Upper-bound Stability Analysis Methods Of Tunnel Faces And Pre-reinforcement Designs

Tunnel engineering is one of key projects in subways,railways and highways.Accurate stability assessment of the tunnel face can effectively improve the safety of tunnel construction and the construction efficiency.With the development of infrastructure in China,tunnel construction will inevitably encounter extremely complex hydrogeological conditions,one of the key challenges in the field of tunnel engineering,in theory and practice engineering,is to propose more effective design methods for stability analysis and pre-reinforcement of tunnel faces.Existing studies are mostly limited to numerical methods or simplified models for stability analysis and pre-reinforcement designs of tunnel faces in homogeneous and isotropic soils,which seriously lags behind the stability analysis and pre-reinforcement design as well as disaster prevention and control requirements in tunnel excavations under complex hydrogeological conditions.Based on this,this work will carry out the research on the three-dimensional upper-bound analysis methods of collapse and blow-out stability of tunnel faces and pre-reinforcement designs under complex hydrogeological conditions by using the upper bound limit analysis theory.The main contents of this work are as follows:(1)Based on the discretization and “point by point” technique,improved three-dimensional collapse and blow-out rotational failure models for tunnel faces were proposed in non-homogeneous and anisotropic soils,and the shallow buried and inclined ground surface were also considered,which filled the theoretical gap in traditional analysis models that cannot consider the characteristics of non-homogeneous and anisotropic soils.In the framework of upper bound limit analysis theory,three-dimensional collapse and blow-out upper-bound assessment methods for tunnel faces were constructed based on the principle of virtual work in non-homogeneous and anisotropic soils.The effects of non-homogeneous and anisotropic soil parameters on the three-dimensional limit failure surfaces and limit support pressure were presented.Random field analysis models of geotechnical strength parameters were conducted to illustrate the applicability of these two proposed upper-bound analysis methods for the collapse and blow-out stability of tunnel faces in non-homogeneous anisotropic geotechnical soils.(2)Considering the actual failure of the tunnel face in the limit state,a blow-out partial failure model for the tunnel face was constructed based on the numerical simulation results.Considering the nonlinear failure characteristics of geotechnical soils in composite soil layers,collapse whole and partial failure models for the tunnel face were proposed in non-homogeneous and nonlinear geotechnical soils.In order to avoid falling into the trap of local optimum value,new upper-bound analysis methods of collapse and blow-out partial stability of tunnel faces under the control of multiple optimization parameters were conducted by using a hybrid optimization algorithm of rough global search combined with local refinement search,which improved the reliability of the theoretical analysis method in the collapse and blow-out stability assessment of tunnel faces.The partial failure characteristics of tunnel faces and the evolution of tunnel face from partial failure to whole failure were presented.(3)A numerical model was estiblished to calculate the anisotropic seepage field behind the tunnel face in water-rich zones.Anisotropic groundwater seepage law and seepage boundaries of the study area were obtained using the proposed numerical model.Based on the seepage law of unit block and Darcy’s law,a theoretical analysis model was proposed to calculate the anisotropic seepage field behind the tunnel face in water-rich zones,which solved the problem of applicability of theoretical analysis methods to different groundwater levels and tunnel diameters.Combined with the proposed collapse failure model,an upper-bound analysis method for collapse stability of tunnel face was proposed in water-rich zones.The effects of relative water tables and anisotropic seepage coefficient ratios on the three-dimensional limit failure surfaces and limit support pressures of the tunnel face were presented.(4)Based on the idea of homogeneous method,a three-dimensional non-homogeneous and anisotropic failure criterion for reinforced soil was first proposed,which enapnded the research of theoretical analysis methods on the design of reinforcement of engineering structures.Based on the Winkler’s spring model,the complete analytical solutions of the work rates done by the axial force and bending moment of umbrella pipes were provided.Based on the proposed collapse failure model,two theoretical analysis models for pre-reinforcement designs of glass fiber anchor bolts and umbrella pipes for tunnel faces were developed using the hybrid optimization algorithm combined with the strength reduction method.Based on the proposed seepage field model,two theoretical analysis models for pre-reinforcement designs of glass fiber anchor bolts and umbrella pipes for tunnel faces in water-rich zones were developed.Effects of bolts and umbrella pipes on the tunnel face stability were presented,and the optimal relative support lengths of bolts and umbrella pipes were determined.