Study On The Mechanism And Control Of Large Deformation Instability Of Tunnel Surrounding Rock

Along with the increase of the time and the water content,the swelling soil cause the surrounding rock to bring on squeezing-out of plastic materials in the direction of the internal tunnel,and the effect seriously threaten safety and stabilization of tunnel engineering.Adopting in-situ monitoring the large deformation characteristics about surrounding rock of tunnels in swelling soil,the data was used to analyze the general rules of the large deformation.The physico-mechanical properties of expansive soil from site were studied by laboratory,many characteristics were obtained in this way,such as expansion?softening and slake-durability which is caused by absorbing water,besides,shear strength curve was measured,and mechanical parameters were acquired.Based on finite deformation,bringing in parameters of expansion and softening,a constitutive model about the large deformation of expansive soil based on Mohr-Coulomb theory,the differences between large and small deformation were appeared by numerical calculation,and the stabilization of surrounding rock support system could be analysed.Based on the characteristic of strong expansion,bringing in coefficient of expansion correction,the coefficient of disturbance belonging to Hoek-Brown model could be corrected,and the applicability of the model could be proved.Establishing the 3-D numerical model of large-section expansive tunnels by using finite difference software,and using seven step excavation method,adopting the methods given above analyses the large deformation regularities of tunnel surrounding rock,studys interactional relationship between surrounding rock and support,optimizes the design parameters of support structure,and proposes the method of controlling large deformation of tunnel surrounding rock.Above all,the feasibility of optimized scheme should be validated by these data from in-situ monitoring,such as surrounding rock deformation,surrounding rock pressure,lining stress,etc.The theoretical basis and technical support can be provided adequately,when monitoring data?lab test results?theoretical model and numerical results can be analyzed synthetically,and the monitoring data of tunnel section is multi-step scroll predicted.The main research results are summarized as follows:(1)The swelling soil has obvious characteristics of expansion and softening,and its percentage of bed-expansion will get larger with the increase of moisture content.According to the analysis of mineral composition,the main composition are albite?quartz?illite and montmorillonite,these matters all have obvious characteristics of expansion,and it is the primary reasons that results in the large deformation of tunnels surrounding rock.(2)According to improving elastic-plastic method of finite deformation theories,based on plastic potential function of Mohr-Coulomb criterion,the constitutive model in consideration of moisture content and softening characteristics was proposed.According to realize programmatic numerical software,and establishing numerical model,the differences of calculating result between the theories of large deformation and small deformation were analyzed by numerical calculation.According to analyze the differences in calculating interactional relationship between surrounding rock and support of the theories,it is obvious that the deformation is bigger with using the large deformation theories than the other theory,it is concluded that the former is more suitable for the mechanic characteristics study of large deformation of surrounding rock in swelling soil tunnels.(3)The revised Hoek-Brown model with expansive characteristics was proposed on the basis of revising Hoek-Brown model.It is shown that the deformation is bigger with using the revised models than the conventional models,the method of lab test can obtain the parameters of the revised models,and the model has practical application value.(4)According to establish 3-D numerical model of large-section expansive tunnels,using the constitutive model of the large deformation of expansive soil for calculating the evolution rule of surrounding rock's large deformation at different excavation.(5)According to optimize the support structure,the plan of reinforcing middle columns primarily is proposed,and it ensures controlling the large deformation of the surrounding rock.From the analysis of in-situ monitoring,it proves that the plan can ensure the stability of the large deformation.(6)The evolutionary neuron algorithm is used to predict the large deformation of Chenggong tunnel,and it multi-step scroll predicted the monitoring data of tunnel section.Comparing monitoring data with predicted results,it is shown that the algorithm can predict accurately,and the method can be used to similar engineering.