction have been found to occur in alluvial (water) deposited silty sands. Currently, the effect of non-plastic fines (particles smaller than No. 200 sieve) on the liquefaction behaviour of sands is viewed to be either negligible or its presence actually inhibits liquefaction. Undrained triaxial compression test results performed on silty sands clearly indicate a direct correlation between the quantity of finer, non-plastic constituents and the liquefaction potential of granular soils. Increasing the fines content increases the liquefaction potential, even though the density increases. Complete static liquefaction occurs at low confining pressures. As confining pressures increase, the liquefaction potential decreases resulting in increased stability. Thus, silty sands exhibit a ‘reverse’ pattern of soil behaviour with confining pressure. Drained tests indicate both a large contractive volume change and a suppressed friction angle at low confining pressures, and this explains the undrained behaviour. It is hypothesized that the mechanism underlying this behaviour is related to the formation of a particle structure between the large and small grains which creates a highly compressible soil fabric. This ‘reverse’ behaviour pattern makes predictions of static liquefaction of silty sands difficult. However, simple modifications to the Single Hardening Model yield surface formulation enables predictions of this behaviour pattern.