Study On Load-carrying Mechanism And Deformation Control Of Double Primary Supports For A Super-large-span Highspeed Railway Tunnel

With the rapid development of social economy,the requirement for transportation in China has skyrocketed,especially for the rapid development of the railway transportation network that is closely related to the people.However,with the continuous improvement of the speed of high-speed railways,the demand for the span of railway tunnels is so increasing that new challenges to the construction of super-large-span tunnels is raised.Therefore,this paper is oriented toward the super-large-span high-speed railway tunnel,and proposes the concept of double primary supports for the XBS-1 and XBS-2 tunnels.By theoretical analysis,numerical simulation and site monitoring,this paper researchs bearing model of double primary supports,multi-step excavation mechanical property of super-large-span high-speed railway tunnel,parameter optimization of double primary supports.The main work and research results are as follows:(1)Based on the existing examples of double primary supports,the design concept of the double primary supports is summarized,and it is considered that the double primary supports can be divided into: stiffness-reinforcement type,stress-release type and integral-reinforcement type.At the same time,Based on the assumption of deep-buried circular tunnel and full-section excavation,combined with the simulation of rock stress release,the elastic solutions and elastic-plastic solutions of double primary supports are calculated,and the calculation examples with above solutions indicate the bearing model of double primary supports is "The first primary support is major bearing structures;The second primary support is auxiliary bearing structures ",and the key of surrounding rock stability and primary support safety is the construction time of two-layer initial support.(2)Based on NATM,the conception of double primary supports for the super-largespan high-speed railway tunnel is proposed.By the calculation of the three-dimensional numerical model,the rock deformation control effect and the mechanical characteristics of support structure under the double primary supports are researched,and concluded that "double primary supports + double-side-drift method for seven pilot" can ensure concurrently the stability of the rock and the safety of primary support.The result also points out that the essential process of double-side-drift method.For structural safety and construction efficiency.the temporary lateral support should be banned,and the The length of dismantling temporary support should be 9m.(3)By the analysis of interaction mechanism between double primary supports and rock,the adjustable parameters of double primary supports are discussed,on the basis of thickness distribution,the work model of double primary supports can be diveded three types:“small stiffness + large stiffness”,“medium stiffness + medium stiffness”,and“large stiffness + small stiffness”,and the construction times of the second primary supports in above type are discussed.It is concluded that “following with next pilot” or“integral construction of upper pilot + integral construction of lower pilot” in “medium stiffness + medium stiffness” is the best way for double primary supports.(4)By site monitoring for the double primary supports at portal section of XBS-2tunnel.For one thing,the vault settlement and horizontal convergence are sorted out and analyzed,it points out that the double primary supports at portal section of the tunnel can effectively control the deformation of the rock.For another thing,contact pressure,steel rib stress,shotcrete stress,security coefficient of double primary supports are sorted out and analyzed,and concluded that the second support is mainly carried by shotcrete,the first primary support was the main bearing structure,the second support mainly acts as a reinforcement to the primary support.