Study On Bearing Characteristic Of Supporting Structure For Super-span Tunnel Under Weak Surrounding Rock

In recent years,with the booming development of infrastructure construction in China,the number of railway tunnels construction in our country is increasing,and the section of tunnels is becoming more and more large.The enlargement of the section will inevitably increase the difficulty of construction and support.Therefore,how to ensure the stability of super large section tunnel and the safety of supporting structure is an important engineering problem that we face.In this paper,the most complex station in China(Badaling railway station)is used as the project background,focusing on the transition section of the tunnel between the main line and the station(referred to as the large span),with a span of up to 32.7meters.Faced with such a tunnel,which is large span,it is necessary to study the bearing characteristics of supporting structure from the support characteristics of various supporting methods.In this paper,combined with a variety of research methods,such as theoretical analysis,field monitoring,numerical simulation and statistical analysis,the parameters optimization of bolt and cable in super-span tunnel,the selection of initial support structure and the ultimate bearing characteristics of the secondary lining structure are studied respectively.the following key research results are obtained:(1)The mechanical characteristics of elliptical tunnel excavation is analyzed.Relying on the theory of elasticity,combined with conformal transformation and complex functions,the laws of the deformation and the stress in secondary stress state of elliptical tunnels are analyzed.For the elliptical tunnel,the shear stress and the radial stress of the surrounding rock are independent of the physical properties of the surrounding rock.There are three factors affecting the circumferential shear stress of the elliptical tunnel:side pressure coefficient,angle and flat rate.The concept of isobaric flattening rate is proposed.When the side pressure coefficient is fixed and the flat ratio is the reciprocal of the lateral pressure coefficient,there is an isobaric ring in the elliptical-tunnel chamber.The quadratic relationship between the settlement of the vault and the flat rate is confirmed by the numerical simulation of the ABAQUS and the theoretical analysis.(2)The parameters of bolt and cable support are optimized.Taking the orthogonal test design as the research means,based on the cooperative support theory of bolt and cable,the support effect of four parameters about the length and prestress of bolt and cable in the super-span tunnel is analyzed.It is considered that the prestress of cable is the key factor to control the deformation of the surrounding rock,and the optimal supporting scheme is obtained by comparison.The average utilization rate of strength of bolt and cable and the stability rate of surrounding rock are studied.It is considered that the average utilization ratio of bolt body strength is higher for the super-span tunnel,and the average utilization ratio of the cable is relatively low,and most support schemes do not reach the synergistic effect.(3)The bearing characteristics of the initial supporting structure are compared.In this paper,the plastic damage constitutive model of concrete is introduced and the stability and bearing characteristics of plain concrete,steel grill frame reinforced shotcrete and merchant steel reinforced shotcrete are analyzed by the numerical simulation method.The failure process of the three kinds of concrete structure is clarified,and it is considered that the arch lumbar of the tunnel is the most destructive area.Under the condition of different lateral pressure coefficient,the relationships between the displacement of the tunnel vault and the contact pressure of three structures are studied then the relationship between the lateral pressure coefficient and the crown settlement and the contact pressure of the vault is analyzed.The reinforced effect of the steel grill and merchant steel of the initial support are summarized.(4)The ultimate bearing capacity of the secondary lining structures has been studied.Taking the large span section of Badaling underground station as the engineering background,rely on the loading-structure model,the safety analysis of reinforced concrete and plain concrete is carried out.The reinforcement effect of the steel bar in the secondary structure is clarified.Using the extended finite element theory and taking ABAQUS as the research platform,the law of the crack development of the secondary lining structure under the diverse ground stress is studied,and the reasons for the failure of the secondary lining structure are summarized.Combining with a lot of statistical data about contact stress between first and second lining,the actual force characteristics of the secondary lining structure of long span tunnel are obtained.