Instability and liquefaction of granular soils under undrained and partially drained states

An experimental investigation of the initiation of instability (liquefaction) in saturated sand under partially drained conditions is presented. The domain of stress space in which this instability develops is identified under various degrees of drainage, and its relationship to the zone of instability observed under undrained shear is explored. It is shown that partially drained conditions may render sand unstable that would otherwise be stable in a completely undrained state. Extremely small void ratio increases that cannot be regarded as physical loosening of sand, if sand is partially drained, contribute to instability. Implications of the findings are discussed in practical problems of liquefaction.Key words: sand, liquefaction, undrained, partially drained, instability, triaxial test.