Study On Mechanism And Engineering Application Of Advanced Support For Lower Overburden Tunnel In Weak Stratum

Abstract:Tunneling with lower overburden in weak stratum, Effective advanced support is essential to ensure stability of surrounding rock and meet the control standards of ground deformation. Based on the summary of deformation mechanism and controlling techniques for weak rock tunnel, and taking several real projects as examples, the reinforcement mechanism of pipe-roof and sub-horizontal jet-grouting were discussed by means of the analytical solution, field test, numerical analyses and so on. The main contents of this thesis are:(1) The deformation of tunnel surrounding rock can be divided into pre-convergence?face extrusion and convergence. Tunneling in weak ground usually results in wider pre-convergence range which takes a greater percentage of the total deformation. A large tunnel face extrusion is accompanied with in soft and weak ground than normal ground. Suggestions on the selection of pre-reinforcement method for tunneling in weak ground with low overburden were proposed.(2) A specific numerical model was established to study the mechanism of long pipe roof based on field test carried out at Shitougang Tunnel. The pipe roof turns out three stress region with respect to the position tunnel face:tension area in front of excavating face, compression area behind the excavation face, and tension area at the tunnel entrance. The length of the tension area in front of tunnel face is constant while the other two tension areas increase with the advance of the tunnel face, and they fall into a regular "slow after the first fast" pattern.(3) In order to overcome the deficiencies of the original elastic foundation beam, Improved elastic foundation beam model has been proposed. The new model considers comprehensive carryover effect of initial support?variable bedding value of surrounding rock in front of the tunnel face and time-space effect of rock or soil stress release. Numerical analysis based on Shitougang tunnel shows that, The modified Winkler elastic foundation beam model can results a good agreement with field testing. The effectiveness of the modified Winkler elastic foundation beam model has been highlighted.(4) Soil samples was taken from the spillway section of Jiangmen Tunnel which is pre-reinforced by sub-horizontal jet-grouting. Physical and mechanical parameters of the soil before and after reinforcement was obtained by lab test. The specific numerical model which consider the fluid-structure interaction is established to study the mechanism of sub-horizontal jet-grouting. Results show that sub-horizontal jet-grouting can play good "arch effect" in transverse direction and significant "beam effect" in the longitudinal direction. As a result, the stability of surrounding rock and the effects of reinforcement has been increased.(5) Sub-horizontal jet-grouting have some unconquerable defects because of limitation on the material properties and construction techniques, such as the tensile and shear strength is very low. The measure of combining with pipe roof has been proposed to add pipe roof under sub-horizontal jet-grouting pile. Numerical simulation shows this method can reduce plastic zone and tensile stress of the pipe, and improve force of columns. The composite structure brings their respective mechanical advantage into full play. It has been applied successfully in the spillway section of Jiangmen Tunnel.(6) Tunnel face reinforcement not only can reduce the face extrusion deformation increasing the stability of working face, but also change the stress pattern of sub-horizontal jet grouting columns. This is another method to overcome the defects of sub-horizontal jet-grouting. The study on reinforcement parameter for sub-horizontal jet-grouting and face shows that the effect on controlling the surface settlement is very limited after the diameter and stiffness of sub-horizontal jet-grouting columns and the ratio of reinforcement area to tunnel face reaching to a certain amount. These values should be chose according to the circumstance.(7) Based on the existing design theory and construction technology of pipe-roof and sub-horizontal jet-grouting, the design parameters of pre-support (reinforcement) should follow the principles of safe, economical and efficient according to engineering ground conditions and control requirements. In special circumstances, comprehensive measures are necessary, and the parameters for control and management should be strengthened. There are95figures,36tables and160references.