Instability and Plastic Flow of Soils. I: Experimental Observations

According to Drucker's postulate, materials should become unstable when the second increment of plastic work is negative. A series of conventional triaxial tests on fully saturated and partly saturated specimens is performed to study regions of stable and unstable behavior. Experiments on specimens exhibiting dilation or compression show that, during drained conditions, stress paths directed in the region where Drucker's postulate is violated do not produce unstable behavior. However, for specimens that compress and have degrees of saturation higher than critical, there is a change in drainage conditions causing the stress path to be directed within the region where plastic loading occurred with a negative second increment of work and instability. Previously stated stability conditions for materials with nonassociated flow are analyzed in view of the experimental results on saturated and partly saturated sands.