| In the construction of shallow buried bias tunnel in weak and broken strata,it is easy to collapse and roof fall due to the poor self-stability of surrounding rock,and the pre-support of pipe shed is a common auxiliary measure to effectively restrain the deformation of surrounding rock and ensure the safety of construction.Based on the new Tongziling tunnel of Fuxiu second line of Yuhuai railway,the pipe shed in shallow buried bias tunnel is further studied Midas GTS NX using 3D finite element software.The stress characteristics of the pipe shed and the influence law of the pipe shed’s circumferential arrangement range,the extrapolation angle,the length of the circulating steel pipe and the permeability coefficient of the reinforced area on the pre-support effect are obtained.The research results have certain theoretical and application value.The main research contents and achievements are as follows:(1)The model of pipe shed in shallow-buried bias tunnel is simulated and calculated by four groups of steel pipe lapped(10 m lapped 3 m each group).The results show that the steel pipe at the arch and deep-buried side is mainly subjected to axial pressure and positive bending moment behind the face of the face.The shallow buried steel pipe is subjected to axial tension and negative bending moment behind the face of the palm,and the internal force of the deep buried side pipe shed is larger than that of the shallow buried side.The maximum vertical displacement of tunnel structure can be reduced by 32.0%and the maximum lateral displacement by 61.6%,which indicates that the effect of pipe shed on the suppression of tunnel lateral deformation is stronger than that of vertical.The deformation value of the initial support and reinforcement area in the pipe shed lap area is obviously smaller than that in the rear non-lap area.(2)By simulating and analyzing the influence of pipe shed circumferential arrangement range and external insertion angle on the effect of pre-support,it is concluded that:the larger the circumferential arrangement is,the more obvious the reinforcement effect is.And improving the overall stability of the tunnel,the support effect is stronger when the pipe shed is only arranged on the vault and the deep buried side than when it is only arranged on the vault and shallow buried side.The influence of the arrangement range of the pipe shed on the maximum transverse deformation value of the tunnel is greater than that of the maximum vertical deformation value of the tunnel.With the increase of pipe shed insertion angle,the support effect gradually decreases,and the increase of lateral deformation is relatively large.A smaller insertion angle should be selected and controlled within the range of 1~?～10~?when pipe shed support is arranged in shallow buried bias tunnel.(3)The supporting utility of different types of pipe shed with different reinforcement zone permeability system and steel pipe length between 10 m～40 m is simulated and calculated.It is concluded that in shallow buried bias tunnel,the reinforcement effect of the tunnel is more remarkable by selecting the pipe shed with short length of steel pipe and multiple groups of circular lap joints.As the length of steel pipe decreases,the axial force value of steel pipe increases gradually.The smaller the permeability coefficient,the lower the settlement value of surrounding rock,initial support and surface,and the higher the vertical compressive stress,initial support clearance convergence,steel tube axial force,reinforcement location shift springback equivalent of arch bottom;The decrease of permeability coefficient has the greatest influence on the tensile force value of steel pipe. |
