Research On Pre-supporting And Excavation Methods’ Optimizationof Large-section Loess Tunnel

The loess has some special engineering properties.The tunnel construction in large-section loess tunnel is always an important issue of tunnel engineering, especially the construction method selection is an important aspect in controlling the deformation of the surrounding rock. When the tunnel in stratigraphic lithology is poorer, thepre-supporting system can better to provide the securityand stability of surrounding rock, to ensure the safety of construction more smoothly.In this paper, it based on the Xujiachuan Tunnel of " Baoji–Lanzhou High-Speed Railway ", with a large-section of the loess tunnel in level IV surrounding rock as the driving force,Through stress and strain monitoring of the construction site andtheoretical analysis of surrounding rock stress and deformation mechanism and the numerical simulation analysis,the tunnel select the reasonable tunnel excavation method.And selectthe optimization of step length and speed for the reasonable method. Through the theory of beam on elastic foundation to analysis the mechanism of the preliminary support of the pipe roofin the constructionprocesses oftunnel. And according to the numerical simulation results to analyses and evaluatesupporting effectofpre-timbering with tubular prop. Finally, the paper determines a construction of the optimum range of pipe roof bolting tunnel by comparing the different pipe roof support angle. The main results and achievements of this paper are as follows:(1) The interactive relationship and characteristics betweendeformation and supporting force of Xujiachuan Tunnel are analyzed throughmonitoring measurement dataat the construction site. The tunnel has characteristicsof initialfast deformation rate and long deformation duration.Deformation characteristics are greatly influenced by rock formations, tendency, so the deformation of surrounding rock, stress of surrounding rock and supporting force has obvious asymmetry.(2) According to the analysis and study on large-section loess tunnel excavation methods, it put forward the three steps of seven method, CD method, CRD method can better works in Xujiachuan Tunnel on level IV surrounding section.It turned out three steps on seven methods on construction safety and schedule can meet the construction requirements through comparing construction safety, the stability of surrounding rock and supporting structure stress with other methods by contrast optimization.(3) Through the optimization analysis by the numerical simulation of three step seven methods in different length or the optimization analysis different excavation steps, finally it is concluded that the optimal measures of deformation controlby comparison on excavation under different conditions, it is appropriate to use 3 m length and 0.6 m excavation steps. It shows that short footage and short steps can reduce the exposure time of surrounding rock and faster to be closed.(4)Throughthe theory analysisof preliminary support of the pipe roof model,it introduce the formulasabout bending moment, deflection and cornerof Pasternak calculation model.And through numerical simulation on the Xujiachuan Tunnel on level V section, the preliminarysupport of the pipe roof system use separate numerical simulation.It is in order to study the mechanism of pipe roof andsurrounding rock deformation, supporting stress and the surface subsidence under the action of the preliminary support of the pipe roof in the process of tunnel excavation.It shows that the effects of pipe roof deformation near the excavation plane gets bigger, bending moment near the excavation surface is negative. During excavation, which have not been supporting section of pipe roof mainly bear the pressure of surrounding rock, the excavation has been supporting the pipe roof and supporting has to bear the pressure of surrounding rock.. The steel pipe can be put behind the force transmission constraints of surrounding rock,when the surrounding was not been excavatied.(5) Through the simulation analysis about support effectof the preliminary supportunder the scope of 60 °,90 ° and 120 °, the analysis shows that under the 120 ° angle of the pre-timbering with tubular prop support is the best.