Study On The Bearing Mechanism Of Composite Lining In Deep-Buried Mountain Tunnels

The design and construction concept of mountain tunnels in China is based on the NATM(New Austrian Tunnelling Method).Through the experience-based engineering analogy method to guide the support design,the surrounding rock grades of a large number of built tunnels are classified for cross reference in the design of the proposed tunnels.However,the geological and hydrological conditions of tunnels are complex and variable,and many construction steps,construction quality fluctuations and backward calculation theory lead to that it is difficult to give the pressure characteristics after the primary support and secondary lining bearing design accurately.Therefore,it is of great theoretical and engineering significance to carry out research on the bearing mechanism of composite lining in deep-buried tunnels for the support design of deepburied tunnels’ sections in China.The paper comprehensively summarizes the existing load distribution rules of primary support and secondary lining in deep-buried tunnels.Theoretical analysis,numerical simulation and model test means to carry out in-depth and systematic research on the loads sharing of primary support and secondary lining were further adopt.Finally,to analyze the causes and give countermeasures based on the results of the above researches,the feasibilities of scheme for engineering problems arising from the Nianpan Tunnel and the Dahongyan Tunnel were verified.The following results were summarized.(1)The loads on the primary support and secondary lining at each monitoring point of the deep-buried tunnels were summarized.The distributions of contact loads of the tunnel arch roof,arch waist,side wall and arch foot both between the surrounding rock and primary support,primary support and secondary lining for different surrounding rock grades and the factors affecting the support loads were analyzed.The bearing pressures of primary support are converted into vertical and horizontal pressures,and compared with the results from the empirical formulas of surrounding rock pressure to verify the applicability of the empirical formulas.In addition,the load sharing ratios between the primary support and secondary lining were studied for different surrounding rock grades of tunnels,and the main bearing structures of composite lining subsequently were then explored.Further,the deficiencies of existing support load theories were derived.(2)Four mechanical models were established based on the relationship between the location of the distal ends of anchors and plastic zone.Based on Hoek-Brown strength criterion,considering the release effect of the stresses during the excavation face advancement,a theoretical method of obtaining composite lining support loads for deep-buried tunnels based on strain softening model and ideal elastoplastic model was proposed.The reliability of the proposed theory was verified by comparing it with the engineering measurement data.Moreover,the GSI(Geological Strength Index)value intervals were equivalent to the surrounding rock grades,and the bearing ratio intervals of tunnel support loads for different surrounding rock grades were obtained.The factors affecting the support loads were also discussed subsequently.(3)The software FLAC3D 6.0 was used to simulate the support structure bearing during the construction of deep-buried tunnels.The focus is on modifying the HoekBrown constitutive model by embedding the parameter GSI in the constitutive model which proceeds to change the situation where the softening process of surrounding rock is entered with arbitrary values.The segmental equation containing the variation of each parameter with softening coefficient was embedded to analyze the bearing capacities of the composite lining for surrounding softening rock tunnel.Finally,the influencing factors of support loads were analyzed and ranked according to the influencing factors on loads on the tunnel support structures in different surrounding rocks.(4)Geomechanical model tests of the deep-buried tunnels were carried out.Considering the factors of the surrounding rock grade and support time,the similar ratio materials for Class III and V surrounding rock,steel arch,anchors,shotcrete,and secondary lining were produced.By using a large geomechanical model test platform to proximally reproduce the whole excavation-support process of deep-buried tunnels,the development history of primary support,secondary lining bearing pressures and vault settlement were obtained.In-depth investigation of the load sharing ratios of the support structure were influenced by the surrounding rock grade and support time factors,and the change rules of the internal forces of bolts,steel arch and secondary lining were investigated.The reliabilities of the calculation theory and numerical calculation were further verified.(5)Based on the Nianpan Tunnel and Dahongyan Tunnel,the two major engineering problems,i.e,softening surrounding rock in the Nianpan Tunnel because of water gushing and excessive force cracking in the secondary lining of the Dahongyan Tunnel,were analyzed and countermeasures had been given based on the formation mechanism of composite lining loads.The feasibility of countermeasures to the problems was further verified by using the field monitoring data.