Laboratory Investigation of the Seepage Induced Response of Granular Soils Under Static and Cyclic Loading

Experimental observations of the seepage induced response of soils under static and cyclic loading are reported. Hydraulic tests were performed using a modified filtration apparatus designed to capture the response of soils subjected to an upward flow. This apparatus could conveniently monitor various factors influencing the onset of seepage induced failures such as spatio-temporal variations in porosity, average and local hydraulic gradients, and the mean effective stress distribution with depth. Under static conditions, heave and heave-piping failures occurred in densely compacted uniform fine-gravels and fine-sands, respectively, and excessive washout (i.e., suffusion) was observed in gap-graded sandy-gravel soil. Despite this gap-graded soil failing in a similar way under static loading, relatively premature suffusion occurred under cyclic loading that could be attributed to the constant agitation of fines and the development of pore pressure within the pore spaces. The reported results and published data under cyclic loading were compared with various static filtration criteria to assess their potential instability, and they revealed that none could accurately capture their cyclic filtration response. However, at the onset of instability, a unique hydro-mechanical correlation could be observed between the magnitudes of local hydraulic gradients and effective stresses calculated using a proposed stress reduction model. Nevertheless, this correlation governing the inception of instability in reported tests established a clear hydro-mechanical boundary with possible implications for practical filter design under cyclic loading conditions.