Crack Propagation Law And Safety Evaluation Of Banxi Tunnel Secondary Lining

As the support of the tunnel,the second lining is of great significance to ensure the stability of the tunnel.However,in actual engineering,the second lining of the tunnel is cracked due to various reasons,which seriously weakens the support effect of the second lining on the tunnel structure.Security and durability pose a huge threat.In view of this,this paper takes the Banxi tunnel in Youyang area of Chongqing as the engineering background.By establishing a three-dimensional tunnel lining model,the crack propagation law of the second lining is studied to obtain the crack propagation law of the cracked lining,and the safety of the Banxi tunnel is carried out.Evaluation and research results have important engineering significance for the prevention and treatment of cracks.The main research contents and results are as follows:(1)By referring to relevant literature and combining with the inspection reports of32 tunnels in Youyang area of Chongqing,the distribution rules of cracks in the second lining of the tunnel and the causes of cracking were analyzed from the geological structure,design,construction and other factors.(2)Using the finite element software ANSYS,a three-dimensional tunnel second lining model was established,considering the sliding effect of steel and concrete,under the conditions of uniform load,partial lining bias,and voids behind the lining,the distribution law and cracking load of the second lining were studied.The study found that under the uniform load,the second lining will not produce cracks when it only bears the surrounding rock pressure;when the lining is partially biased,the cracks are mainly distributed inside the lining bias,and when the back of the lining is hollow,the cracks are mainly distributed in the lining cavity Outside.(3)Using the finite element software ANSYS to simulate the cracking process of the intact lining,the cracking load and the ultimate bearing capacity are obtained,and the second liner finite element model of the cracked tunnel is established.A total of 13 working conditions are designed and the preset crack length,width,depth and position are changed,The crack propagation law and bearing capacity of cracked lining are studied.The results show that,from the perspective of crack size,the crack depth has the greatest influence on the crack propagation law and bearing capacity of the second liner,followed by the length,and the width has the smallest effect.When it reaches 6m or the depth reaches 15 cm,the cracked lining will produce new cracks under the action of surrounding rock pressure;from the crack position,the preset cracks are often cracked first,but when the cracks are located in the arch waist,the top of the arch is cracked first,That is,the occurrence of cracks at the arch waist has little effect on the crack propagation and bearing capacity of the second liner.(4)Using numerical simulation,the internal force of the cracked lining at different preset crack lengths,widths,depths and positions is obtained,the calculation method of the safety factor of the cracked lining is corrected,and the safety factor is calculated to obtain the structural safety warning value.The results show that When the crack length reaches 6 meters or the depth reaches 1/3 of the thickness of the lining,the safety factor is close to the standard value,so the crack length of 6 meters and the ratio of the depth to the thickness of the lining is 1/3 as the structural safety warning value.(5)Through the numerical simulation results and the Banxi tunnel inspection report,the safety evaluation of the Banxi tunnel is carried out.The results show that 3cracks in the Banxi tunnel have reached the safety warning value,and reinforcement measures are required.