Development of the shear displacement of sandy soil due to absorption under constant shear stress for creep failure

Measurement of the displacement and pore pressure in physical model slopes and natural slopes revealed that not only an accelerative increase up to failure but also a decelerative increase occurred in surface displacement under a constant groundwater level, which could be recognized as creep deformation under constant stress. The displacement increased significantly at first in both types, which made it difficult to evaluate whether the displacement developed to the point of failure at the start of the increase. It was necessary to find an indicator for evaluation as the first step of the prediction of an onset of slope failure. Measurement of shear and normal displacements of the sandy specimen in an inclined direct shear box with increasing water content was conducted to examine the indicator. The increase in the shear displacement was categorized into three types: the accelerative increase up to failure with increasing volumetric water content (VWC), the accelerative increase up to failure under constant VWC just prior to failure, and the decelerative increase under constant VWC. It was recognized that a constant VWC corresponded to constant suction from the experimental data. Shear displacement increased up to failure with the monotonic decrease in void ratio just prior to failure in the case of increasing VWC. The void ratio monotonically decreased under constant VWC in the case of shear displacement termination under constant VWC, while it significantly varied at first and then converged to a constant value just prior to failure in the case of shear displacement increase with constant VWC. The ultimate void ratio under the same stress conditions might have been unique. These facts revealed that the void ratio can be recognized as an indicator of the failure of specimens under creep deformation according to absorption. When the void ratio converges to a constant value under constant VWC, the shear displacement might increase up to failure, while the void ratio varying under constant VWC indicates that the shear displacement terminates before failure.