Research On Reinforcement Range And Stability Of The End Of Ultra-deep Shield Working Well

Shield initiation and arrival is one of the main steps of shield tunnel construction,and it is also the part where safety accidents are likely to occur.The success or failure of shield initiation and arrival will directly affect the smooth construction and completion of the shield tunnel.With the integration and development of China’s shield technology and modern urban underground construction projects,the shield tunnel burial depth and cutter diameter are increasing,and the ground conditions and construction environment are becoming more and more complex and changeable,which fully demonstrates the superiority of shield tunnel construction technology,but also reveals many defects and shortcomings.In this paper,through engineering field practice,consulting and collecting engineering actual measurement data related to this study,and taking the shield start working shaft of the Jinan Yellow River Tunnel as the research background,the strengthening pattern,strengthening limits and steadiness of the Jinan Yellow River Tunnel project are studied by means of theoretical derivation,finite element analysis and comparative analysis of engineering field monitoring,mainly as follows:(1)According to the engineering overview,using the classical strength theory,the actual project is simplified by using elastic mechanics and higher mathematics to determine the force analysis model,derive the analytical formulas of different mechanical calculation models,and carry out theoretical calculation and analysis on the shield initiation of the Jinan Yellow River Tunnel.(2)Summarizing the advantages and disadvantages of the existing end soil reinforcement technology and its stratigraphic applicability,combined with the Jinan Yellow River Tunnel shield start section of mainly powder clay stratigraphy,the use of triple pipe high-pressure rotary pile and artificial freezing-assisted reinforcement,the combination of the two methods has the advantages of high strength,good impermeability,etc.,can effectively ensure the safety of breaking the enclosure structure.(3)From the results of the finite element numerical simulation,it can be seen that the maximum values of various displacements and stresses show a trend of decreasing first and then approaching a stable value as the reinforcement length increases,both in the transverse and longitudinal reinforcement ranges.Continuing to increase the reinforcement length has no significant effect on the displacement field and stress field of the end reinforced soil.The longitudinal reinforcement length is 14m,and the transverse reinforcement length is 5m,which means that the strength and stability requirements of shield initiation can be met.Under the same reinforcement length,when the tunnel burial depth is taken as 1.5D,the reinforced soil body shows an obvious trend of overall slip when the enclosure structure at the cavern door is broken.(4)The stability coefficient of the end soil before reinforcement tends to be 0.7785217,and the theoretical reinforcement coefficient should be F_r=1/0.7685218=1.28448.After the end soil is reinforced,the mechanical properties of the soil are significantly improved,and the stability coefficient is about 2.53461,which meets the requirements of calculation accuracy and is much larger than the theoretical reinforcement safety coefficient of 1.83,ensuring the stability of the end strata when the shield enters the hole.The stability and safety of the shield structure end formation are ensured.(5)Combining the finite element numerical calculation results and the engineering actual measurement results,the monitoring data during the shield initiation and excavation process are comprehensively analyzed,and the change trend of both is basically the same,which indicates the reasonable accuracy of the numerical simulation model establishment and geotechnical parameters,and can provide theoretical guidance and reference for similar projects.