Typical drain defects
Underground sewer pipes are subjected to various forces, the most predominant being ground movement, tree root ingress, chemical attack and accidental overload. Due to one of these drain defects, a crack may appear in a pipe. Effluent then leaks out of the crack and erodes the soil surrounding the pipe.
Most pipes depend on the soil for structural support, and for the distribution of overburden loads. The erosion alters the equilibrium and causes the development of eccentric point loads. These point loads cause further deterioration of the pipe, which may give rise to fractures. The fractures allow further washout and a vicious cycle of deterioration is established. Eventually, the pipe collapses.