Calculation Method And Experimental Study On Ultimate Bearing Capacity Of Cavern Roof

With the rapid development of construction engineering and road and bridge construction,the application range of pile foundation has been continuously expanded.Due to the large amount of karst landforms lie in the central and western regions of China,it is inevitable to encounter karst foundations in engineering construction.how to determine the ultimate bearing capacity of cavern roof is the key to solving the influence of caves on pile foundations.At present,the calculation theory of foundation bearing capacity above caves is not enough.Improve and meet the needs of the project,the relevant specifications on this aspect is also superficial,according to the existing norms for the design of the underlying caverns in the cave area,in some cases,there will be insufficient display capacity,so the study of the bearing capacity of cavern roof is particularly important.Based on the previous research results,this paper uses the limit equilibrium method to obtain the calculation method of the ultimate bearing capacity of the karst cave foundation under the complete rock foundation and the punching failure mode.The calculation method of ultimate bearing capacity of cavern roof in plastic failure mode is proposed by using the slip line method.Using the S-shaped growth curve fitting,through reasonable assumptions,the calculation method of the ultimate bearing capacity of the cave roof of the punching shear failure mode is proposed.The calculation method of ultimate bearing capacity of cavern roof under symmetrical load and load position deviation is derived by using limit analysis method.The rationality of the theory is verified by experiments,and the indoor model test of the ultimate bearing capacity of the roof considering load deviation and span ratio is further carried out.The results show:(1)The theoretical values of the ultimate bearing capacity of the intact rock foundation are consistent with the measured values using the global shear and plastic failure modes.When calculating the ultimate bearing capacity of the roof of the cavern under the center load,the values calculated by considering the tensile and shearing effects are too conservative compared with the measured values.When the two are combined,the theoretical results are closer to the actual measurement result.When the top plate of the cavern is subjected to die shearing and shearing,the S-shaped growth curve can be fitted to obtain reasonable results.(2)When the cave diameter L is 6D and the thickness of the top plate is 1D-4D,the ultimate bearing capacity of the karst roof under the symmetrical load increases linearly with the increase of the thickness of the roof plate,according to the calculation method of the roof under the symmetrical load.The theoretical results are close to the experimental results.When the roof of the karst cave is damaged by punching,the top punching body obtained under the load offset can be simplified to the axisymmetric problem,and the theoretical calculation results are more accurate.Under the same thickness-to-span ratio,the ultimate bearing capacity of the karst roof is obtained.As the load position offset increases,it increases nonlinearly.(3)With the same roof thickness,with the increase of the span ratio,the ultimate bearing capacity of the cave roof is continuously reduced.When the ratio of the thickness of the roof to the diameter of the hole is greater than 2,the ultimate bearing capacity of the roof is close to the bearing capacity of the complete bedrock,and the influence of the cave can be ignored.With the increase of the thickness-to-diameter ratio,the ultimate bearing capacity of the cavern roof and the corresponding critical cave span increase linearly.(4)When the cave diameter L is 1-4D,the range of the punching failure zone on the horizontal plane does not exceed 1.5L.When the load offset reaches 1.5L,the ultimate bearing capacity of the roof is consistent with the bearing capacity of the complete bedrock.And as the load offset increases,the failure mode of the roof is a gradual process from punching failure to incomplete punching failure(punching + tear failure)to plastic failure.