Study On The Stability Of Tunnel Vault Rock Mass Based On Upper Bound Theorem Of Limit Analysis And Response Surface Method

With rapid development of underground projects,especially in tunnel engineering,various adverse factors including geological structure and groundwater have brought severe challenges to the safe construction of tunnels.Once improper disposal,it is easy to induce various collapse accidents.Therefore,the evaluation of tunnel stability under complex geological conditions has attracted wide attention.How to ensure the stability of surrounding masses under unfavorable conditions has always been the core problem in the field of engineering.At present,the theoretical researches on tunnel stability mainly focus on the homogeneous layer,while the studies on the instability mechanism under the condition of water-bearing composite stratum are obviously insufficient.In fact,under the circumstance of combination between different strata and the softening effect of groundwater on rock and soil mass,the deformation and failure mechanism of tunnel vault is extremely complicated.Therefore,it possesses important application value to study the tunnel stability under the condition of water-bearing composite formation.To evaluate tunnel vault stability and taking the composite strata and groundwater into consideration,the instability mechanism and reliability are selected as the main topic,different methods including theoretical analysis,numerical simulation,model test,engineering cases and design specifications are adopted,the instability mechanism and reliability of tunnel roof under different buried depths and various section types are established primarily.Then,the evolution law of width-height ratio,collapse range and failure probability with different parameters is revealed,and the most unfavorable formation combination under different physical and geometric parameters is studied.Besides,according to the basic principle of reliability-based design,the minimum support force satisfying a certain reliability index is obtained.On this basis,the precontrol,process control and collaborative control measures for tunnel crown are proposed,which are applied in river resource allocation project of Fujian Province.Detailed conclusions can be illustrated as follows:(1)The basic collapse types are discussed form five aspects of failure scale,failure location,failure shape,failure mechanism ad failure form.Subsequently,by collecting76 tunnel collapse cases,the distribution characteristics are quantified from tunnel span,surrounding rock classification and influence factor,in which,underground water displays most unfavorable effect on the tunnel stability.(2)Considering the failure feature of deep-buried tunnel and the influence of composition strata and groundwater.Above all,the two-dimensional curve failure mechanism of tunnel vault under different section types of deep buried tunnel is constructed based on the nonlinear Mohr-Coulomb failure criterion and upper bound theorem of limit analysis.The upper bound solution of failure mechanism can be obtained by the variational method.The evolution law of width-height ratio and collapse shapes with different parameters is studied.Finally,the validity of the proposed model is verified by comparing the existing theories,model tests,engineering cases and design specifications.(3)The collapse mechanism of shallow-buried tunnel roof under different section types is constructed primarily,the action mechanism of surface load and geometric parameters including groundwater level,stratum thickness,tunnel radius on the widthheight ratio is analyzed.Then,the influence of mechanical and geometrical parameters on the collapse shape is studied.Finally,the effectiveness of the analytical model is verified by comparing with theoretical calculation,numerical analysis and engineering case.(4)Based on the difference between the support force and the potential collapse weight,the reliability function of the deep and shallow buried tunnel vault is constructed,and the relevant parameters are divided into random variables and nonrandom variables.The variation coefficient,correlation coefficient,distribution type of the random variables and the mechanism of the non-random variables on the failure probability are analyzed.Then,for the deep-buried tunnel,considering combination between the strength and weakness strata,a series of four-group simulations,including homogeneous,hard overlying and soft underlying,soft interlayer and hard underlying,are designed to analyze the most unfavorable formation combination.For the shallowburied tunnel,a total of 12 groups cross tests are designed to determine the most unfavorable formation combination,including four groups of strong and weak formation combinations(homogeneous,hard overlying,soft interlayer and soft overlying)and 3 groups of formation thickness combinations(equal thickness,thick above and thin below,thin above and thick below).Eventually,according to the basic concept of reliability-based design,the minimum design support force to the tunnel roof under a target reliability index is obtained.(5)The safety control principle to tunnel roof collapse is expounded and the bearing characteristics of pre-control and process control measures are revealed.Firstly,in terms of pre-control measures,the mechanism of rock bolt design parameters on collapse shape and failure probability is analyzed,and the mechanical response mechanism of pre-timbering with tubular prop is studied by considering the circumferential spacing,pipe diameter and excavation footage.Subsequently,in terms of process control measures,the theoretical calculation model of surrounding rock load is established,and the stress and deformation characteristics of the initial supporting structure are analyzed.Finally,the combined support system of pre-control and process control can effectively prevent the deformation and failure of tunnel vault.(6)Taking river resources allocation project of Pingtan and Minjiang in Fujian Province as an illustrated example,the collapse mechanism of vault in deep-buried horseshoe and shallow buried rectangular tunnels is studied based on numerical simulation and upper bound theorem,by making reference to reliability theory,the failure probability and reliability index are analyzed.Besides,the distribution interval of collapse range under a certain probability condition is calculated.It can provide a theoretical basis for the prevention of tunnel vault collapse disaster.