Abstract: Pile foundation is installed on the roof of karst cave under certain conditions. It is great significance to investigate the stability and the safety thickness of karst cave roof, when it is subjected to the coupling effect of the earthquake and pile-tip load. Based on the similarity theory and the separation similarity design method, dynamic failure characteristics of the cavern roof were studied by varying the roof thickness and cave diameter using the shaking table. Results show that the failure modes of the top of the cave roof are closely related to the thickness of the roof and the size of the cave in the earthquake. With a certain thickness of roof, when the cave-diameter was smaller than 2d (where d is the pile diameter), shear failure happened in the roof. On the contrary, when the cave diameter was larger than 4d, punching-shear failure occurred. This indicates that the larger the diameter is, the greater the proportion of punching volume is. According to failure characteristics of the karst roof obtained by shaking table tests, the theoretical calculation model was proposed for minimum roof safety thickness, combining with the quasi-static method. Besides, the proposed model considered characteristics of rock mass, seismic intensity and pile diameter. It was shown by numerical studies that the minimum safety thickness was larger than that under static condition, and it increased with the increase of seismic intensity. Under the same condition, the calculated values of punching and shearing were larger than those of shear failure, which indicates that in the earthquake environment, the greater the cave size is, the higher the thickness of the required roof is. Therefore, this study can provide a valuable reference for developing the design approaches for the karst-pile in the earthquake region.

Key words: karst pile foundation, earthquake action, shaking table test, failure characteristics of cave roof, safety thickness