Model Test Study On Influence Of Concealed Karst Cave On Stability Of Shield Tunnel Surrounding Sock

In the process of shield construction,it is inevitable that it will traverse various complex geology,and shield construction in karst development area is an engineering problem that must be faced.The existence of the karst cave will make the shield machine prone to deflection,sinking,and even cause the surrounding rock to collapse,which will have a significant impact on the construction progress,economic cost,and safety of life of the shield tunnel project.If we can reasonably predict the ground settlement caused by shield excavation,the surrounding rock instability criterion,and the critical safety thickness of the roof of the tunnel and karst cave,it will provide help for the safe construction of the actual tunnel project,reduce the project cost,and reduce the environmental impact.Based on the shield tunnel project between Qianyan Station and Quanshui East Station in Dalian Metro Line 5,relying on Liaoning Provincial Key R&D Program Guidance Plan Project（2017231001）and China Railway Seventh Bureau Group Science and Technology Research and Development Plan Project（17A01）,this paper carried out a similar model test study on the influence of karst caves on the stability of the shield tunnel surrounding rock.The specific research contents are as follows:1.Based on the similarity theory and the actual engineering data of Dalian Metro,the generalized parameters of similar materials in soil are proposed,and detailed physical parameters are determined.Researched the model box production,shield excavation device design,tunnel layout plan,measurement method and other work in the simulation process.2.According to the test results,the final settlement curves of the formations with different depths all conform to the Gaussian shape.As the depth of the formation increases,the maximum settlement value of the settlement tank gradually increases;The closer to the tunnel margin,the greater the impact of shield excavation.The horizontal lateral impact of shield excavation is about 5 times the excavation radius;the existence of karst caves amplifies the impact of shield excavation on the bottom pressure Impact,increasing construction risk.3.When the size of the cave and the thickness of the roof are considered variables,the influence of the excavation of the shield tunnel on the settlement of the stratum.The increase in the size of the karst cave and the decrease in the thickness of the roof will increase the formation settlement,S_max,0/S_max,zand 1-h_z/h at the depth of the stratum above the tunnel still maintain a linear relationship,and the relevant parameters of the linear function are affected by the size of the cave and the thickness of the roof.By comprehensively comparing the law of stratum settlement under the combination of the size of the cave and the thickness of the roof,based on the multiple linear regression method,the prediction formula of the maximum settlement of the stratum caused by shield excavation under the condition of the underlying cave is established,and the applicable range is given.4.Considering that the size of the cave and the thickness of the roof are variables,the influence of the excavation of the shield tunnel on the stress of the surrounding rock.The larger the diameter of the karst cave and the smaller the thickness of the roof,the greater the impact on the surrounding rock pressure of the tunnel;As the diameter of the karst cave increases,the pressure caused by shield excavation on the roof of the karst cave decreases,and the increase in the diameter of the karst cave causes the pressure on the top of the karst cave to be released;As the thickness of the roof of the karst cave increases,the shielding pressure on the roof of the karst cave increases,and the bearing capacity of the roof of the karst cave increases.5.Based on the cusp catastrophe model,the bifurcation set equation that satisfies the catastrophe is established,and the rock instability conditions and the roof critical instability balance equation are obtained.When the karst cave is under the tunnel,the karst cave roof is simplified into two mechanical models,an elliptical plate and a fixed beam,and the rock formation instability conditions are further derived from these two models,combined with the critical equilibrium equation of the roof,the calculation formula of the critical safe thickness is established respectively.Verification and verification are carried out through analysis of calculation examples to provide references for the design and construction of karst tunnels.