Development of the surface displacement during repeated rainfalls in sandy model slopes: condition for the increase of the displacement to failure

Abstract

It had been often reported that surface displacement showed an accelerating increase just prior to failure of a slope. Many methods for predicting failure time of a slope were proposed based on the modelling of the accelerating displacement just prior to failure. The mechanical condition which caused the accelerating displacement up to failure has been unclear until now. Measurement of 2-dimensional displacement was conducted on the model slopes under repeated rainfall, and measured data was analysed to examine the condition for accelerating surface displacement up to failure. Measured normal displacement varied with increasing surface displacement and did not converge to constant finally against the idea that the normal displacement approached to constant at steady state in direct shear condition on the slip surface parallel to the slope surface. It was supposed to be due to the difference in the direction of the slip surface to that of the slope surface. The angle α between the direction of slope surface and that of synthetic displacement on the surface, which synthesized the surface and normal displacements on the surface, was introduced to explain the variation of normal displacement with the increase of shear displacement on the slip surface in direct shear condition just prior to failure. The angle α varied at first with increasing surface displacement and converged to constant or quasi-constant just prior to failure on model slopes. It was recognized that the normal displacement converged to constant just prior to failure on the slip surface which was inclined against the slope surface. The angle α could be an indicator to judge whether the surface displacement increased significantly up to failure or not in the case that the slope failed under artificial rainfall.