Characteristics of hidden karst collapse in the Zhongliangshan area of Chongqing and an approach of geophysical surveys

The Zhongliangshan area in Chongqing has a low mountain landform, composed of the Guanyinxia anticline. It is a high-level karst with steep topography and complex geological environments, In recent years, with increasing human engineering activities, karst ground collapses occur frequently, leading to serious geological hazard, posing threat to safety of people’s life and property in this area. This work attempts to clarify the characteristics of these karst collapses. Through sampling and analyzing soil layers in the collapsed and non-collapsed areas, it is found that collapses mainly occur in loose silty clay areas with karst well developed, and suffosion is the main factor for collapse occurrence. Forward simulation on the geological model indicates that the high-density resistivity method can only detect anomalous range of 2-3 times the electrode distance, it is difficult to detect fine fractures, but it can reflect the approximate location of karst cracks, namely, it permits to delineate the funnel-shaped geological form. Then by using the seismic imaging method to divide the loose layer of surface soil , the hidden karst collapse area can be delineated. In the field survey to a residential district of the Zhongliangshan area, the high-density resistivity method is used to preliminarily screen hidden karst, and then the seismic imaging is conducted to find the area of shallow loose soil layer. In terms of such a integrated approach, the hidden collapse karst area is delineated. Finally, 10 boreholes are drilled to verify the survey results, among which 5 boreholes reveal the characteristics of the loose soil structure, in agreement with the results of geophysical surveys. It demonstrates that the approach of geophysical surveys aforementioned is feasible in detection of hidden karst collapses.