| As the karst geological condition is extremely complicated in the Western China,pile foundation is widely used. As pile cave exists, increased the difficulty of pile design and construction, which lead to engineering quality and safety problems easily, restricted the infrastructure construction in karst area. Therefore, it is quite necessary to research the problems on the interaction between piles and karst caves, bearing characteristics, failure mechanism and as well as the stability of karst cave roof. Based on the former researches and combined with National Natural Science Foundation of China Project ―The study of pile vertical bearing mechanism and design calculation method in Karst area‖(Number:51278187), The following work is done on karst roof under pile:Firstly, summarized main factors influencing the bearing mechanism of karst pile, analyzed the load transfer mechanism of piles in karst. Pile load transfer process, pile side resistance and pile end resistance calculation model are discussed. Combined with the rock failure mechanism, the model of punching shear failure, shear failure and bending failure mode of karst roof in karst area were predicted. Then both the beam-slab model and the arch structure mode are discussed, the formula of the bearing capacity of punching shear failure is obtained, as well as the punching cone generatrix equation in the beam-slab model. The formula of the bearing capacity and roof safety thickness of karst cave based on catastrophe mechanism is obtained too.Secondly, according to similarity theory, punching properties of karst cave roof under pile tip are investigated through four series of model tests in different thick-span ratios. The load-displacement curves of pile foundation and roof and the strain displacement curves of roof bottom were showed. Based upon these curves, the bearing mechanism of punching feature in pile-cave system was analyzed. The results indicate that punching failure of the test plate is the result 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 while the displacement distance decreases. Shear effect becomes more apparent, while bending effect gradua lly disappears. The punching cone generatrix is fitted by analyzing the punching cones in four different thick-span ratios.Finally, introduces the point safety factor method to analyze the stability of piles foundation over limestone caves in karst areas, Based on the existing theory, the point safety factor method is deduced, which codes the FISH command stream of safety factor, then by using the FLAC3 D numerical analysis software, combined with a engineering project, the cave model of point safety facto r contour is realized. Next, the cave model of point safety factor contour is mutually authenticated with the vertical displacement vector of model, distribution of stress, etc. which provides theoretical basis for engineering practice. |
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