Discrete Element-based Optimization Of Overrunning Pipe Shed Support Parameters For Soft And Weak Surrounding Rocks In Tunnel Cavity Sections

The construction of tunnels in soft surrounding rocks is a difficult problem in underground construction,and the use of pipe shed overrun support can control the deformation of surrounding rocks very well.At present,many achievements have been made in the analysis and calculation of the mechanical model of pipe shed support,but the mechanical model of support in soft and weak surrounding rocks is worth improving.In view of the discontinuity of the weak surrounding rock,the discrete element software is used to study the support effect of the pipe shed on the weak surrounding rock,and parameter optimization analysis is carried out,which provides theoretical guidance for the pipe shed support project in the weak surrounding rock and has certain practical significance.The following research work was carried out in the paper:(1)By investigating the cases of pipe shed support in actual projects,the influencing factors affecting the effect of pipe shed support are divided into three categories: external influence,influence of pipe shed’s own parameters and construction influence,and each factor is specifically described;the vault settlement,peripheral convergence,ground settlement and pipe shed deflection are taken as the judging parameters of pipe shed support effect,and based on this,the comprehensive influence equation of pipe shed support effect is listed,and it is clear that the design and optimization of pipe shed support should follow the principles of safety,deformation control and systemic.(2)Considering the deformation characteristics of weak surrounding rock,the characteristics of grouting reinforcement and the spatial and temporal effects of surrounding rock stress release and the change of bed coefficient,the mechanical model of pipe shed is optimized and solved based on Pasternak elastic foundation beam.The influence of four parameters,namely,pipe shed diameter,spacing,excavation advance,and forward bed coefficient,on the support effect of pipe shed is studied,and a sensitivity ranking method with the maximum deflection of pipe shed as the system characteristic is proposed,and the sensitivity ranking from largest to smallest is diameter,spacing,excavation advance,and forward bed coefficient.(3)For the characteristics of soft surrounding rock with discrete nature,a discrete element simulation study of tube shed oversupport was conducted using 3DEC numerical software,which concluded that the tube shed grouting is integral,and the deformation control of soft surrounding rock tunneling project is better after considering the integral nature of tube shed grouting,and the settlement of surrounding rock under pre-support is reduced.(4)By changing the parameters of the overrun pipe shed support which affects the integrality of grouting,the working conditions of pipe shed support were simulated under four parameters of different pipe shed diameter,pipe shed spacing,grouting reinforcement thickness and elastic modulus of grouting reinforcement area at different excavation feeds,the maximum deflection of pipe shed under different working conditions were counted,the influence of different pipe shed support parameters on the support effect was explored,and the optimization analysis of pipe shed support parameters was carried out.