Once a cavity created in ground grows along with time with its volume extending to surface of the ground then a cave-in would occur consequently. Cases of such cave-in occurred in community area have been reported that they are typically attributable to cavities in the ground created by soil and ground water infiltrated together into the canal of sewer pipe through cracks or damaged connections of old sewer pipes. Currently, the reactive measures against the occurrence of cave-ins are usually

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... cused on fast recovery of the part of cave-ins rather than taking actions to identify exact causes thereof or finding proper approaches therefor. Thus, the basic mechanism enabled to explain the causes or factors creating cavities in ground or the growing speed of the volume of such cavities or the relaxation in ground compaction needs to be identified. Therefore, this study was designed to simulate the damage of old sewer pipes in ground, one of major causes inducing the cave-ins. For this purpose, a model of soil tank was prepared and through which the model test employed standard sands was carried out to simulate the effects of insufficient compaction of background and changes in ground water level on the formation of cavities in a ground. The influence of the degree of ground compaction and changes in ground water level on characteristics of soil loss will be examined and, results obtained from the experiment conducted in this study will be used as basic data to perform quantitative analysis of decreasing ground density in the area causing cavities to clarify the mechanism of cavity formation.