Stability Analysis For Tunnel Of The Tertiary Semi-diagenetic Strata Based On Fluid-Solid Coupling Effects

With the development of underground engineering in our nation, increasing number of railway tunnels are constructed in Tertiary strata with high hydraulic pressure. Furthermore, the Tertiary strata are different from other strata due to the distinctive geological characteristics, such as the low degree of diagenesis, being susceptible to be weathered in the air and being weakened when encountering water, which results in collapse, significant deformation, water inrush and sand inrush during tunnel construction. Additionally, mud-pumping is prone to arise during tunnel operation periods, this has increased the risks of stability of excavations. Therefore, the research on engineering geological characteristics of the Tertiary strata will further deepen the understanding of this strata and being of great significance for the construction practice.This paper based on the ground investigation and design of Zhongtiaoshan tunnel of Menghua railway line, following by comprehensive geological and hydro-geological investigation, laboratory test and the infiltration mechanism of groundwater seepage. It is proposed to apply finite difference software FLAC3D5.0 to simulate project by three-bench seven-step construction method. In order to investigate the effect on stability due to tunneling, several aspects including the nearby seepage field, surrounding rock deformation, primary support and the deformation of plastic zones are taken into consideration. The related conclusions are addressed as follows:(1) By means of adjusting physical, chemical and mechanical properties of rock mass structure seepage field causes changes in stress field. Moreover, the stress field will, in turn, affect seepage field by changing the permeability, the permeability coefficient and storage coefficient of rock mass structure.(2) By comparing the construction conditions with or without fluid-solid coupling effects, it is concluded that the displacement of surrounding rock, stress of the primary support and the range of plastic zone is much larger under fluid-solid coupling circumstance. Therefore, it is necessary to consider the dynamic water pressure of groundwater seepage in project design. Exceptional attention needs to be raised on the waterproof and drainage works of the arch crown, the arch shoulders and the inverted arch in that seepage vector concentrated areas.(3) By studying the influence of initial water head on the stress and the displacement of surrounding rock and primary support, it is concluded that the displacement of the arch crown settlement, peripheral convergence, arch bottom uplift and extrusion deformation of excavation face are increasing linearly with the rise of initial water head. The influential region of the plastic zone and the stresses of the support structure are increased gradually as well.(4) By studying the influence of the coefficient of permeability on the stress and the displacement of surrounding rock and primary support, it is concluded that when the permeability coefficient of surrounding rock is more than 10"5 cm/s, the effect on the distribution of stress and the displacement of surrounding rock and support structure is very small, However, when the permeability coefficient of surrounding rock is less than 10"5 cm/s, the maximum principal stress of the primary support decreases, the displacement of surrounding rock remains the same, the influential region of the plastic zone increases as the coefficient of permeability decreasing.(5) By studying the influence of the lateral pressure coefficient permeability on the stress and the displacement of surrounding rock and primary support, it is concluded that with lateral pressure coefficient increasing, the arch crown settlement decreases while the displacement of the peripheral convergence and arch bottom uplift increases. In addition, the maximum principal stress of the primary support is concentrated on the arch shoulders and the value of arch waists increases. The distribution shape of the plastic zone develops from a butterfly-shaped to a ring-shaped, and the volume of plastic zone is least when λ=1.0.(6) According to the simulation of Zhongtiaoshan tunnel and the special engineering characteristics of the stratum, the corresponding construction suggestions are put forward to ensure construction safety, such as horizontal jet grouting piles reinforcement, waterproof design and "depressurization to the first".From the above research, we can provide the theoretical basis for the controlling of induced deformation of surrounding rock and the stress which is caused by various control factors in different areas in the actual construction project. Consequently,this plays a fundamental role to ensure the transfixion of Zhongtiaoshan tunnel has been finished successfully.