Study On Entrance Technology Of Large-Span Shallow Tunnel Portal Under Unsymmetric Pressure

With the rapid development of social economy, more and more high-speed railways will be built, especially in mountainous terrain to build high-speed railway, the tunnel will be the key of projects of construction. As the tunnel entrance has its own characteristics compared with other sections:surrounding rock is generally seriously weathered; there are many joints, faults, fractured zone and the phenomenon of the shallow and eccentric compression is very common. Because the high-speed railway has a large excavation section, slightly improper construction can cause accidents, such as serious rock mass deformation, destruction of the supporting structure, cave-collapse and slope-slippage after tunnel excavation. So the construction entrance technique of the portal section needs further studying urgently.The present situations of tunnel portal construction and theory at home and abroad are summarized in this thesis, including in the adverse topographic and geologic conditions, the auxiliary methods and the excavation methods commonly used in large-span tunnel portal. Based on the problem in the construction of tunnel portal, this thesis puts forward the viewpoint that making entering portal safely by controlling for stability of face-upward slope and choosing reasonable construction methods and auxiliary methods is the key of the tunnel entrance safety.This thesis summarizes the factors that affect the stabilization of tunnel portal slopes. Researching the excavating effects on the slope in the different wall conditions、different gradient of the slopes、different reinforcement methods and different tunnel buried depths by establishing different two-dimensional numerical simulation for the slopes. Combined with a tunnel, making three-dimensional numerical simulation for the section of tunnel portal in the real topography and surrounding rocks conditions, first using the grouting pre-support technology steady the slope, second supporting the hole by the arch and pipe shed, last excavating the dark tunnel, against the features for shallow bias crushing and other unfavorable topographic and geologic conditions of the portal section. Comparative analysis is made by analyze the surface displacement, displacement and stress of surrounding rock and the supporting force change of using double-side-drift excavation combined with different auxiliary methods. From the simulation results, analyze the weak link in the construction, and for the effect of auxiliary methods in the construction of the tunnel portal,...