Influence of soil and hydraulic conditions on the processes of internal erosion, cavity formation and collapse behind coastal structures

Collapse of cavities formed by internal erosion has occurred at the back sides of seawalls and quaywalls all over the world. Here, the process of internal erosion was investigated by conducting prototype-scale experiments. A series of processes of cavity formation and collapse were reproduced considering different grain size distributions and water level conditions. Suction, that is, negative pore water pressure relative to atmospheric air pressure, affected the formation of cavities and the timing and scale of their collapse. The scale of the collapse was large under high suction. The types of hydraulic force also affected the cavity formation. A symmetrical cavity was formed under one-dimensional water level fluctuations, whereas the cavity expanded in size toward the waveincident boundary under incident waves. Cavity collapse was found to be caused by a decrease in suction accompanied by a rise in groundwater level. The collapse occurred within the area where ground surface displacement was observed, and the maximum ground surface displacement (dmax) was correlated with the ratio of the maximum width of the cavity and its depth (B/L) regardless of the ground height and water level fluctuation range. Countermeasure against the internal erosion using a two-layer filter was systematically studied in a series of sand filtration tests and prototype-scale hydraulic experiments. The filter maintained a sufficient deterrent effect under various hydraulic and dynamic forcing when the uniformity coefficient of the filter (DF60/ DF10) was higher than 3.0, and the median particle diameter ratio (DF50/DS50) was less than or equal to 20.