Study of Critical Hydraulic Gradients for Seepage-Induced Failures in Granular Soils

This paper reports on a series of laboratory hydraulic tests on a select range of granular soils compacted at relative densities between 0% and 100%. The critical hydraulic gradient at the onset of seepage failure (i.e., heave and suffusion) is considerably smaller than unity for internally unstable (i.e., nonuniform) sand–gravel mixtures due to stress reduction in their finer fraction. For example, stable uniform fine sands have been shown to exhibit heave at hydraulic gradients $\ge 1.0$, whereas sand–gravel mixtures suffer from suffusion at hydraulic gradients $\ll 1.0$. The boundary friction from the cell walls of test equipment would influence the development of heave, while suffusion is controlled by interparticle friction. In this study, the critical hydraulic gradient is modeled theoretically by considering the effects of interparticle and boundary frictions, and stress reduction in the soil. The experimental results from both this and past studies are used to verify the proposed model, which showed good agreement with experimental observations with less than 5% standard error.