Study On The Evaluation Method Of Ultimate Bearing Capacity Of Tunnel Lining And Its Application

Due to the experience and analogy of tunnel engineering design,for conservative reasons,the lining design is often too thick and the reinforcement is too dense.However,such a design is not always guaranteed to be safe,sometimes it will backfire.Therefore,it is necessary to explore reasonable design method from the actual stress condition of composite lining.In Code for design on tunnel of railway,the lining structure is designed based on the plastic stage design method.Then the safety factor is introduced through the engineering analogy,and the component has the concept of overall safety.Compared with the allowable stress design method,this method has made obvious progress,but the minimum safety factor is determined by experience,and the effect of the lining deformation and crack is not considered in the calculation of the safety factor,which makes it impossible to judge the advantages and disadvantages of different types of lining through the safety factor.Based on the above two problems,the ultimate bearing capacity curve of lining section(M-N curve)is proposed based on material constitutive theory.Then the feasibility of optimizing the composite linings of the single track tunnels,double-track tunnels and loess tunnels of the Menghua Railway was studied.The main researches are as follow.(1)through the M-N curve,the calculation formula of the eccentric compression members in the specification is carded out,and the limitation of the lining calculation based on the plastic stage design method is pointed out.Different types of lining components(reinforced concrete lining and plain concrete lining)and lining members with different stress conditions(large eccentric compression lining and small eccentric compression lining)are different for the minimum safety factor and the calculation formula for the safety factor.Therefore,the advantages and disadvantages of different types and different stress states can not be judged through the comparison of safety factors.The plastic stage design method is used to calculate the lining section,so the influence of cracks in the lining section can not be considered when calculating the safety factor of the lining section.(2)based on the constitutive relation of concrete and steel bar,the ultimate bearing capacity curve of lining section is made,and the cracking of lining section is considered.For plain concrete,when the crack width is 0.2 mm,the maximum bending moment of the lining section can reach about 65%of the maximum bending moment.Reinforcing bars obviously inhibit the development of cracks in lining sections.Based on the curve of ultimate bearing capacity of lining section,the concept of crack safety index for lining section is put forward.Based on the ultimate bearing capacity curve of lining section and lining crack safety index,combined finite element analysis software,the feasibility of reducing the lining support parameters and improving the initial support parameters is studied under the conditions of grade IV and V surrounding rock of the tunnel(single line tunnel,double line tunnel and loess tunnel).And,the feasibility of using the plain concrete for the two lining is studied.After optimization,the safety index of lining section cracks has been improved;the safety index of single-line tunnel,double-line tunnel crack and double-line loess tunnel is greater than 4,and the composite lining optimization scheme is feasible.