Influence of underground structures on cavity formation due to various conditions of water flow

Abstract

Many cave-in accidents happen nowadays in urban roads. Cave-in means sudden collapse of the ground like a pitfall. The accidents are a serious problem because it sometimes injures people. It is known that expansion of underground cavities causes of such accidents. Underground cavities have expanded in the ground without being noticed. Then finally surface ground is collapsed when cavities have reached near the surface ground. Many cavities were found close to underground structures. Some of those cavities were caused due to breakages of buried structures such as sewer pipes. Soil is flowed out from cracks of pipes with water, and then cavity is formed. However, in many cases, obvious breakages were not found in underground structures when a cavity was formed. It is not clear why cavities were generated around buried structures even if they didn’t have clear damages, but is empirically suggested that water flows more easily around underground structures than in other part of the ground. Soil is carried with water flow around buried structures, which might cause a cavity around underground structures. In order to study the influence of an underground structure on formation of cavities due to water penetration, a series of model tests was conducted with a small soil chamber in various conditions. The soil chamber has an opening at the bottom, from which soil can be flowed out. A block simulating the underground structure was placed into the soil chamber. Then repetition of small amount of water flow or steady flow was given in the ground. Then deformation of the model ground was observed and the weight of drained soil was measured. Position of the block, relative density of soil and direction of water penetration were changed. From a series of model test, it was suggested that position of the block and direction of water flow had large influence on cavity formation. In addition, the ground around the block had high water content.