Study On Interaction Mechanism Between Surrouding Rock Hydro-collapse And Lining Structure Of Loess Tunnel And Its Evaluation Method

During the construction of basic transportation facilities in the Loess Plateau area of Northwest China,it is inevitable that the tunnels pass through the collapsible loess stratum.The potential collapsible deformation of the foundation will seriously affects the safety and stability of the tunnel structure during its operation.Therefore,it is particularly important that the foundation collapsible deformation of loess tunnel can be reasonably predicted and the harmfulness for lining structure can be correctly evaluated in the stage of investigation and design.However,the current criterion for evaluating collapsible deformation of loess foundation is only applicable to the building foundation under shallow and loading conditions,which is difficult to make a reasonable evaluation for the loess tunnel foundation under deep and unloading conditions.Therefore,the safe and effective treatment measures for loess tunnel foundation can not be selected based on the current evaluation method of collapsible deformation.In response to this question,the field test,numerical simulation and theoretical analysis were carried out in this thesis for researching the hydraulic-mechanical conditions and development process of loess tunnel foundation collapsibility,and the mechanical response law of tunnel structure.Finally,an evaluation method of foundation collapsibility which was suitable for large cross-section loess tunnel of passenger dedicated line is established.The main achievements of this thesis are as follows:(1)An large-scale field immersion test was carried out in large-thickness collapsible loess site with existing tunnels,which was the first test at home and abroad.Through in-situ observation of water infiltration and migration in strata,collapsible deformation development of strata and its effect for tunnel structure during long-term immersion on the surface above tunnels,the hydraulic-mechanical conditions of collapsibility of tunnel basement stratum and the mechanical response of lining structure under actual engineering conditions were revealed.The research results provided an important basis for the study of interaction mechanism between collapsibility of loess surrounding rock and lining structure and its evaluation method.(2)A method of calculating collapsible deformation based on the current stress state and collapsible deformation characteristics of soil element was proposed,which was embedded in FLAC~3DD finite difference program to realize the numerical simulation analysis of collapsible deformation of loess tunnel.The simulation program cound reflect the significant decrease of strength,the marked increase of deformation and the different collapsible deformation of strata under variant stress state in the process of hydro-collapse of loess surrounding rock.Based on the inversion analysis of in-situ immersion test in loess tunnel site,the stress field of surrounding rock,the distribution of tunnel basement pressure and the variation law of stress and deformation of lining structure during hydro-collapse of surrounding rock were studied,and the interaction mechanism between collapsibility of loess surrounding rock and lining structure was revealed.(3)According to the characteristics of surrounding rock pressure increasing and surrounding rock restraint softening during the hydro-collapse of loess surrounding rock,the load-structure model analysis method was established for calculating the mechanical response of lining structure during the hydro-collapse of loess surrounding rock.The change process of the internal force of lining structure and the base pressure during field immersion test were analyzed,and the rationality of this method was verified by comparing with the corresponding measured results.Furthermore,this method was used to analyze the mechanical response characteristics of lining structure and the variation law of base pressure under different conditions of foundation hydro-collapse.(4)Considering the unloading effect and the distribution characteristics of the base pressure of the loess tunnel,the calculation method of the compressive stress of loess tunnel foundation with large section was established,and the method for determining the collapsible deformation of the loess tunnel foundation under actual compressive stress was put forward.Considering the adaptive ability of the lining structure under different collapsible conditions of the foundation,and combining with the corresponding post-construction settlement control standards of subgrade,the control standards of residual collapsibility of loess tunnel with large section of high-speed railway was put forward.Finally,the evaluation method of collapsible deformation of loess tunnel foundation with large section under unloading condition was established.