| The classification of surrounding rock of railway tunnel in China is based on the stability of surrounding rock of single-track railway tunnel,and the surrounding rock is divided into six grades by two basic indexes of rock hardness and rock integrity.However,a large number of engineering practices shows that the tunnel excavation section parameters such as span and flatness have an important impact on the mechanical properties and deformation of surrounding rock,and the existing classification method of railway tunnel surrounding rock does not consider these factors.Therefore,it is of practical significance to study the influence of tunnel section parameters such as span and flatness on surrounding rock stability.In this thesis,based on the principle of orthogonal experiment,numerical analysis,analytic hierarchy process and strength reduction method are used to study the sensitivity of section parameters such as excavation span,flat rate and initial support thickness of support parameters affecting the stability of surrounding rock of tunnel,the mechanical characteristics and deformation mechanism of surrounding rock of large-span railway tunnel,the reasonable value of flat rate of large-span railway tunnel and the correction method of span to the basic quality index BQ value of surrounding rock.The main conclusions are as follows.(1)Based on the principle of orthogonal experiment and numerical simulation method,the vertical displacement of vault of surrounding rock tunnel at different levels under specific span and flat rate is calculated and analyzed by range analysis method,and the importance order of the influence of excavation span,flat rate and initial support thickness on the stability of surrounding rock is qualitatively revealed.Through regression analysis,the contribution of three factors to the vertical displacement of vault is quantitatively analyzed.(2)The finite difference method is used to numerically simulate the construction process of tunnels with different flat rates under different surrounding rock levels.Combined with the displacement,stress,plastic zone of surrounding rock caused in the construction process and the safety of primary support structure,the mechanical properties and deformation mechanism of surrounding rock under different section types are obtained.(3)Considering the surrounding rock stability of the four-line long-span railway tunnel,the safety of the initial support structure and the economy of the tunnel project,the multiobjective optimization of the section type of the four-line long-span railway tunnel under the condition of grade I-V surrounding rock is carried out by using the analytic hierarchy process.The reasonable flat rate of the four-line long-span railway tunnel under the condition of surrounding rock at all levels is obtained,which are 0.625 for I ～ II rank surrounding rock,0.65 for III rank surrounding rock,0.675 for IV ～ V rank surrounding rock.(4)The shear safety factors of surrounding rock during the excavation of railway tunnels with different spans and different surrounding rock levels are calculated by using the strength reduction method,and the quantitative standard of surrounding rock classification is established based on the safety factor index of small-span railway tunnels under different working conditions and the corresponding basic quality index BQ value of surrounding rock.Through regression analysis,the mathematical relationship among safety factor,BQ value and span is established,and then the correction method of surrounding rock quality index based on span is obtained. |
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