Study On Stability Of The Entrance Of The Tunnel Under Pipe-Roof-Support System In Terms Of The Constructive Influencing Factors

Excavation work of tunnel entrance is a throat engineering in the tunnel construction process and the importance of it is self-evident.But surrounding rock of tunnel entrance has shallow depth as well as weathering generally,engineering instability accidents like collapse and roof caving often appearing in the process of excavation.So the excavation stability study of surrounding rock in tunnel entrance has been one of important subjects.There are many fa ctors that can affect stability of tunnel entrance,including non-construction factors like geological structure,rock properties,ground water and so on.In addition,construction factors have also impact on stability of tunnel entrance,taking excavating types,excavation distance and preliminary support of pipe roof for example.If the selection of excavation supporting methods is unreasonable,it is easy to cause the engineering safety accidents which cause unnecessary casualties and property losses.According to the practical engineering problem,this study will focus on those construction factors which affect excavation stability of tunnel entrance in the preliminary support of the pipe roof.Basing on the strength reduction FEM,this paper will take use of the orthogonal experiment design method and finite element software ABAQUS to carry out related research.The research is about construction factors which influence the stability of tunnel entrance under preliminary support of the pipe roof.This paper will choose influence factors which include excavating types,excavation distance,thickness of primary support,diameter of pipe,range of pipe roof and grouting radius of pipe roof,and will designate the scope of each construction factors according to the related study.Then the paper will arrange numerical simulation according to the principle of orthogonal design.On the basis of the above,this study will take full advantage of the finite element software ABAQUS to establish three-dimensional numerical models and analyze various factors on the side slope stability of tunnel entrance and on the stability of the tunnel cavern.When analyzing the side slope stability of tunnel entrance,the stability safety coefficient will be chosen as the analysis indicators.The degree of the influence of six construction factors on the stability safety coefficient will be researched,and determine the influence rank of six kinds of construction factors.In this way,it will concluded that the main influence factors and secondary influence factors.Meanwhile,this paper will analyze the influence of different values of construction factors on the stability safety coefficient,and it will draw the trends of stability safety coefficient as the change of value of influence factors.When analyzing the stability of tunnel cavern,this paper will choose four displacement measurement points: top of arch,bottom of arch,left hance and right hance.Then the displacement of those points will be chosen as index of stability to study the degree of the influence of six construction factors on each measuring point displacement,and determine the influence rank of six kinds of construction factors.In this way,it will concluded that the main factors and secondary factors.In addition,this paper will analyze the influence of different values of construction factors on the displacement of those points,and it will draw the trends of the displacement of those points as the change of value of influence factors.