AbstractNumerous studies have been conducted to evaluate the influence of internal erosion on the geotechnical properties of soil under unidirectional seepage. Yet cyclic gradients prevail in natural or engineering systems, and the mechanical consequences of soil subjected to cyclic hydraulic gradient reversal have not been addressed in the literature. This paper reported the first batch of internal erosion tests on a modified stress-controlled triaxial apparatus that allows alteration of seepage direction and hydraulic gradient reversals. Three different schemes of hydraulic gradient application, namely monotonic stepwise loading, cyclic reversal after monotonic loading, and direct cyclic loading, were adopted. In each test, the cumulative particle loss, deformation, hydraulic conductivity, and posterosion shearing behavior were evaluated. It was found that the cyclically reversed hydraulic gradient resulted in more severe erosion at the same level of mean hydraulic gradient. Microscopic analysis was also conducted to investigate the mechanism of internal erosion under cyclic hydraulic gradient reversal, in which mitigation of clogging within the soil matrix was observed and found to facilitate the progression of internal erosion.