Experimental Study And Calculation Method On Ultimate Bearing Capacity Of Rock Foundation Over Cavern

A lot of practical engineering experience shows that the rock foundation at the bottom of caved pile end in karst area has certain self-bearing capacity and can meet the demand of superstructure load.If fully utilized,the construction difficulty and cost can be reduced.However,the calculation method of bearing capacity of underlying hollow rock foundation is relatively backward in practice.Therefore,it is of great significance to carry out the research on the ultimate bearing capacity of underlying caved rock foundation to perfect the theory of bearing capacity of special foundation and to guide the engineering design of underlying caved rock foundation.The bearing capacity of hollow rock foundation is not only related to rock mechanical properties,rock structural surface,cavity distribution and rock surface fluctuation of foundation,but also related to roof thickness,foundation size,cavity shape and rock failure mode.However,most of the current researches focus on the study of roof thickness on the ultimate bearing capacity of caved foundation,and there are few reports on the analytical theory of the ultimate bearing capacity of caved foundation caused by the offset of span and load position.In this paper,first of all,through the indoor model test,use the materials such as gypsum,cement,sand,clay simulating karst rock,small loading plate test,for different thickness of top plate,hollow span,load position offset hole under the roof load characteristics are studied,for each load point of cavity roof failure mode and ultimate bearing capacity value,and analyzes the different cavity roof thickness,hole span,load load position offset of affecting the ultimate bearing capacity and failure mode of roof is empty.Then the failure mode is simplified based on the test results,A reasonable velocity field is constructed by using the limit analysis method,and a formula for calculating the ultimate bearing capacity of the void foundation considering the displacement of load position and the void span is established,The upper-bound solution is obtained by using the optimization program,and the rationality of the upper-bound solution is verified by comparing with the test results,and the influence of relevant parameters on the ultimate bearing capacity of the hollow foundation is further analyzed.The following conclusions are drawn:(1)With the increase of displacement of load position,the failure mode of hollow foundation changed from punching failure to logarithmic spiral failure,and finally developed into Prandtl failure.(2)When the load position offset is constant,the ultimate bearing capacity of rock foundation increases linearly with the increase of the bedrock thickness.When the bedrock thickness reaches 3d,the bearing capacity under different load position offsets is close to the complete bearing capacity of bedrock.(3)when the thickness of rock foundation h is constant,the ultimate bearing capacity of rock foundation increases nonlinearly with the increase of load position offset e.When the eccentricity reaches a certain distance,the bearing capacity of hollow rock foundation has no effect and reaches the bearing capacity of complete bedrock.When h/d=1,e=5d reaches the critical value.When h/d=2,e=4d reaches the critical value,and when h/d=3,e=3d reaches the critical value.Therefore,not only the thickness of rock foundation but also the displacement of load position should be considered in the design.(4)When the thickness of rock foundation h and the displacement of load position e are fixed,the ultimate bearing capacity of rock foundation increases nonlinearly with the increase of internal friction angle.When the internal friction angle is less than 20 °,the growth curve of ultimate bearing capacity is relatively gentle.When the internal friction angle is more than 20°,the growth curve of ultimate bearing capacity is more and more steep.Therefore,the parameters should be reasonably selected in the design(5)Under the same roof thickness h/d,the ultimate bearing capacity coefficient(Nc)decreases rapidly with the increase of the void span l in the early stage of the ultimate bearing capacity coefficient curve.When the void span l reaches the critical span,the ultimate bearing capacity coefficient(Nc)tends to be stable.Based on the data,the relationship between the critical span of void and the thickness of roof is obtained.When the void span l is 0.5d and the roof thickness h/d is 1.0?2.5,the influence of the hole on the ultimate bearing capacity of the foundation should be considered.