Abstract
Local subsides or cave-in’s in the road often occur in urban areas. The complicated underground situation as well as the necessity of urgent road restoration does not usually allow full investigation for the real cause. The detailed mechanism of the phenomenon has not been, therefore, well understood. On the other hand, in many cases, the loss of soil through the defects of old sewage pipes is thought to be one of the most important factors. In order to understand the possible influence of old/damaged buried pipes on the surrounding backfill, a survey on cave-in’s in the road and sewer pipes under the location was conducted in seven cities. In this paper, some typical patterns of pipe defects found in the survey are reported and the potential properties of pipes and surrounding soil causing a cave-in are discussed. Furthermore, in order to understand the mechanism and governing factors of cavity formation/expansion in the ground, a series of model tests simulating flowout of soil through a crack/gap in a damaged sewer pipe was conducted. It was found that a cavity and surrounding loosening in the ground can extend rapidly upward when the ground consists of poorly graded sand, especially when it is fully saturated. Quantitative evaluation was also given to the loosening around a cavity.
