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%.