A series of processes of internal erosion, cavity formation and its collapse was investigated under various soil and hydraulic conditions by prototype-scale experiments. It was clarified that the formation of the cavity, the timing and the scale of its collapse are influenced by a height of suction varying with particle size distribution of sand. For instance, the scale of cavity collapse tended to be large if suction was relatively high. The type of hydraulic force was also shown to affect the process of internal erosion. The cavity collapse was found to be caused by decrease in suction in association with water level rise.
The relationship between dimensions of the cavity and ground surface displacement was also studied based on the experimental results to gain fundamental knowledge for the prediction of cavity growth and collapse. The results demonstrate that the maximum ground surface displacement (dmax) is correlated with a ratio of cavity width (B) to its depth (L) regardless of the ground height and water level fluctuation range. The correlation equation between B/L and dmax was obtained for each sand with different suction.
This research systematically studied two-layer filter as a countermeasure against the internal erosion. Appropriate particle size distribution of the geotechnical filter was obtained based on the results of a series of element tests and the prototype-scale experiments. Namely, it was found that the geotechnical filter keeps sufficient deterrent effect irrespective of particle size distribution of sand if the uniformity coefficient of the geotechnical filter is larger than or equal to 3.0 and the medium particle diameter ratio (Medium particle size (Filter)/ Medium particle size (Sand)) is less than or equal to 20. Based on the experimental results, this report shows a criterion for the design of the geotechnical filter for the prevention of internal erosion, cavity formation and collapse.

Key Words: internal erosion, cavity, collapse, suction, geotechnical filter, uniformity coefficient