Influences of Suction on Plate Load Tests on Unsaturated Silty Sands

Abstract

In this study, results of plate load tests performed on saturated and unsaturated silty sand samples are presented. Very little is known about how to interpret the plate load test when performed on unsaturated soils comprising silt–sand mixtures because the few published studies have been limited to either unsaturated clean sands or clays. The new plate load test results are interpreted using a bearing capacity equation, applicable when nonuniform χs profiles prevail in the soil, where χs is the contribution of suction to the effective stress. The interrelation between χs, strength, and bearing capacity is shown. There is good agreement between measured and computed bearing capacities. Assuming χs remains constant during a plate load test, compared to assumptions of constant suction or constant moisture content, simplifies interpretation without significant loss of accuracy. For a particular soil density, the bearing capacity for an unsaturated condition is found to be much larger than that for a fully saturated condition. It is also shown that the effect of suction on the bearing capacity is dependent on the initial location of the hydraulic state on the soil–water characteristic curve. Failing to account for suction and the hydraulic loading history may lead to incorrect interpretations. It is also demonstrated how plate load tests may be used to infer the value of χs at the soil surface.