Study On Construction Mechanics Effect Of Bias Multi-arch And Bifurcation Tunnel

The bifurcation tunnel is composed of large arch tunnel,multi-arch tunnel,small interval tunnel and separated tunnel.It has the characteristics of these tunnel structures,but it is by no means a simple superposition of the type of tunnel structures.Due to the different topographic and geological conditions of the tunnel,the construction method and supporting form of the bifurcation tunnel are different,its construction methods and construction conditions have itself unique characteristics.The stress distribution and structural stress state of surrounding rocks in the transition section is complicated.The middle partition wall and the middle rock column will bear greater surrounding rock pressure.Therefore,it is of great significance to study on construction mechanics effects of bifurcation tunnel.In this paper,The bifurcation tunnel project as the research background,using finite element numerical simulation,combined with monitoring measurement,longitudinal space mechanical effect of multi-arch section and small interval section is analyzed,the multi-arch section,the transition section,the small interval section construction excavation method is also analyzed.Combined with monitoring data,the deformation characteristics of tunnel surrounding rock are analyzed.The main contents of this paper are as follows:(1)Based on the design scheme of tunnel,this paper analyzes the mechanical response characteristics of surrounding rocks under different longitudinal spacing of the excavation surface in multi-arch section and small interval section.It is concluded that: the vertical displacement of the vault is the largest at its burrow,which gradually decreases with the depth of the tunnel.After the excavation stopped with the established construction method,the multi-arch section tends to be stable at 1.0B in front of its excavation surface,the small interval section tends to be stable at 10 meters in front of its excavation.The increase of spacing will enlarge the distribution of the antecedent plastic zone,but the plastic strain will decrease.The main stress of the middle partition wall increases continuously when the spacing≥1.5B and≤2.0B,and decreases when the spacing≥2.0B and ≤3.0B.The increase rate of the main stress of the middle rock column changes significantly when the spacing≥ 0.5B and≤1.5B,and does not change significantly when the spacing≥1.5Band≤2.0B.The reasonable longitudinal construction spacing of multi-arch sections should be≥3.0B,the reasonable longitudinal construction spacing of small interval sections should be≥2.0B.(2)based on the analysis of reasonable longitudinal construction spacing,the construction method of multi-arch tunnel and small interval tunnel is summarized.From the four aspects of displacement,stress state,force of supporting structure and distribution of plastic zone,the influence of construction method on mechanical characteristics of multi-arch section,bifurcation transition section and small interval section were analyzed.(3)combined with monitoring data of the bifurcation tunnel,the longitudinal deformation profile of the monitoring section was analyzed,and the deformation characteristics of surrounding rock of the transition section were obtained: the small interval section of the interface was the weak area of surrounding rock stability of the tunnel,but its influence range was limited,and the corresponding construction countermeasures were proposed:(1)After the middle partition wall reach to the design strength,the small interval sections should be excavated forward;(2)The beginning of the small interval sections’ lining can be reduced to a level of surrounding rock support;(3)Pay attention to the construction quality of the system bolt or tension reinforcement of the middle rock column.