Mechanisms and characteristics of sand seepage deformation under groundwater level fluctuation scenarios

Abstract

Seepage deformation in sand results from complex water-soil interactions, which are the primary reasons of sand surface collapse, as well as instability and deformation in dam foundations, building foundations, and slopes. Frequent fluctuations in groundwater levels cause changes in the direction, velocity, and pore water pressure of groundwater within the sand. Further research is essential to fully understand the characteristics and mechanisms of sand seepage deformation under varying groundwater conditions. In this study, natural undisturbed sand samples were collected. Laboratory seepage deformation tests were conducted to simulate continuous rises and falls in groundwater levels, exploring the response characteristics of internal erosion and hydraulic behavior of the sand under varying groundwater flow rates and directions. The results show that: As groundwater flow rate increases, the sand undergoes multiple episodes of seepage deformation, which includes the processes of structural stability, seepage deformation, and seepage failure. Initially, the hydraulic gradient for seepage deformation is small, and the particles carried by seepage are small. With a further increase in groundwater flow velocity, the hydraulic gradient rises, larger sand particles are migrated by seepage, and seepage failure may eventually occur. When the karst groundwater level is lower than the elevation of the sand bottom (H2 < z2) and the sand bottom is in a negative pressure state, the hydraulic gradient of seepage deformation is usually smaller than that observed in the other two states of positive pressure. In these cases, pore water pressure exerts an upward buoyant force, while in the negative pressure state, the pore water pressure transforms into downward suction. This downward suction aligns with the direction of gravitational forces and downward seepage force acting on the sand, making seepage deformation of the sand more likely. Sands with greater unevenness, finer particle, and lower density are more prone to seepage deformation but failure at different hydraulic gradients.