Influencing factors and formation mechanism of covered karst collapse in Liujiazhuang, Qufu City

By the end of 2019, four karst collapses had occurred in Liujiazhuang village, Qufu City, resulting in seven collapse points that caused damage to houses and farmland, ultimately leading to the relocation of the entire village. To investigate the influencing factors and formation mechanism of karst collapse in Liujiazhuang village, Qufu City, a comprehensive study was conducted, including ground surveys of karst collapses, geophysical exploration, hydrogeological drilling, geotechnical tests, and dynamic monitoring of karst groundwater levels. The study systematically examined the degree of karst development, thickness, structure, and physical and mechanical properties of the overlying soil layer, as well as the dynamic characteristics of karst groundwater. The results show that karst collapse is influenced by fault structures. Rocks in the fault structures and their influencing zones are fractured, and groundwater dissolution is intensified, facilitating the formation of dissolved pores and karst caves. The collapse points are located near the faults and their secondary faults. Karst development in the shallow layer is intense and can be categorized into strongly developed, moderately developed, and generally developed karst fissure zones. The collapse points are located within a well-developed karst fissure zone. The overlying soil layer is composed of low-cohesion sand and soil with higher cohesion. The soil layer structure can be divided into single-layer, double-layer, and multi-layer structures, with collapses mainly occurring in the double-layer soil structure. The thickness of the overlying soil layer varies significantly, ranging from 35 to 45 meters at the collapse points. The annual variation of the water level can be divided into three stages: wet season, normal water season, and dry season. Collapses mainly occur when the karst water level drops rapidly and significantly during the agricultural irrigation period and when the karst water level rises sharply to the rock-soil interface after concentrated rainfall. The groundwater dynamic model is categorized into confined-unconfined fluctuation and confined fluctuation periods. Collapses occur during the confined-unconfined fluctuation period of the karst water level. The formation mechanisms of karst collapse vary depending on the structure of the overlying soil layers. In the single-layer structure mode, collapses are primarily caused by piping. Their formation and evolution process can be summarized as follows: karst water level drop→soil particle loss→soil cave formation, development, and expansion→soil cave roof instability→karst collapse. In the double-layer structure mode, collapses are mainly caused by negative pressure suction, seepage piping, and load softening. The formation and evolution process can be summarized as follows: karst water level drop→negative pressure suction→soil peeling→soil cave formation→rainfall infiltration→seepage piping→soil cave development and expansion→load softening→soil cave roof instability→karst collapse.