Study On Mechanics Behavior Of The Excavation And Stability In Mountain Highway Tunnel Portal Section

With the development of transportation in China, more and more long tunnelsappeare in the traffic engineering construction in Southwest China. As key works of thetunnel construction excavation in portal section is often in the shallow burying biaswhicn is a typical adverse geological condition. Discussion on the dynamic variation ofsurrounding rock and primary support during the tunnel excavation and supportprocesses under bias shallow-buried geological condition, and further comparison andoptimization analysis of excavation methods have positive significance in promoting theconstruction of mountain tunnel.Taking the Qishupo tunnel exit section of Chengwang expressways channel asengineering background, based on Qishupo tunnel geological survey and designinformation, and referring to code for design of highway tunnel, this paper doesnumerical simulation for the tunnel. Meanwhile, combining monitoring andmeasurement during tunnel portal excavation process, further comparative analysis ofdeformation features of the surrounding rock are finished. The main contents of thisarticle include:1. According to the actual construction, applying the finite element analysissoftware ANSYS10.0, this paper does two-dimensional numerical simulation onQishupo tunnel portal section, and focus on analying stress and deformation ofsurrounding rock and supporting structure in excavation of up and down stairs. Severalconclusions are drawed: Inside of spandrel have greater tensile stress, outside ofspandrel have greater compressive stress, stress concentration are on both sides of Archin the top step at the excavation of tunnel. After down srair excavation, inside rocktension move down to the arch at the waist, while lateral side rock compressive stressextending into the tunnel vault. The selection of tunnel excavation and support structureis reasonable, under compare analysis of tunnel section and the actual monitoring andmeasuring of results. As the tunnel is in a larger unsymmetrical pressure, somelimitations exist in tunnel monitoring in the horizontal convergence measurement, wecan improve levels of measurement of convergence.2. The simulation shows deformation and stress of surrounding rock are greaterduring up stair excavation, and stress concentrations occur on arch springing. So weneed optimize the case of tunnel cross section while ensuring tunnel constructionmethod and parameters do not change. It forms a more rational excavation cross-sectionby changing the inverted arch shap.The optimization analysis results show that internalforces of surrounding rock in the excavation of the tunnel reduce by30%, the furtherdeformation of surrounding rock in the down stair excavation may also get bettercontrol.3. Dynamic simulation analysis of construction of tunnel portal section are finsihedthough establishing three-dimensional numerical analysis model. The simulation showsstress variation of surrounding rock and spatial effects in tunnel excavation process withthe tunnel buried deep growing. Based on control theory and simulation analysis of excavation of tunnel portal section, we can get an effective control on surrounding rockdeformation during the construction, but the stress of the surrounding rock is largecomparing step excavation. At the same time, optimization measures of primary supportare proposed and analysed considering stress concentration phenomenon, this mayprovide some useful references for similar projects in the future.