Abstract: In this paper, the case of collapse is only considered.Under this condition, the soil under the pipe is belived fell down or swept away by flood, therefore the pipe is considered hanging in air a distance between two supports.A model in which the gravity of the pipes, weight of the covering soil, the high pressure and the friction between soils taken into consideration is introduced. The results show that when the hanging length is over 30 m, the pipeline is unsafe.