Laboratory model tests on punching properties of karst cave roof under pile tip considering various thickness-span ratios

According to the similarity theory, punching properties of cave roof under pile tip are investigated through three series of large-scale laboratory model tests with different thickness. The load-displacement curves of pile foundation and roof, the ultimate strain curves of roof bottom and the failure characteristic of cave roof with different thickness are obtained. The test results show that the punching failure occurs within the circle table punching body when the thickness of the cave roof equals to 1d or 2d; however, the bending failure rather than the punching failure occurs when the thickness of the cave roof equals to 3d. The turning point of the elastic stage is transformed into the plastic stage in half the position of the ultimate bearing capacity according to the load-displacement curves, indicating that the plastic ultimate bearing capacity is two times the elastic ultimate bearing capacity. At last, the calculation formula of the minimum safety thickness is induced based on the theories of bending for thin plates and punching failure. The theoretical calculation results are in good agreement with the test results, which can meet the requirements of engineering calculation accuracy. The test results in this paper are based on critical situation, and they can provide reference for the preliminary design of piles in Karst areas.