Soil Arching Effect Induced By Shield Tunneling In Sandy Ground And Its Application

Soil arching effect widely exists in geotechnical engineering.At present,due to the lack of understanding of the soil arching formation,evolution and stress transfer mechanism during the tunnel excavation,the research on the related issues of tunnel earth pressure calculation theory is restricted to some extent.In this study,the arching effect caused by tunnel excavation is analyzed in detail by adopting the methods of numerical analysis and theoretical calculation.Based on this,the new models for predicting the earth pressure on the deep tunnel and the limit support pressure on the tunnel face are established.Finally,the methods for estimating the limit support pressure on the tunnel face and earth pressure on the deep tunnel are obtained.The main study contents and research results in this thesis are shown as follows:(1)The stress redistribution and the soil arching evolution induced by earth pressure balance shield(EPBS)tunneling are revealed based on three-dimensional(3D)finite element method(FEM)models.The changes of the earth pressure,the coefficient of lateral earth pressure,and the settlement of soil mass above the tunnel during tunneling were analyzed and the soil arching zone was determined.3D stress-transfer mechanism in soil arching zone during tunneling was revealed.In addition,the influences of shield-driving parameters(i.e.support pressure,grouting pressure)on soil arching evolution were also investigated.The results show that the loosened zone extends to nearly 0.73D(D=tunnel diameter)above the tunnel crown in vertical direction.The height of the arch zone above the loosened zone is about1.27D.The horizontal soil arching in front of EPBS occurs near the ground surface.With EPBS advancing,the horizontal soil arching gradually transforms into the vertical soil arching.After the installation of lining,the earth pressure is finally mainly transferred by vertical soil arching in the transverse section.The proper combination of shield-driving parameters for controlling the expansion of the loosened zone is support pressure of 0.6P₁-2.2P₁and grouting pressure of 1.0P₂-1.8P₂,where P₁is initial horizontal stress at the tunnel axis and P₂=1.2P₁.(2)A multi-arch model for calculating the vertical earth pressure on a deep tunnel in dry sand is proposed.The model is composed of three parts:the upper end-bearing arch,the stability zone and the lower friction arch,in which the end-bearing arch and the friction arch are calculated by semi-circular arch theory and horizontal layering theory respectively.Moreover,the key parameters(i.e.,width and height of friction arch zone,thickness of end-bearing arch zone and lateral stress ratio in friction arch zone)in the proposed model are suggested according to the existing literature,and a formula for predicting the height of the friction arch zone is derived.The experimental and numerical results and Terzaghi'solution are adopted to assess the validity of the proposed model,which indicates that the proposed models not only coincide with the test results of average vertical earth pressures on tunnels,but also reflect the nonuniform distribution characteristics of vertical stress on the circular tunnel.(3)A multi-arch model to predict the limit support pressure(LSP)acting on a tunnel face for deep shield tunnelling in dry cohesionless soils is suggested.The model consists of the upper end-bearing arch,the middle friction arch,the stability zone,and the lower wedge.A formula for calculating the height of the friction arch zone H was derived,in which the ranges of the parameters were suggested based on the results of existing model tests.The rationality and effectiveness of the new model were verified by comparison with model tests,existing theoretical methods and numerical simulations.Additionally,the effects of a series of parameters on the LSP were also discussed.The results show that the soil internal friction angle?,the soil-cutterhead friction angle?,and the soil volume bulking factor?(when?>0.04)have significant effects on the LSP acting on the tunnel face.(4)A composite function model to describe the ground reaction curve(GRC)is proposed.Characteristics of the GRC were analyzed in detail combining with the development of the soil arching and the shear surface.The parameters in the proposed model for two-dimensional(2D)and three-dimensional(3D)cases were determined by suggesting the new multi-arch models and the results of existing trapdoor model tests.A formula for predicting the height of the friction arch zone was derived.The effectiveness of proposed models was proved by comparing the calculated results using the proposed models with the measured data of laboratory model tests.The results show that the proposed model can not only reflect the variation trend of the full GRC well,but the variations of minimum and ultimate soil arching ratios and load recovery index with the increase of buried depth.(5)This study investigated the deformation and stress characteristics of twin tunnels induced by close distance earth pressure balance shield(EPBS)under-crossing in watered sandy stratum,located in Changsha,China by using the methods of field measurement and numerical analysis.Based on the soil arching theory,the variation of the earth pressure acting on the existing tunnel during the shield tunneling was analyzed and explained in detail,and the horizontal columns constructed by Metro Jet System(MJS)method were used to stabilize the sandy soil below the existing twin tunnels.Then,the transversal deformation,internal forces and torsion behavior of the existing tunnels induced by the new tunnels excavation were obtained.Additionally,a parametric study was conducted to investigate the impact of the intersection angle between the new and existing tunnels on the deformation behaviors of the existing tunnel.The results show that the settlement development of the existing tunnels was found to experience four stages:(?) subsidence,(?) heave,(?) second subsidence,and (?) steady state,respectively.The settlement caused by the second shield under-crossing was found significantly larger than that caused by the first tunnel under-crossing.The transversal deformation and internal forces of the existing tunnel show obvious asymmetrical characteristic due to oblique intersection.The parametrical analysis indicates that the intersection angle significantly influences on the maximum settlement of the existing tunnel bottom.Nevertheless,the maximum settlement of the existing tunnel crown is unaffected by the intersection angle.