Soil Arching Effect And Mechanical Responses Of The Existing Shield Tunnel Due To Lateral Braced Excavation

The excavation leads to uneven deformation of the retaining wall,which is prone to inducing the soil arching effect behind the wall,and its dynamic evoluti on has an important impact on the force-deformation of the adjacent existing shield-driven tunnel.Unlike the soil arching effect occurring in the trapdoor test or tunneling developing directly upward,the lateral excavation-induced soil arching effect tilts upward and thus it was defined as the lateral soil arching effect.However,the formation-evolution law of the lateral soil arching effect and its influence on the adjacent existing shield tunnel is not clear yet.This paper is based on the South extension project of Heping Avenue in Wuhan.The excavation-induced lateral soil arching effect herein is analyzed deeply by field measurements and numerical simulations,based on which the force transfer mechanism between the retaining wall,the soil arch,and the existing shield tunnel is further investigated and the external load-deformation-internal force law of the adjacent existing tunnel is revealed.Given the inconspicuous soil arching effect,this paper proposes a theoretical approach to the excavation-induced transverse force-deformation of the adjacent tunnel considering the soil-tunnel interaction,verifies the calculation method through the results of field measurements and centrifuge tests,and analyzes the correlation mechanism between the retaining structure-soil-tunnel in combination with field measurements.This study can provide support for the prediction and control of the lateral excavation-induced tunnel deformation,and the main contents and results are as follows:（1）Typical undisturbed clay samples were obtained by the thin-walled sampling method in the north of Wuhan Heping Avenue South Extension Project.Basic physical and mechanical properties of soil were explored through a basic physical property test,standard one-dimensional consolidation test,triaxial test,and parameters of hardening soil model with small strain（HSS model）were obtained.Proportional relationships among stiffness parameters were investigated.It is found that the correlation between stiffness parameters of typical clays in Wuhan is E_oed^ref/E_s,1-2=0.78-0.84,E_{5 0}^ref/E_oed^ref=1.06-1.14,E_ur^ref/E_{5 0}^ref=4.1-10.9.（2）The lateral soil arching effect is accompanied by shear stress transfer along the upper sliding,which transfers part of the pressure from the soil below the upper sliding surface to the upper part,causes the top soil of the retaining wall to squeeze each other,increases lateral earth pressure,and form a horizontal compression and vertical extension zone.The lower sliding plane is determined by the maximum shear strain and the sudden change point of lateral earth pressure,which transfers part of the earth pressure above the sliding plane to below.The loosened zone,arch zone,and stable zone are determined by the boundary where the ratio of horizontal earth pressure is equal to 1,the inflection point of lateral earth pressure at different depths,the lower sliding plane,and the wall.The excavation presents a three-dimensional deformation mode.The two-dimensional soil arch on each cross-section overlaps longitudinally to form a three-dimensional soil arch with two arch feet near the excavation corner and transfers the earth pressure to the arch feet through the soil arch zone.（3）Three-dimensional numerical simulation was employed to investigate the lateral soil arching effect and the mechanical responses of the existing tunnel.The force transfer mechanism among the retaining wall,soil arch,and existing shield tunnels is further elucidated.The force-deformation-internal force development of the adjacent existing tunnels is studied.From shallow to deep along with the depth,it can be divided into a passive zone,active zone,and passive zone according to the variation of lateral earth pressure.Ad jacent existing tunnels usually exhibits unloading near the excavation and loading far from the excavation.Tunnels located in the arch zone or stable zone have increased radial load on the tunnel invert or crown due to tunnel-soil interaction and stress transfer induced by the lateral soil arching effect.The tunnel located in the loosened zone and close to the excavation is subjected to the stress transfer brought by the minor principal stress arch in the loosened zone and the radial additional load on the tunnel crown increases.（4）A theoretical approach for calculating the transverse force and deformation of the existing tunnel subjected to lateral excavati on is presented,which takes into account the effects of retaining wall deflections and tunnel-soil interaction,and is verified by the results of the field measurement in the south section of Wuhan Heping Avenue South Extension Project and a centrifuge test in dry sand.Based on the analysis of field test results,the correlation mechanism between the retaining pile,soil,and tunnel is analyzed,the relationship between the transverse force,deformation,and internal force of the tunnel subjected to later al braced excavation are revealed,the relationship between the distribution of additional bending moments and the assembling form of the segments is found,the relationship between the longitudinal deformation of the tunnel and the longitudinal strain is described,and the quantitative evaluation index of the soil-tunnel coupling effect is presented.