Study On Progressive Failure Of Soil Particle When The Shield Tunnel Across The Hole And The Design Of Reinforcement Refinement

Research on the soil stability and refined reinforcement design problems of in-out head-hole for the practical shield tunnel project is investigated systematically in this thesis. A reinforcement instance of Nanjing Metro tunnel sect of No.TA15 is used to illustrate the propositional principle. Based on the idea of comparing the micro-based model to the macro phenomena, and then modified method through the practical engineering example, Nanjing subway microscopic particle flow model and the reinforcement model are established along the typical fine sand silt layer of hilly and plain areas. Corresponding to the research way in which Microscopic particle flow-Shear zone-Macro failure- Refined reinforcement design example, this paper do further parameter calibration, comparative analysis and field verification for failure mode analyse of shield tunnel in-out head-holes, stability calculation method and reinforcement size determination. The relevant research results are obtained as follows:(1) The micro-sandy particle flow model of Nanjing tunnel in-out head-hole is established based on Nanjing subway deposition along the hilly plain area of special geological features and geological conditions. The special progressive destruction characteristics and gradually failure mode analysis is carried out across the soil disturbance and damage process with shield advance. Micro soil parameters and effective properties are initially identified. Undisturbed soil are operated by the shield cutter cutting, grasping, planning, vibration, shock, compaction, peeling and other dynamic and static loads. The pore water pressure appears the processes of dissipation and local sharp increase. The internal friction angle showed a linear increasing trend with particle friction coefficient increase; Cohesion among the particles also shows a linear increase relationship with the increase of bond strength.(2) The soil stability factor, parameters and the scope of reinforcement theory of in-out head-hole for shield tunnel are given. Through angle of view from micro to macro and comparison methods, stability and parameter sensitivity analysis is carried out more detail in Nanjing shield tunnel. Some points are gained as follow:It should find out whether the tunnel design can increase the effective depth depends on control or improve the stability of the soil in-out head-hole or not; The risk is greater with the tunnel depth is more shallow, or the stability of soil in-out head-hole is relative more poorer; It need to improve indicators of the internal friction angle and cohesion to lower the risk of collapse. In this case study, the change of internal friction angle and cohesion had the most significant effect on the stability of the soil in-out head-hole, and Poisson's ratio change has not obvious affect the stability of the soil in-out head-hole. (3) Through analysis of the development and changes of the displacement field for the soil in-out head-hole of Nanjing subway shield tunnel, it shows that more pronounced trends in regional and directional movement is appeared. The particles motion law express themselves obviously characteristics from non-directional to directional under specific shield cutter working conditions. The sands and granules are pushed closer. With the process of reconstruction, stripping, rapid reformation of distinct slip bands and the shear slip went through the surface are developed. Finally, two concentrated squeeze regions and sliding surfaces appear more obviously in the head-hole:the basic sliding surface distribution was oblique lines above the top edge position; and the sliding surface approximate appear the logarithmic spiral or circular distribution below the top edge location. The measured volume of soil loss is consistent with the volume size of dynamic surface settlement; it provides a theoretical support case for the law.(4) A modified elastic plate limit equilibrium analysis model is proposed for shield tunnel in-out head-hole. The reinforcement of soil thickness is calculated. The conditions amendment of the theory is done compared with the examples,The analysis shows:In the condition of taking the same parameters and the safety factor, the calculation of the classical reinforcement thickness of the limit equilibrium model get 4.68m, the reinforced thickness of the modified elastic plate model get 5.15m, the reinforced thickness of the traditional elastic plate model get 7.54m. Nanjing subway project select the actual reinforcement thickness of 6.0m, and achieve success out of holes.It can be seen that the reinforced thickness of the calculation result of the classical limit equilibrium model and modified elastic plate model results slightly smaller and close to the value of the actual project of Nanjing subway, but the traditional calculation of elastic plate model results significantly safer by contrast with the practical engineering applications.(5) Based and expand the classical theory of plastic zone around lining to the engineering application areas, the calculation value of the reinforcement width and thickness can be gotten based on the plastic theory and the axial symmetry/non-axisymmetrical boundary equation.In the same calculation conditions of taken the same safety factor and other parameters, the calculated reinforcement thickness of cross-section were 2.21m,2.26m and 3.17m corresponding condition ofλ=1.0,λ=0.33 andλ=0.25, Nanjing subway tunnel sect of No.TA15 engineering finally attributed success in across reinforcement section of 3.0m. It achieves the safe and successful crossing with good agreement between the established model and the actual verification.