| Karst landform is widely distributed in China,particularly in the central and western regions.With the development of West China,more and more bridges,buildings and other projects have been built on the karst ground.To treat karst ground with pile,we usually let the pile pass through the cavity and anchor it into the stable bedrock,it's expensive.In fact,if the cave roof was thick enough,the pile foundation can be directly placed above the cave roof.In engineering practice,we usually ignore the influence of cave only when the ratio of roof thickness to cave span is not less than 1.There is no practical and proper calculation method to elevate the bearing capacity of the cave roof.Combined with the National Natural Science Foundation “The Failure Mode and Safety Thickness Calculation Method of Karst Rock Under Pile Tip”(project number: 51208195),the bearing capacity characteristics and the calculation method of the bearing capacity of the karst cave roof under the condition of intact bedrock are systematically studied,the main research contents are as follows:(1)Through the indoor model test,the bearing capacity of karst cave roof under different roof thickness and load eccentricity is studied.The failure modes and ultimate bearing capacity of karst roof at various loading points are obtained.The influence of the thickness of the cavern roof and the load eccentricity on the ultimate bearing capacity and failure modes of the karst cave roof are analyzed.According to the standard,the safety factor of different karst cave roof thickness is discussed.(2)Based on the limit equilibrium method and the limit analysis method,the calculation method of the ultimate bearing capacity of the karst cave roof under the condition of no load eccentricity(e=0)is discussed.Based on limit equilibrium method,calculation method of ultimate bearing capacity are obtained considering tensile stress and shear stress act on failure surface simultaneously;Based on limit analysis method upper bound theorem,by assuming the angle between the failure surface and the bottom of the punching shear is(?/4-?/2)or seeking partial derivative of diameter of punching body bottom,two upper bound solution of ultimate bearing capacity are obtained.(3)When the load eccentricity e?0,according to the test results based on limit analysis method,the failure modes of punching shear body are obtained by calculus of variation,by seeking partial derivative of diameter of punching body bottom,the upper bound solution is obtained when e?0.Then the results were simplified,the simplified calculation formula is obtained.At the same time,a method for estimating the failure range of punching shear is given.(4)The experimental results are compared with the theoretical calculation results to verify the rationality of the calculation method of the ultimate bearing capacity of karst caveroof.When e=0,based on limit equilibrium method,separately considering tensile stress and shear stress calculation results is too conservative,the calculation is slightly conservative when considering tensile stress and shear stress at the same time;Based on the limit analysis method and the Griffith criterion and by seeking partial derivative of diameter of punching body bottom,the calculated results agree well with the experimental results,by assuming the angle between the failure surface and the bottom of the punching shear is(? /4-?/2),the calculation results are higher than test results. |
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