Abstract: The formation and expansion of a soil cavity is among the basic conditions of the overburden karst collapse. To reveal the mechanism of karst collapse induced by groundwater pumping, the authors analyse the expanding condition of soil cavity by analytical method and put forward the concept of spalling force. It is indicated that the expanding condition of soil cavity is the spalling force exceeding tension strength of soil: increasing along with the continuous expansion of soil cavity, spalling forces caused by pumping water are heterogeneous around the soil cavity, so once expansion start, the soil cavity shows an accelerated developing trend because of the increasing spalling forces.

Key words: pumping, soil cavity, expansion condition, karst collapse, spalling force