Wave-induced liguefaction and Flow Deformation in Sand Beds

本报告首先描述了在具有水平表面的沙堆上进行的一系列离心机波槽试验的结果。实测的不同土层深度的超孔压痕迹清楚地表明，在剧烈的波浪条件下，液化首先发生在浅层土层，然后向下传播，最终使整个土层进入液化。利用高速摄像机拍摄了波浪作用下流体/土体界面的运动过程。结果表明，在浅层土壤发生液化时，流体/土壤界面开始发生明显的振动，随着液化前沿的向下推进，土壤振动的幅度随时间显著增加，当整个土层处于完全液化状态时，振动幅度趋于恒定。这些有趣的观察结果导致了一个简单但可行的理论模型的发展。它是Lamb(1932)双流体层理论的推广，在这个意义上，液化的土壤被视为具有特定质量密度的无粘流体，并被亚液化的弹塑性土壤沉积所覆盖。通过一系列的离心机波浪水槽试验，研究了较粗糙的散体坡脚部及其周围的土体失稳。试验结果表明，在自由场的浅层土壤发生液化时，碎石丘的脚趾开始以唯一可辨别的速度下沉。事实上，随之而来的是在波浪加载过程中液化区的广泛扩散，从而导致整个碎石丘深入液化的地基沙中。

This report first describes results from a range of centrifuge wave tank tests performed on sand deposits with a level surface. The measured traces of excess pore pressures at different soil depths clearly indicated that under severe wave conditions, liquefaction first took place at shallow soil depth and then propagated downwards, eventually bringing the entire soil bed into liquefaction. The motion of the fluid/soil interface during wave loading was captured by means of a high-speed CCD camera. The images taken showed that the fluid/soil interface began vibrating significantly upon the occurrence of liquefaction at shallow soil depth, and that the amplitude of soil vibration increased markedly with time in accordance with the downward advancement of the liquefaction front and leveled off at a constant value when the entire soil bed was in a state of complete liquefaction.These interesting observations led to the development of a simple yet workable theoretical model. It is an extension of Lamb's (1932) two-fluid-layer theory in the sense that the liquefied soil is regarded as an inviscid fluid with a particular mass density and is underlain by a deposit of sub-liquefied, elastoplastic soil. The performance of the theoretical model compares favourably with what has been observed in the centrifuge wave experiments.Furthemore, efforts were directed to investigating the soil instability at and around the toe of much coarser granular slopes, through a series of centrifuge wave tank tests. The results from the experiments showed that upon the occurrence of liquefaction at shallow soil depth in the free field, the toe of the rubble mound started settling at an only discernible rate. This was in fact followed by extensive spread of the liquefied zone in the course of wave loading, thereby causing the entire body of the rubble mound to settle deep into the liquefied foundation sand.