| Long-term crustal movement of tunnel stratum results in complicated geological environment and changeable rock mass characteristics,leading to unknown factors to tunnel engineering stability.It is crucial to investigate which role tunnel surrounding rock is playing in maintenance of tunnel stability.Over past years,it believes in underground structural mechanics that the function of surrounding rock integrates three into one,which are load source,bearing structure and engineering materials.And surrounding rock is regarded as the main structure of tunnel bearing system composed of naturally surrounding rock and artificially support structure.The load borne by surrounding rock comes from the secondary stress generating after tunnel excavation disturbance.Based on this,this thesis forms a new understand according to structural mechanical principle,namely,surrounding rock functions as the dominant structure of the bearing system during underground tunnel construction,bearing the natural far-field stress as the load.On the one hand,surrounding rock functions as only bearing structure,which is consistent with the function of general civil structure;On the other hand,surrounding rock is different from general civil structure regarding the material source,such as formation process and material composition,in which the former is determined by geological history and is naturally formed.Therefore,in order to distinguish from artificial structure,the surrounding rock is called as natural structure.On basis of this,this dissertation attempts to develop an analysis method of safety factor of tunnel bearing system,this is to say,this dissertation studies several aspects of the tunnel natural structure,i.e.,evaluation of relevant parameters,boundary formation process,mechanical state,calculation of resistance,calculation of factor of safety,and estimation of the allowable value of the safety factor;It provides a way to promote the qualitative analysis method based rock mass classification systems to a quantitative analysis method based indices of safety factor.The main content of this dissertation is as follows:First,the conversion relationships among the main three rock mass quality indices are investigated,revealing the corresponding relationships among the rock mass quality grades decided by three rock mass classification systems.A new diagram is provided to demonstrate the relationships among the three rock mass classification systems,improving -system’s demonstration of the limit state of the natural structure.In light of this,the revealed conversion relationships are combined with the formulation provided by Mohr-Coulomb criterion,demonstrating the envelope between rock mass parameters corresponding to different rock mass classification systems,verifying the feasibility of the relationships among the rock mass classification systems.Based on this,a method is proposed to calculate rock mass parameters based on the rock quality index .Second,aiming at axisymmetric tunnels,based on the respective results of the Mohr-Coulomb criterion and the Hoek-Brown criterion,numerical simulations of tunnelling are conducted.It is found that regarding high-quality rock mass,the disturbance amplitudes of rock mass,which is located at 6 times tunnel radius away from the excavation center,are close to 2.8%,while those regarding general or weak rock mass are obviously more than 2.8%.Different ranges of numerical model size are concluded corresponding to different qualities of rock mass.Aiming at non-axisymmetric tunnels,a method of estimating the natural structure boundary is proposed,and main factors related to the natural structure boundary are analyzed.It reflects that lateral pressure coefficient has a clear influence on the natural structure boundary,while tunnel excavation geometry has almost no influence.And methods are summarized to approximate tunnels from non-axisymmetric conditions to axisymmetric conditions,providing an effective design of numerical model size in the preliminary stage of tunnel engineering.Third,considering initial instability mode of rock mass,tunnel minimum support pressure is calculated,and a method is proposed to estimate the allowable value of radial deformation of the natural structure.The proposed estimation method is verified,finding that results obtained from this proposed method are more conservative than numerical simulation outcomes;Factors then are analyzed in terms of this proposed method,showing that regarding the radial deformation allowable value,each of three broad factors,i.e.initial conditions,excavation geometry and rock mass characteristics,has positive or negative impact;Regarding the normalized support pressure,each factor has some impact except for two factors,i.e.,Young’s modulus and Poisson’s ratio.Based on the proposed method,the convergence confinement method is integrated to calculate safety factor of the natural structure.This is combined with the formulation of support structure safety factor,providing a quantitative analysis method of stability of tunnel support system composed by two main support components,i.e.,natural structure and support structure.Fourth,a new strength reduction method is developed based on a traditional strength reduction method,improving reduction calculation of rock mass properties.This is integrated with the strain threshold introduced by Sakurai,developing an iterative procedure to compute the natural structure safety factor as well as detailed steps.Considering the -system’s diagram developed in Chapter 2,minimum safety reserve of the natural structure is analyzed,values of relevant parameters are selected to meet the requirement of the lower limit of the natural structure safety factor.These values are input into numerical simulation procedures,different lower limit values are obtained according to different cavern spans.The developed approach of the natural structure safety factor lower limit is validated through a practical engineering example,and then suggestions are provided to optimize support design in the preliminary stage.Fifth,the equivalent calculation method is utilized to simulate the reinforcement effect of support structure on the natural struct ure,and construction schemes under different excavation methods are designed,considering two main factors,namely,whether support structure is installed,and different installation timings of support structure.Numerical simulations are used to investigate the effects of the two main factors on the longitudinal deformation profile,and further on the safety factor of support structure and of natural structure.It is found that the increasing values of the support structure safety factor,considering support installation,only occupies no more than 41.30%of those considering no support installation;while the two main factors have effects of 3% at most regarding the natural structure safety factor.Last,a typical cross section,subject to complicated environment in Luoyixi Tunnel,is taken as a practical engineering example.Numerical models are constructed to be utilized in the numerical simulation of the excavation process of Luoyixi Tunnel as well as the support structure installation process.The natural structure boundary of this numerical model is determined,and the allowable value of the natural structure safety factor is evaluated.Then a quantitative analysis of tunnel stability is performed according to two types of the support structure,finding that when giving a later installation timing of support structure,the support structure safety factor becomes higher,meaning a higher degree of support structure stability;while the natural structure safety factor becomes lower,meaning a lower degree of natural structure stability. |
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