| In order to solve the problem of urban traffic congestion,many cities began to build a large number of rail transit.Shield method has become the main means of urban underground space construction by virtue of its own advantages.When the earth pressure balance shield passes through the saturated silty soil layer,the high-water pressure difference between the groundwater and the soil chamber will produce a large penetration force pointing to the excavation face,which will cause greater deformation of the soil and lead to the instability of the excavation face.In order to ensure the safety of construction and explore the instability mode of excavation face,the calculation of limit support pressure of excavation face and the law of soil deformation under the condition of seepage,the stability of shield excavation face in saturated silt layer is studied.The main research contents and results are as follows:(1)Based on the standard consolidation test,the curve of elastic modulus of Qiantang River silt with buried depth is analyzed.It is found that the change is nonlinear in the range of 0～20m.When the axis of the shield tunnel is located at 7.5m underground,the five different modulus values at the axis are minimum 27.81 MPa and maximum 45.96 MPa.(2)In the parameter analysis of strain softening constitutive model,it is found that the shear strain is 1.5 times of the axial strain.Based on the deviatoric stress-strain results of Hu Qi ’s triaxial unloading test,the strain softening internal friction angle of Qiantang River silt was analyzed.When the shear strain is 4.7 %,the internal friction angle reaches a peak of 25 °.When the shear strain is 7.4 %,the internal friction angle reaches a residual value of 21 °,which decreases linearly between 4.7 % and 7.4 %.(3)Based on the centrifuge model test of Tang Lvjun,39 groups of numerical models are established by using the finite difference software FLAC3 D.The influence of constitutive model and elastic modulus of soil on the stability analysis of excavation face is studied.The constitutive model suitable for saturated silt and the soil parameters most suitable for soil characteristics are obtained.It is found that compared with Mohr-Coulomb constitutive model,the application of strain softening constitutive model reduces the error between numerical simulation and centrifuge test results and improves the accuracy of numerical simulation results.The minimum error of limit support pressure is 3.1 %,and the surface subsidence error is 5.67 %.The elastic modulus of soil has little effect on the limit support pressure of the excavation face,but has a non-negligible effect on the soil deformation.(4)Parameter sensitivity analysis is carried out on the stability of shield tunnel excavation face.A total of 88 groups of analysis models were established to study the influence of different excavation face permeability,soil strength parameters,buried depth ratio and water depth ratio on the stability of shield tunnel excavation face.Based on 88 groups of analysis models,the calculation formulas of limit support pressure without seepage and seepage force under seepage are proposed.When the internal friction angle φ = 30 °,the buried depth ratio is 1,the water depth ratio is1.5～4.5,the limit supporting pressure is 72.68 ～ 208.154 k Pa without seepage,and the seepage force is 33.25～106.75 k Pa with seepage.(5)Through the analysis of lateral and longitudinal settlement of soil surface under different conditions,it is found that the width coefficient of settlement trough and soil loss rate under the condition of seepage are greater than that under the condition of no seepage.The maximum settlement point of the surface is about 0.5D in front of the excavation face without seepage,and the maximum settlement point of the surface is in the range of 0.5D～0.9D in front of the excavation face with seepage.For shallow shield tunnel excavation in saturated silt layer,the width coefficient i of the settlement trough is 3.03～3.93,the average value is 3.41;the range of soil loss rate η is0.21%～0.47%,the average value is 0.33%.The settlement trough width coefficient i of the excavation face without seepage is 2.06～2.46,with an average value of 2.30.The soil loss rate ranges from 0.08% to 0.19%,with an average of 0.12%.(6)On the basis of the research results of numerical simulation,by analyzing the displacement cloud diagram of numerical simulation,with the help of Canny edge detection method,further fitting analysis is carried out to obtain the instability angle of the excavation surface without seepage(the angle between the wedge slope and the horizontal plane is the instability angle β),and the instability angle of the excavation surface seepage.When the internal friction angle φ = 15°～30°,the instability angle of the excavation face without seepage is 47°～53°,and the instability angle of the excavation face with seepage is 39°～42°.Obviously,the instability angle under seepage is smaller than that without seepage,which is smaller than the 60° used in previous studies,indicating that the predecessors underestimated the range of instability zone.Then,based on the classical ’ wedge-prism ’ limit equilibrium model,the instability mode of the excavation face of the shield tunnel and the calculation formula of the limit support pressure are modified.It is proposed that the limit support pressure under the seepage condition of the excavation face should be the sum of the limit effective support pressure and the seepage force under the seepage state,and the explicit solution of the limit effective support pressure is derived.Comparative analysis of previous results and numerical simulation results to verify the theoretical correction formula.Figure [64] Table [19] Reference [99]... |
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