Features of the shallow karst development and control of karst collapse in Wuhan

In order to reveal the shallow karst development laws and provide the basis for prevention and control measures of karst collapse disaster, the development characteristics of covered karst in Wuhan are studied by the means of comprehensive analysis and mathematical statistics method. There are 6 independent carbonate rock belts with the trend of NWW to SEE. According to the characteristics of the overlying carbonate rock covers, 5 karst geologic structure types are divided. The shallow karst types in Wuhan are mainly solution fissure, grike, grooves and sink as well as small caves. The cave encountering rate of borehole (ECR) is 46.0 %-50.1 %; the linear karst rate of borehole (LKR) is 5.93 %-6.00 %. In shallow karst belt, the height in 1/3 karst caves is less than 0.6 m, a half less than 1.0 m, 90 percent less than 3.0 m. The depth from bed rock surface to cave roof in 1/3 karst caves is less than 2.5 m, a half less than 4.5 m, 90 percent less than 12.5 m. The fillings of karst cave are mainly clay with limestone detritus and block. And 70.8 % of karst caves is fully filled, about 20 % is unfilled, less than 8 % is half filled. The average depth below bedrock surface of fully filled caves is 5.13 m, half filled cave 5.71 m, and unfilled 7.69 m. From the fully filled to half filled or unfilled caves, the depth below bedrock surface of karst cave roof gradually increases, reflecting the caves are filled downward from the top and the fillings are from overlying cover layers. The shallow kart in Wuhan belongs to the “vertical seepage karst zone” and is the result of vertical seepage action. On vertical section, the shallow carbonate rock can be divided into strongly developed karst upper zone and weakly developed lower karst zone. In plane view, 3 risky areas of collapse can be divided, namely the high, the middle and the low risk area. The prevention and control principles of karst collapse disaster are different in different risk areas. High risk area is the focus of karst collapse disaster prevention, and the basic principle is to prevent the overlying sand erosion. The prevention and control principle of the middle risk areas is to protect the integrity of the old clay or red layer. In low risk area, we should pay attention to the soil caves located in the interface of old clay and carbonate rock far from the city. The governing works should be based on the karst geologic structure and it should be guided by the principles in make relevant governing measures. Moreover, weak karst zone should be reasonably selected and utilized in engineering construction.