Influence of Shallow-buried Large-section Loess Tunnel Construction on Mechanical Behavior of Pipe Shed
Considering the primary support lag effect,excavation disturbance effect and pipe shed grouting diffusion effect of loess tunnel,this paper establishes a mechanical model of pipe shed support for large-section loess tunnel based on the theory of Pasternak elastic foundation beam,and derives the analytical formulae of pipe shed displacement and internal force.Taking the shallow-buried section at the portal of Yijun Tunnel on Xi'an-Yan'an HSR as the study object,this paper studies the influence mechanism of excavation method and excavation footage on the advanced support mechanics of pipe shed.The study results show that:(1)Due to the poor grouting diffusion effect in loess stratum,the circumferential bearing arch effect of pipe shed is not significant,and beam effect appears in the longitudinal direction,which transmits the loose load generated by excavation to the primary support structure and surrounding rock behind the tunnel face through bending deformation.(2)The internal force of pipe shed fluctuates in a sine function along the longitudinal direction,and tends to be stable within a certain range after entering the surrounding rock of the tunnel face.The maximum bending moment occurs in the unsupported excavation section,and the maximum shear force occurs at the junction of the supported and unsupported sections.The shear force of the pipe shed is greater than the bending moment,so attention shall be paid to the shear performance of the pipe shed section in the project.(3)The influence of excavation method on the pipe shed displacement and internal force is greater than that of excavation footage.Excavation by benching tunnelling method can significantly reduce the pipe shed displacement and internal force,while reserving core soil in the benching tunnelling method can reduce the pipe shed displacement,but has little influence on the internal force.The study results can provide reference for the advanced support design of pipe shed in loess tunnel.
Xi'an-Yan'an HSRlarge-section loess tunnelpipe shedPasternak elastic foundation beam
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