Formation mechanism and development process of karst collapses in Tanluo town of Nanning City

Karst collapse is a common geohazard in karst areas. Proper study on its formation mechanism may help to prevent and control those incidents. In June 2012, a cluster of karst collapses happened in Tanluo town of Nanning City, with 19 collapse pits occurred in 26 days. In order to understand the formation and development mechanism of Tanluo Karst Collapse Cluster (TKCC), many works including field investigation, geophysical exploration, hydrogeological and engineering geological drilling, dynamic groundwater monitoring, rocks and soil sample tests were carried out. The results indicated that multiple adverse factors that had contributed to the development of the TKCC, of which the specific water-soil-rock interaction and human activities were the major ones. Groundwater is shallow in the study area. The topsoil is cohesive with low permeability and easy to collapse. The underground karst is well developed. However, the TKCC was induced by overexploitation of groundwater. Further investigation shows that the formation and occurrence of the TKCC was contributed by the coupled effect of creep seepage, increased load and water level fluctuation which resulted in hydraulic pressure difference. The collapse patterns are the combination of gravitation-, erosion-, vacuum cavitation-and burst-collapses. Based on the occurrence sequence of the karst collapses, it is inferred that the development process of the TKCC can be divided into three phases and each stage has its different development mechanism. The original soil balance was destroyed by different types of geologic forces so that the karst collapse could happen.