Static liquefaction of very loose sands

The results of an experimental study on two different sands are presented. Drained and undrained triaxial compression tests were performed on Nevada sand at four different initial relative densities, namely 12, 22, 31, and 42%. Drained and undrained triaxial compression tests were performed on Ottawa sand at an initial relative density of 0%. All tests were performed under monotonic loading. At these low densities anomalous soil behavior was observed. Undrained triaxial compression tests exhibited complete static liquefaction (zero effective confining pressure and zero stress difference) at low confining pressures. As confining pressures were increased, the effective stress paths indicated increasing resistance to liquefaction by showing increasing dilatant tendencies. This behavior is opposite to that of normal soil, where increasing confining pressure results in decreasing dilatant tendencies. The presence of fines (particles smaller than 0.074 mm) in the soil was identified as the likely cause of this behavior. It was hypothesized that the fines and larger particles create a particle structure with unusually high compressibility at low confining pressures. Drained test results indicated inherently large contractive volumetric strains, which under undrained conditions create sufficiently large pore pressures to produce complete static liquefaction. Anomalous behavior was exhibited by the drained friction angles at low densities, where the friction angle was found to first increase with increasing confining pressure, followed by the normal pattern of decreasing friction angles.