Uplift mode of shield tunnel caused by seismic liquefaction in liquefiable soil
Aiming at the uplifting behavior of shield tunnel in liquefiable soil strata during earthquake,to explore the uplifting conditions,the uplift mode was discussed by combining finite difference model with shaking table test.Based on the finite difference software FLAC3D,an analysis model of seismic dynamic liquefaction was established.The Finn liquefaction constitutive model which can reflect the increase of pore pressure under seismic load was adopted.The reliability of the analysis model was verified by shaking table test with the excess pore pressure ratio as the evaluation index.Different combination cases were set for the numerical model calculation.The influence of standard penetration test-blow counts(SPT-blow counts)and tunnel buried depth on the tunnel uplift was analyzed.The uplift mode of tunnel caused by earthquake was further explored.The relationship curves of the two were obtained respectively.Through force analysis and formula derivation,the critical excess pore pressure ratio of tunnel uplifting was obtained,which was mutually confirmed with the numerical calculation results.The method of judging tunnel uplifting caused by seismic liquefaction was obtained.The results show that the smaller SPT-blow counts and tunnel depth are,the larger the uplift is.The uplift is divided into two parts,one caused by seismic shear deformation and the other by soil liquefaction.The former is related to the buried depth of the tunnel,and the latter is influenced by the soil condition(SPT-blow counts).The critical condition of tunnel uplifting caused by seismic liquefaction is that the buoyancy of the tunnel needs to be greater than the sum of the self-weight of the tunnel and the effective pressure of the overlying soil,which can be judged by the relationship between SPT-blow counts or the excess pore pressure ratio and the buried depth of the tunnel.For example,for a shield tunnel with diameter of 6.2 m,buried depth of 10 m and groundwater level of 1 m under the fortification intensity of 7 degree,the uplift caused by soil liquefaction occurs when SPT-blow counts is less than 7 or the excess pore pressure ratio is greater than 0.432.This method can provide reference for tunnel aseismic design.7 tabs,16 figs,26 refs.
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