Study On Mechanical Behavior Of Portal Section Of Loess Tunnel With Small Clear Distance Under Unsymmetrical Pressure Of Landslide

With the construction of a large number of expressways and railways in our country,tunnel engineering has also been vigorously developed.In order to solve the problems of bridge tunnel connection,overall alignment planning and optimization of highway lines under special terrain,a variety of special forms of tunnels have emerged.As a new type of tunnel structure,small spacing tunnel can adapt to complex terrain,save land resources,and has a better optimized alignment.It is convenient to connect with bridges and has been widely used in recent years.However,due to the complexity of small spacing tunnel structure,there are great differences in different practical projects.There are many key problems to be solved in the design and construction,which need further study and discussion.Taking Wuliang tunnel of Longzhang expressway as the research background,this paper studies the loess tunnel with small clear distance under bias pressure of landslide mass through field monitoring,theoretical analysis and numerical simulation.(1)Through numerical simulation,the surrounding rock deformation,supporting structure stress,size and distribution of plastic zone in the construction of up and down bench reserved core soil method,three bench partial excavation method,one side wall heading method and two side wall heading method are compared.It is concluded that the performance of two side wall heading method and one side wall heading method is close and better than up and down bench reserved core soil method and three bench partial excavation method.The single side heading method is selected as the optimal construction method.(2)Compared with the unsymmetrical pressure small clear distance tunnel,the deformation caused by the left tunnel or the right tunnel as the second tunnel is always larger than that caused by the first tunnel,which indicates that the excavation of the tunnel first will reduce the strength of the surrounding rock mass and increase the deformation of the surrounding rock caused by the second tunnel excavation,which is larger than that of the excavation disturbance of the second tunnel on the first tunnel.Compared with the different construction sequence of the single side wall heading method,it is concluded that the sequence of first outside then inside,first shallow then deep is more reasonable.(3)By establishing the calculation model of surrounding rock pressure of the left and right tunnels of shallow unsymmetrical pressure small clear distance tunnel,the calculation formula and applicable conditions of surrounding rock pressure are obtained.It is concluded that the excavation of the back tunnel will reduce the side pressure coefficient of the first tunnel,reduce the horizontal pressure,and increase the vertical pressure of the vault.This paper analyzes the size and law of surrounding rock pressure under different bias angle,buried depth and clear distance,which provides reference for the design and construction of small clear distance tunnel.(4)Through the study on the stability of surrounding rock of small clear distance loess tunnel crossing landslide,it is concluded that the landslide changes the maximum horizontal deformation and maximum settlement position of surrounding rock of small clear distance tunnel,and the plastic zone of surrounding rock extends downward at the intersection of sliding zone and tunnel,reducing the plastic zone of arch shoulder and increasing the plastic zone of arch foot.Under the disturbance of the dynamic load,the most obvious change is that the plastic zone of the middle rock column is connected by the dynamic load.Therefore,in the process of tunnel construction,it is necessary to strengthen the support of the arch shoulder,arch foot and the middle rock column of the deep buried side tunnel.