Mechanism of karst collapse caused by engineering construction in Wuhan City

Wuhan City is situated in a karst collapse-prone region where karst collapses frequently occur and have caused serious losses of life and property in the past. In recent years, due to extensive infrastructure and construction projects along with its rapid urbanization taking place in the city, problems associated with the engineering construction induced karst collapse in the karst areas have been becoming increasingly prominent, which seriously affects the urban planning and construction process. There are in total 36 karst collapses occurred 36 in the city, of which 28 events were caused by engineering construction, and 8 cases by natural factors. It is noted that the collapses occurred after 2001 are all induced by the engineering construction. They collapses can be divided into 4 categories according to construction types, i. e. twenty one collapse induced by the drilling, pile foundation construction, five events by draining underground water and two by heavy vehicle load. Geomorphologically, the Wuhan City is located in the transition zone between Jianghan plain and the mountainous region of eastern Hubei. Twenty five out of the 28 karst collapses caused by engineering construction occurred at the first terrace of Yangtze River (alluvial plain area), where the overlying quaternary soil layer has the dualistic structure characteristics of "clay in upper layer and sandy soil in lower layer"; the rest 3 collapses occurred at the ridge hill areas, where the overlying quaternary soil layer has the single clay structure. Based on the previous research and through the comprehensive analysis of the collected data, this paper studied typical characteristics of karst collapse caused by engineering construction, in order to reveal its geological background conditions and control factors. Some examples are also presented to analyze the formation mechanism and collapse mode in different geological conditions. Research findings and analytical results collectively show that, (1) the karst collapse caused by drilling was mostly "Destroy of Stratified Roof-Vertical Seepage Pressure" collapse mode or "Disintegration-Damage at The Top of Soil Cave-Increasing Loading "collapse mode; (2) the karst collapse caused by pile foundation construction was mostly "Destroy of Stratified Roof-Seepage Liquefaction" collapse mode; (3) the karst collapse caused by draining underground water was "Suffosion-Suction Erosion" collapse mode; (4) the karst collapse caused by load vibration was "Suffosion-Vibration-Increasing Loads" collapse mode. In addition, this paper demonstrated that the karst collapse in Wuhan was mainly caused by drilling or pile foundation construction, because it not only destroyed the geotechnical structure and hydrodynamic conditions, but also happened rapidly and violently. The affected area of draining underground water was so wide, and could not be easily detected due to its concealment. Load vibration could only take effect after the formation of soil cave. It is expected to provide useful information for the prevention and control of karst collapse in Wuhan.