Distribution characteristics and factors influencing karst collapse in Dachengqiao, Ningxiang, Hunan

In the past few years, 484 karst collapses with a total collapse area of over 300 km2 have occurred in the Dachengqiao area of Ningxiang county. They caused direct economic losses of more than 1 billion RMB and they have generally been increasing in number. Dachengqiao is located in the transition zone between the hills of Hunan and the Dongting lacustrine plain. The area’s geomorphology is mainly hills, with a few gullies and the Weishui alluvial plains; the local strata are mainly Permian, Cretaceous and Quaternary with scattered Carboniferous outcrops. Basic data about the collapses and their current status was obtained through data collection, field investigation, geophysical prospecting, drilling, pumping tests, long time observation, and groundwater level measurement. The environmental and geological conditions of the karst collapses in study area were analyzed according to the correlation theory of karst collapse. The temporal and spatial distribution of the collapses was determined and the factors influencing collapse were ascertained. Main conclusions are as follows，(1) Most collapses occurred during the rainy season. For instance, 282 collapses (58% of total) took place between April and July. The number of collapses exhibited an increasing trend.(2) All of the karst collapses were located near cones of depression caused by drainage and dewatering. 428 collapses (88.43% of total) were in the intensely karstified zone. The collapse distribution density was greatest in gully areas, and higher in areas with non-karst overburden than in those without. 404 collapses were caused by diluvial soil. (3) The main factors influencing karst collapse include the intensity of the karstification, the characteristics and thickness of the overburden, the double-layer soil structure widely found in gullies and on the alluvial plain. Drainage and dewatering in mining area were the main factors inducing collapse. The uneven distribution of rainfall, surface water leakage and river training also had important influence on the collapses. All of these factors accelerated the karst collapse process.