FURTHER DISCUSSION ON MECHANISM OF COVERED KARST GROUND COLLAPSE

In order to better provide a theoretical basis for the treatment and prevention of the covered karst collapse(CKGC),according to the differences of material composition, geological structure, physical and mechanical properties of overburden on karst, for the first time, CKGC is divided into three types of mechanism, which are respectively called by hourglass type collapse, soil-cave type collapse and soft soil flow type collapse. In the distribution area of sandy soil, hourglass type collapse is caused by sand particles loss to the karst channels and karst caves because external factors triggering. Gravity and effective space provided by karst are necessary conditions for the occurrence of hourglass type collapse, and the role of groundwater accelerates the occurrence and shortens the process of hourglass type collapse. The ground subsidence phenomenon caused by soil-cave roof arching failure is called soil-cave type collapse occurring in clay and dense sand soil. The natural soil-cave is formed by three ways of the surface and groundwater erosion action, sand loss and soft soil flow. The roof of soil-cave is stable in the period of the formation and development of soil-cave, and ground subsidence occurs when the cave roof arching failure by triggering of external factors. The soil-cave type collapse has the characteristics of concealment and burst. The ground subsidence phenomenon caused by the soft soil flow is called as soft soil flow type collapse, is the result of the soft soil loss through karst channel and cave. The triggering factors of CKGC include groundwater action(vertical and horizontal seepage and underground water level frequent fluctuation at the soil/rock interface),external load(dynamic load and static load) and soil-cave roof resistance reduction(strength and thickness decrease and structure destruction of the roof).These external factors trigger the occurrence of CKGC.