Abstract: The occurring and developing of soil cave is the base of karst collapse; and critical soil cave is the final developing phase. In time and space, the critical soil cave can divide the whole developing process of karst collapse into two phases: inter-collapse phase and surface-collapse phase. The inter-collapse phase is the developing and enlarging process of soil cave in over-layer soil, and the failure of superstratum soil above soil cave is similar to the failure of apical plate surrounding rock of underground cavity. Therefore, based on the equilibrium arch theory of М•М•Протодьяконов , the maximum height of limit equilibrium arch of superstratum, which is the limit equilibrium height of critical soil cave, can be determined. In addition, an evaluation of a critical soil cave of karst collapse at Tangshan Palaestra in Hebei Provinces, was completed using limit equilibrium height expression. The results show that the height of critical soil cave by calculating accords with fact, and the error only is 4.3%.

Key words: karst collapse, inter-collapse phase, limit equilibrium height expression of critical soil cave, Tangshan Palaestra