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.

中文翻译：

---

地下水位波动情景下砂渗变形的机理与特征


砂土渗流变形是复杂的水土相互作用造成的，这是砂土表面坍塌的主要原因，也是坝基、建筑地基和边坡不稳定和变形的主要原因。地下水位的频繁波动会导致沙子中地下水的方向、速度和孔隙水压力发生变化。进一步研究对于充分了解不同地下水条件下砂渗变形的特征和机制至关重要。在这项研究中，收集了天然未受干扰的沙子样本。进行了室内渗流变形测试，模拟地下水位的连续上升和下降，探索了不同地下水流速和方向下砂土内部侵蚀和水力行为的响应特征。结果表明：随着地下水流速的增加，砂土发生多次渗流变形，包括结构稳定、渗流变形和渗流破坏等过程。最初，渗流变形的水力梯度较小，渗流携带的颗粒很小。随着地下水流速的进一步增加，水力梯度上升，较大的沙粒通过渗流迁移，最终可能发生渗流失效。当岩溶地下水位低于沙底高程（H2 < z2）且沙底处于负压状态时，渗流变形的水力梯度通常小于其他两种正压状态下观察到的梯度。在这些情况下，孔隙水压力施加向上的浮力，而在负压状态下，孔隙水压力转化为向下的吸力。 这种向下的吸力与作用在沙子上的重力和向下渗流力的方向一致，使沙子更容易发生渗流变形。不均匀度较大、颗粒较细、密度较低的砂土在不同水力梯度下更容易发生渗流变形和失效。