Undrained Monotonic and Cyclic Strength of Sands

Abstract

Results of torsional shear tests on specimens of reconstituted 20‐30 Ottawa sand are presented to resolve inconsistencies in prevailing interpretations of previously published test results regarding: (1) The conditions leading to limited or steady‐state flow deformation under monotonic loading; and (2) the state conditions marking the initiation of strain softening behavior under either monotonic or cyclic undrained loading. It was found that the effective stress path in monotonic undrained shear appears to constitute a state boundary that controls the initiation of strain softening under undrained cyclic shear loading. Evidence is presented in support of a new concept of the collapse of sand fabrics. It is used to explain the sudden increase in pore‐water pressure associated with the initiation of strain softening behavior on loading, or with the development of a condition of zero effective stress after repeated cyclic unloadings beyond a critical level of mean effective normal stress. The concept also explains why steadystate conditions in drained shear are not necessarily the same as in undrained shear. The implications of the new findings on the determination of the liquefaction potential of sands are discussed.