Lattice-shaped Diaphragm Walls Used As Bridge Foundation:Soil Foundation Interaction Mechanism And Aseismic Capability

Rectangular closed diaphragm walls(RCDW)is a new type of liquefaction mitigation foundations,which can mitigate liquefaction in soil core.RCDWs are used as bridge foundation due to its advantageous properties,such as good structural integrity,high stiffness,construction efficiency,low cost.However,the RCDW-soil interaction and aseismic capability have been studied scarcely,and the seismic performance of RCDW with superstructure have not been studied yet.Therefore,the design of RCDW in liquefiable ground was lack of theoretical support,which limited the practical application of RCDW.This paper presents three-dimensional(3D)solid-fluid fully coupled effective stress numerical models,based on three centrifuge tests,to study the RCDW-soil interaction,aseismic capability,and mechanism of liquefaction mitigation in sloping liquefiable ground.Validity of the three numerical models were validated by the data of dynamic centrifuge test.Then the calibrated models were used to study the RCDW-soil interaction,aseismic capability under different earthquake waves with different peak accelerations.The mechanism of liquefaction mitigation of RCDW was revealed by analyzing the differences of the seismic performance of soil in soil core and far field.A new energy-based pore water pressure model was proposed by the analysis of relationship between the arias intensity(I_a)and pore water pressure using the cyclic triaxial numerical tests.A new method was proposed to evaluate liquefaction based on the energy-based pore water pressure model.Moreover,based on the settlement evaluation of foundation in non-liquefiable ground,this paper proposed a settlement evaluation of RCDW in liquefiable ground,which take consideration of effect liquefaction on load transfer functions.Finally,two numerical models with superstructure were proposed to study the effect of superstructure on seismic response of RCDW,based on the centrifuge tests and numerical models without superstructure.(1)Three 3D solid-fluid fully coupled effective stress numerical models were proposed based on three centrifuge tests.The saturated Fujian standard sand was modeled by the multi-yield surface plasticity constitutive material,which can represent the large post-liquefaction deformation.The RCDW was simulated by the elastic material.The water body was simulated by the modified method based on the method proposed by Vytiniotis.Modified shear beam boundaries were used to simulate the laminar container.Parallel computation was used to save computing time.The feasibility of the numerical models was validated by comparing the numerical results with the experimental data.(2)Then the calibrated models were used to study the seismic response,capacity of liquefaction mitigation of RCDW under different earthquake motions with different PGAs.The results showed that RCDW can mitigate liquefaction or decay initial liquefaction.The seismic response of soil and RCDW became severely under ground motions with larger I_a.The performance of RCDW was more sensitive to ground motions with more low-frequency content.The performance of RCDW was also significantly affected by the initial direction of ground motions.(3)The mitigating liquefaction mechanisms revealed by analyzing the results under different peak acceleration.The mitigating liquefaction mechanisms were that(a)RCDW can reduce the shear strain of soil core,due to its huge stiffness and good structural integrity;(b)The self-weight of cap was transfer to soil in soil core,which enhance the resistance to liquefaction.(c)RCDW can work as a cutoff wall against pore water migrating from the soils outside of RCDW to soil core;(d)Displacement of RCDW can be reduced by the support of soil in soil core;(e)RCDW can resist the liquefaction-induced lateral spreading,due to its huge stiffness and good structural integrity.(4)Two numerical models with superstructure were proposed to study the effect of superstructure on seismic response of RCDW,based on the centrifuge tests and numerical models without superstructure.The results inidicated that the seismic response of soil in soil core and RCDW became severely with the increase of superstructure mass;The increase phenomena were obseverd in excess pore water pressure time histories;The performance of RCDW with two champers was more sensitive to the period of superstructure.The lateral displacement of superstructure had a linear relationship with the lateral displacement and angle displacement of RCDW,the correlation coefficients increase with the increase mass and decrease with the period of superstructure.(5)This paper proposed a new energy-based pore pressure model for Fujian standard sand,based on the cyclic triaxial numerical tests.The effect of consolidation pressure and consolidation ratio on pore water pressure was studies.The based on the verified model,a new method was proposed to evaluate the liquefaction for free field and soil core.The method took into consideration of the permeability,surface maximum acceleration and shear stress reduction coefficient.(6)Based on the settlement evaluation of foundation in non-liquefiable ground,this paper proposed a settlement evaluation of RCDW in liquefiable ground,using the load transfer functions for the sandy soil.The method took consideration of effect liquefaction on load transfer functions.The method was verified by comparing the numerical results and the experimental data with the computed results.In conclusion,the results of this paper provide a certain theoretical support for the design,construction and maintenance of the RCDW in liquefiable site,and greatly promote the application of the new type of liquefaction mitigation foundation.