Abstract: According to the similarity theory, punching properties of karst cave roof under pile tip were investigated through four series of large-scale laboratory model tests under different thick-span ratios. The load-displacement curves of pile foundation and roof and the strain displacement curves of roof bottom were obtained, and the bearing mechanism of punching feature in the pile-cave system was analysed. The experimental results show that there are two elastic-plastic zones in the pile curve during the loading process. According to the load-displacement curve of the roof, the displacement rebounds at thick-span ratios of 0.125 and 0.200 during the loading process. Meanwhile, the strain change is consistent with the development of cracks at the bottom surface. Punching failure of the test plate is the result of the combined action of the shear effect and bending effect. With the increase of thick-span ratio, the ultimate bearing capacity of the pile-cave system increases slightly, whereas the displacement distance decreases. Shear effect becomes more apparent, while bending effect gradually disappeares. The shapes of the punching cones change significantly with varying the thick-span ratio. When the thick-span ratio is higher than 0.250, the punching cone is relatively thick and destruct along the boundary. While with the thick-span ratio less than 0.200, the punching cone is not restricted. Finally, the relationships among the punching cone generatrix, the thick-span ratio ξ and the pile diameter d are established through analysing the punching cones under four different thick-span ratios. Therefore, this study provides the basis for the theoretical research and engineering practices of karst caves under pile tip in the karst area.

Key words: karst area, pile-cave system, model test, punching failure