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 (H-2 < z(2)) 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.

Major flood propagation processes often cause instability and damage to the ancient waterfront city walls. To quantitatively reveal the impact of major floods on the stability of ancient city walls, this paper takes Lanxi's ancient city wall as a study object and constructs a numerical model to investigate the influence of the major flood process in 2017 on the wall stability and reveals the varying laws of its seepage, displacements, maximal shear stresses and safety factors with flood propagation time on the basis of flood level data, combining indoor experiments and field observations. The results show that flood level variations significantly affect the PWPs (pore water pressures) of the fillings behind the wall. During the flood period, the maximal horizontal and vertical displacements are mainly induced by soil extrusion and deformation, and the maximal shear stresses of the outer and inner wall also significantly increase. The changing rates of the wall's safety factors measurably exceed that of the flood level. The flood level variation range dramatically affects the safety factors when it changes near and above the wall foot. The minimum of the safety factors decreases with the increasing flood level falling rate when it drops near the wall foot at different rates. The ancient city wall usually does not experience serious instability under a single major flood. This study can provide a theoretical basis for the selection of reinforcement measures for flood control ancient city walls and the protection of ancient waterfront buildings.

Foundation settlement and collapse disasters resulting from seepage deformation in hydraulic-filled islands and reefs have been observed in the South China Sea, but the underlying failure mechanism and characteristic remain unclear. This study aims to investigate the influence of compactness and fine particle content on the seepage deformation of gap-graded coral sand and revel the characteristics and mechanism of seepage deformation of gap-graded coral sand through laboratory seepage deformation tests. The results indicate that the seepage deformation failure mode of gap-graded coral sand is influenced by the content of fine particles which undergo an evolution process from continuous piping to discontinuous piping to boiling. Particle loss is affected by the constraints between coarse particles, and the ability of different particle contact forms to restrict the loss of fine particles is different. Moreover, irregular particle morphology increases intergranular constraints, enhancing the coral sand's resistance to seepage deformation compared to standard quartz sand. Based on these findings, the instability coefficient was used to consider the influence of particle morphology and inter-particle contact on the seepage deformation. A hydraulic criterion for the internal stability of coral sand was established, demonstrating its versatility. Furthermore, the applicability of existing geometric criteria in evaluating coral sand was analyzed. The existing methods were found to be inaccurate in evaluating the internal stability of coral sand specimens with a fine particle content below 20 %.