Failure Mechanism And Safety Assessment Of Metro Tunnel Lining

Rail transit,with its convenient,fast and efficient features,has became an important measure to solve urban traffic problems,and plays an irreplaceable role in ensuring the normal operation of the city.The engineering practice shows that the tunnel structure disease is an nonnegligible problem during metro operation,which would do harm to the safety of train operation.However,there is not much research on the disease status of the metro tunnel which including the expansion mechanism and distribution patterns of the cracks in the lining structures and the safety evaluation of structures.With the increase of the rail transit operation scale,the research on the safety evaluation of tunnel structure appears to be especially necessary.In this paper,the disease status of operated metro tunnel was firstly investigated by taken Beijing subway as an example.Secondly,the internal force solution for the primary support and secondary lining of the composite lining of mined tunnel was obtained.Furthermore,the ultimate internal force,crack extension process and its influencing factors of the tunnel lining section were analyzed.Then the structural and geological factors that influenced the failure of subway tunnels lining were analyzed.The fracture distribution mode of tunnel lining was proposed.Finally,the safety evaluation method of tunnel lining structure was given.The main contents and achievements of this paper are as follows:(1)The site investigation and data indicated that the fissures of tunnel lining were the locations of seepage,including shield tunnel segment joints,bolt hole,the 'three kinds of joints' in mining method and the lining cracks.The transversal convergent deformation overrun and ballast hanging were more common in shield tunneling,with the water leakage as the main disease type.In mined method,lining cracking of the tunnel appears to be the main disease type and mainly affected by the reinforcement amount in lining.The longer the tunnel operated and the lower the measuring position,the more serious the lining carbonization.(2)Based on the elastic foundation curved beam theory and the initial parameter method,on the assumption that the two meet the compatibility of deformation on tunnel profile radial direction,without considering friction between the primary support and secondary lining,the complex lining structure internal force is solved.The inner force of the secondary lining is closely related to the radius and form of the tunnel section,the thickness of the primary support and secondary lining and the concrete strength grade.Compared with the single-layer lining structure,due to the existence of secondary lining structure,the calculation of the internal force for primary support has been revised accordingly.(3)Based on the theory of concrete,the ultimate internal force of secondary lining structure of tunnel under eccentric tension and compression was analyzed.According to the theory of concrete fracture,the Matlab search program for the crack propagation was obtained,and corresponding propagation curves of each failure condition were also derived.When the axial force is tensile,the crack propagation experienced five nonlinear stages include 'no cracking,rapid expansion in the scope of reinforcement protection layer,slow expansion,rapid expansion after crossing the steel bar,and slow expansion before adjacent failure'.Otherwise,the crack experiences through nonlinear expansion until it runs through the whole cross section.(4)Based on the analysis of crack propagation in lining section,the influence of sectional shear force,concrete strength grade,thickness of secondary lining,reinforcement amount,yield strength of reinforcement and thickness of protective layer on crack propagation was analyzed.The conclusions included:larger shear force is unfavorable to the crack resistance;the crack arresting ability of compress condition with high-grade concrete was better than that of the tensile situation;the velocity of crack extension was directly proportional to the lining thickness;the larger amount of reinforcement and protective layer played an active role in crack restriction;the higher yield strength of steel bar had no advantage in the crack restriction when the elastic modulus of material is same.The effect of control of crack is better with larger thickness of steel bar protection layer.The main influencing factors of crack extension are different in the different stagesduring the crack extension process.(5)Based on the principle of orthogonal analysis,112 sets of conditions were established.The influence of structural and geological conditions on lining structure failure were studied by extended finite element method.After that the crack distribution pattern in the failure of the lining structure of the subway tunnel was putted forward.The lateral pressure coefficient of the surrounding rock and the section form of the tunnel were the main factors that affect the convergence deformation and state of the cracks.The influence of structural and geological conditions on the convergence and fracture state of lining structure was different.The distribution mode of lining cracks is most affected by the section form.The principal influence facor was different in each crake propagation stage.(6)The fuzzy comprehensive evaluation method was adopted to establish the three levels index system which includes the convergence deformation and the crack state of the tunnel section.According to the Q type cluster analysis of SPSS and the membership function of each index,the safety grades of subway tunnel was divided into A,B and C based on the safety matrix.At last,the safety level map of subway tunnel which is more convenient for application was given.