Study On Construction Method And Mechanical Behavior Of Preliminary Support Of Loess Tunnel With Super-large Section

With the development of the western region in the past 30 years,super-large section loess tunnels have emerged continuously,but it also brings a lot of loess engineering diseases.Based on the characteristics of rapid development and long duration of surrounding rock deformation of loess tunnel,three-bench with inverted arch excavation method is adopted in some tunnels of Menghua Railway,which can realize rapid construction,early closure of preliminary support and effectively control the deformation of surrounding rock.Based on the Yuejia No.1 Tunnel of the 7th Bid Line of Menghua Railway,the excavation method,preliminary support of super-large section loess tunnel was researched by using the method of combing the field monitoring with finite element numerical simulation.And the reasonable excavation method and support parameters are finally put forward.(1)With the mumerical simulation,the deformation of surrounding rock and the force law of preliminary support of traditional three-bench and three-bench with inverted arch method in Ⅳ-lever surrounding rock are compared and analyzed.It is concluded that three-bench with inverted arch method can control the deformation of surrounding rock more effectively than the traditional three-bench method.The corresponding maximum dome settlement,maximum inverted arch uplift and maximum horizontal convergence is reduced by 14.6%,27.2%and 25.7%respectively,and the overall force of preliminary support is smaller.(2)With the numerical simulation,the optimization design of the upper step height under the method of three-bench with inverted arch is optimized from the aspects of deformation of surrounding rock,plastic zone,stress condition of preliminary support structure and considering the construction space of each step comprehensively.the optimal step height of 0.3H～0.4H is proposed.(3)Based on the three-dimensional topographic modeling of Yuejia No.1 shallow-buried loess tunnel,the method of controlling V-lever surrounding rock deformation of super-large section loess tunnel from two aspects of excavation method and preliminary support is considering.Combining with field monitoring data,it is shown that the composite support technology of "advanced pre-support and lattice girder and spray net and anchor pipe" combined with three-bench with inverted arch method can effectively control the deformation of surrounding rock.(4)Based on the ubiquitous joint model,the influence of loess vertical joints on the deformation of surrounding rock and the force status of preliminary support structure is considered with numerical simulation method,and compared with field measured values.The results show that vertical joints in loess leads to greater deformation of surrounding rock and force of preliminary support,and the horizontal displacement of surrounding rock is the most significant,which is more consistent with the monitoring data.(5)The monitoring results of shallow-buried losses tunnel show that the settlement of dome is relatively small and the settlement convergence is faster because of the reinforcement effect of advance support on surrounding rock.However,the horizontal displacement shows a larger displacement when the depth of loess tunnel is shallower,and it still shows an increasing trend after the preliminary support is closed.Therefore,in the construction process of shallow buried section of super-large cross section loess tunnel,it is necessary to strengthen preliminary support and monitoring measurement of surrounding rock convergence,and strictly realize advance support and early closure of preliminary support.