Flow Failure of Saturated Sand under Simultaneous Monotonic and Cyclic Stresses

An estimate of the residual strength achieved by saturated Toyoura sand during flow failure is proposed, based on laboratory tests where not only monotonic loading is applied alone, but a simultaneous cyclic stress is also superimposed. This superimposed or complementary cyclic stress represents the minor seismic shaking following strong shaking that is still acting on a liquefied sand driven by the in situ static stress. Hollow cylindrical samples are torsionally sheared under monotonic conditions and axially loaded under cyclic conditions. Field-like stress conditions are reproduced on a 45(degree) plane. The cyclic axial load produces more torsional deformation and reduction of the residual strength. A procedure for the estimate of this reduction is proposed. The empirical formula and test results are validated only for Japanese standard Toyoura sand specimens prepared by moist tamping.