Study On Structural Optimization Design Of Composite Lining In Mountain Tunnel

Since the introduction of NATM in Da Yaoshan tunnel,composite lining has been widely used in mountain tunnel engineering in China.However,from the view of many completed tunnels,the design concept of composite lining is quite different and the engineering analogy method based on experience is still universally used in its design.There are still some basic problems to be further studied in the design of composite lining.In this paper,composite lining of mountain tunnel is taken as the research subject,and primary support,lining and waterproof layer are studied respectively by means of in-situ test,theoretical analysis and numerical simulation.According to the actual stress state of composite lining,the feasibility of optimal design of composite lining is discussed.Based on displacement back analysis method,calculation methods for determining lining reasonable construction time and its sharing ratio of surrounding rock pressure are proposed.According to lining safety checking formula in the current code,an evaluation method of lining safety by using the ultimate bearing capacity curve of lining section is discussed.Through theoretical analysis and numerical simulation,the influence of waterproof layer on the mechanical characteristics of composite lining is studied.The main research contents and results are shown as follows:(1)Relying on the new railway coal transportation engineering from Inner Mongolia Autonomous Region to Jiangxi Province,the stress states of primary support and lining in grade IV and V surrounding rock are studied through in-situ test,and the feasibility of optimal design of composite lining is discussed.The results show that with different tunnel geological conditions,pressures behind primary support are different,and monitored surrounding rock pressure is not consistent with calculated value by the current code,especially the horizontal pressure.Pressures behind lining are similar and the values are small,and pressure distribution is uniform along tunnel circular direction.The shotcrete axial force accounts for 70% ? 90% of primary support axial force.The steel grid needs to be combined with shotcrete,which plays a significant role at tunnel arch.Compared with reducing steel grid space,increasing shotcrete thickness has a better effect on restraining surrounding rock deformation.The stress degree of lining reinforcement and concrete are no more than 30%,so the lining is in a safe state with high safety reserve.Generally,it is feasible to adopt the composite lining optimal design scheme of strengthening primary support,reducing lining thickness or using plain concrete lining.(2)Based on the deformation monitoring data of primary support,its safety was evaluated by displacement back analysis.And the corresponding surrounding rock stress field is obtained by primary support residual deformation through displacement back analysis.This stress field is applied to primary support and lining to calculate lining stress,so as to determine lining reasonable construction time and its sharing ratio of surrounding rock pressure.Taking in-situ test tunnel as an example,the effectiveness of displacement back analysis method in determining lining construction time and its sharing ratio of surrounding rock pressure are verified by comparing in-situ monitoring and back analysis results of lining stress and surrounding rock pressure.(3)According to lining safety checking formula in the current tunnel code,the concept of safety envelope curve of lining section is proposed,which can easily and intuitively judge whether lining is safe or not.A method of calculating safety factor by using the ultimate bearing capacity curve of lining section,and then to evaluate lining safety is discussed.The safety of plain concrete and reinforced concrete lining can be evaluated uniformly by the ultimate bearing capacity curve of lining section considering material nonlinearity.The section ultimate bearing capacity curve of high-speed railway tunnel standard lining based on limit state design formula can provide certain reference for lining design.(4)The stronger combined action of waterproof layer is,the smaller curvature and deflection of composite lining will be.However,the decrease of curvature and deflection is at the cost of increasing the tensile zone of lining section.The stronger the combined action is,the greater the restraint effect on lining is,and the lining tensile stress under the action of temperature load during its construction time is larger.Strong combined action increases the probability of pure tensile stress of lining section and there is a risk to cause full deep cracks.While waterproof effect is optimized,the combined effect of primary support and lining interface should be reduced as much as possible.Sprayed waterproof layer improves the overall bearing capacity of composite lining,but its strong combined effect may lead to pure tensile stress in lining section,which is adverse to lining safety.Hanged waterproof layer can reduce constraints on the lining and the possibility of full deep cracks of lining section.