Study On Ground Deformation Caused By Subway Tunnel Construction In Changchun Soft Mudstone Formation

Rheology of soft rock, as an important part of rock mechanics, is closely related to the long-term deformation of subway tunnel and safety of tunnel engineering. The construction methods of Changchun subway No. 1 line tunnel are shallow mining method and shield method, and construction interval of the two method are adjacent, of which engineering geological conditions are similar to each other. Settlement of ground surface, settlement of vault and tunnel convergence are monitored on-site during the construction. The subway tunnel is contained in the predominantly highly weathered mudstone formation, which has rheological property. The direct shear creep tests in laboratory were carried out to study the rheological behavior of mudstone. The deformation feature of ground was analyzed through Peck method and stochastic medium method(SMM), and the results were contrasted with the field monitoring data. With considering the rheology of surrounding rock, the relations of interaction between surrounding rock and supporting structure were analyzed based on the convergence-confinement method.In this dissertation, the main study work focuses on the following several aspects.1. Highly weathered mudstone samples were taken on the site of subway tunnel excavation face, of which the basic physical properties were tested firstly. The direct shear tests was carried out to get the instantaneous shear strength. On this basis different grades of shear stresses were loaded to carry out the direct shear creep test, and the creep curves of strain vs time were obtained. Data of stress and strain at the same time were extracted to draw a series of stress-strain isochrones, from which long-term shear strength of mudstone can be gotten. It can be found that the long-term cohesion and long-term friction angle are lower than that instantaneous, so it is necessary to treat the long-term shear strength parameters as the design value.2. Through the curves of between strain and time at different levels of shear stress, it can be found that when shear stress is lower, the mudstone is in the period of attenuation creep; as the shear stress increases, the mudstone enters the steady creep period gradually; when the shear stress goes up to some extent, the mudstone is destroyed within a short time, meaning that mudstone has already entered the acceleration period. For the deformation characteristics of highly weathered mudstone samples, two different creep constitutive model are used to describe the creep process, which are power law creep model and Nishihara model. The result shows that Nishihara model is better in describing the creep characteristic of highly weathered mudstone. Numerical model of direct shear creep test was founded through FLAC 3D program, and the result agrees with the survey value approximately.3. Two widely used methods in the field of ground movement, Peck method and stochastic medium method(SMM), were used to analyze the deformation characteristic of ground movement which was caused by the construction of subway tunnel. The settlement trough width, the width coefficient, and formation loss ratio, were sum up through the domestic and international scholars' study. Based on the theoretical analysis and case study analysis, the theoretical relationship between SMM and Peck method is discussed. It can be found that the SMM can be regarded as integral superposition result of Peck method, and Peck method is the special case of the excavation section being infinitesimal unit. By means of geometrical analysis, the formula of influence angle of compounding stratum can be obtained, which improves the application range of SMM.4. Based on stochastic medium method(SMM), the calculation process of ground deformation about non-uniform convergent circular tunnel was programed through the MATLAB compiler. The SMM calculating data and Peck calculating data were contracted with the field monitoring data, and the result shows that SMM and Peck method can be both used to describe the deformation characteristic of ground well. The relationship curves of maximum ground settlement vs tunnel vault settlement, maximum ground settlement curvature vs tunnel vault settlement can be built through the MATLAB compiler. The control value of tunnel vault settlement was obtained at last.5. The relations of interaction between surrounding rock and supporting structure were analyzed based on the convergence-confinement method, which contains 3 curves: longitudinal deformation profile(LDP), ground reaction curve(GRC), support characteristic curve(SCC). The numerical model of subway tunnel excavation was established by FLAC 3D program, and the deformation process of subway tunnel was recorded as the excavation surface moved forward, through which the LDP can be gotten. Based on the stress release method, the virtual support force of subway tunnel was released in two-dimensional model. The GRC which had a time effect could be obtained with the creep constitutive model of surrounding rock. Based on thick-walled cylinder theories, the composite SCC of shallow mining method and shield method were gained respectively. The relations of interaction between surrounding rock and supporting structure were analyzed through combining the GRC and the SCC. And efficient supporting time can be determined on this basis.6. Through the comparative analysis of construction influence of shallow mining method and shield method, the conclusion can be drawn that the deformation of shield method is smaller than that caused by shallow mining method, and the support strength is higher than that of shallow mining method. So the shield method is better than shallow mining method in the aspect of safety and fitness.