| Shield construction has become the mainstream technology of urban subway construction in my country.In recent years,the large-section shaft and roadway projects of many domestic mines have also begun to promote the use of this advanced construction technology.However,due to the many "congenital" process defects in the shield technology itself,a variety of associated risks follow.One of the most common engineering risks is the deformation and damage of surface structures caused by shield tunneling through pile foundations.Especially in the process of urban tunnel construction,as a result of the shield tunneling with an unloading effect,the force balance between the original soil and the pile is broken,and the frictional resistance of the pile decreases.The original bearing capacity of the pile will undergo an irreversible downward change,causing destructive effects on the stability of existing structures on the ground.As a result,a "destabilization phenomenon" for existing structures on the surface caused by shield tunneling is observed,which has become the norm.Exploring and understanding the mechanical mechanism of the interaction between shield construction and adjacent piles,formulating scientific and effective technical means,and fundamentally avoiding the occurrence of such disasters have always been one of the technical problems to be solved urgently in the field of shield tunneling.This paper takes the shield tunnel of Suzhou Metro Line 6 passing through the pile foundation of adjacent existing buildings as the research background.The comprehensive research methods,such as theoretical analysis,numerical simulation,laboratory model test,on-site detection and monitoring,etc.are used.On the basis of systematic research on the interaction mechanism of tunnel shieldsoil-existing pile foundation,combined with the obtained theoretical and experimental results,a specific treatment plan is formulated.It has been successfully applied in the shield construction of Suzhou Metro Line 6.and the tunnel can smoothly pass through the pile foundation of the adjacent existing buildings.This paper has achieved the following innovative results:(1)A test system suitable for simulating the disturbance of shield tunneling to existing pile foundations is designed and constructed,and several laboratory experiments are conducted.The experimental results visually and quantitatively exhibit the evolution process of the tunnel shield-soil layerexisting pile foundation interaction.Hereafter,through the finite difference analysis of the experimental data,the mechanism of action is revealed from the perspective of experiment-simulation.Based on the centrifugal testing machine(50 g·t)research platform in the geotechnical centrifugal chamber of Tsinghua University,several geotechnical centrifugal model tests(N=80g)of tunnel-soil-existing pile foundation interaction are designed and carried out.A set of tunnel model devices that can truly and accurately simulate shield tunneling are self-designed and produced,and digital image correlation technology(DIC)for the first time is used under high-speed centrifugal conditions.The changes in various performance indicators such as the displacement and internal force of the existing pile foundation,soil layer deformation and pore pressure,and the gradual evolution characteristics between the existing pile foundation and the surrounding soil layer during the shield tunneling with a step-by-step mean are quantitatively and visually studied.With the help of the FLAC3D 6.0 finite difference program,considering the fluid-solid coupling effect,the advanced elastic-plastic Cap-Yield(C-YSoil)soil constitutive model is selected,and back-calculation analysis of numerical model parameters is conducted.The influence mechanism of shield excavation disturbance on the performance of existing pile foundation under different tunnel-pile horizontal clearance conditions is further explored,and the stability of the tunnel excavation surface is also analyzed.(2)A simplified analytical calculation method considering the influence of multi-factor shield construction and soil disturbance on the lateral horizontal response of adjacent pile foundations is proposed.The prediction of the horizontal mechanical behavior of existing pile foundations can be realized.The calculated results are basically agreeable with the experimental results.The interaction mechanism of tunnel shield-soil layer-existing pile foundation is further revealed from the perspective of calculation.Based on the "two-stage" analysis method and the basic solution of Mindlin,the theoretical calculation formula of the additional total horizontal free displacement field caused by the multi-factor shield construction at the axis position of the existing pile foundation is derived.According to the beam element and foundation model theory,a tunnel-soil layer-existing pile foundation interaction mechanical mechanism analysis model based on the Timoshenko Beam-Kerr elastic foundation model is constructed.Using the finite difference solution method,a simplified analytical calculation method considering the inflence of shield tunneling on the lateral horizontal response of adjacent pile foundations under different boundary conditions is deduced.Through comprehensive comparison and analysis with finite difference program.boundary element program,centrifuge test and field measurement results,the reliability and rationality of the derived analytical calculation method are verified,and the sensitivity analysis of related parameters is systematically carried out.(3)A set of construction methods to eliminate the destructive impact of shield construction on adjacent existing pile foundations are proposed and successfully applied.By adopting improved synchronous grouting materials and technology,and scientifically adjusting the pile density and pile parameters of MJS piles,the "zero" impact of shield excavation on adjacent piles has been achieved,and the project has achieved success.Aiming at the typical engineering problems of tunnel shield-soil-existing pile foundation interaction,combined with the interaction mechanism revealed by various means such as centrifugal test,numerical simulation and theoretical calculation,a safety control scheme,i.e.,shield tunnel synchronous grouting material and process improvement+MJS construction method pile partition prereinforcement,for the construction disturbance of the existing pile foundation adjacent to the shield tunneling is proposed from the perspective of weakening the disturbance source and blocking the disturbance path.The reliability and engineering practicability of the proposed treatment scheme are comprehensively verified through field tests,detection and monitoring,and Midas GTS NX numerical simulation method,and the correctness of the tunnel-soil-existing pile foundation interaction mechanism revealed in the above chapters are further judged,which provides a useful reference for the construction design of tunnel-soil-existing pile foundation interaction engineering in the future. |
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