Liquefaction Mitigation Mechanisms Of Rectangular Closed Diaphragm Wall Used As Bridge Foundation In Sloping Liquefiable Deposits

Extensive damage to bridge foundations has occurred due to liquefaction triggered by earthquake,and it has been verified that liquefaction can be mitigated by diaphragm walls.Rectangular closed diaphragm walls(RCDWs),a new type of diaphragm wall,have better potential to mitigate liquefaction than diaphragm walls.RCDWs are often used as bridge foundations because of their advantageous properties,such as high stiffness and construction efficiency,low cost,and minimal vibration and noise.However,there is currently a poor understanding of the liquefaction mitigation capacities and seismic behavior of RCDWs used as bridge foundations in a liquefiable deposit,and this has limited the practical application of the same.In this paper,three dynamic centrifuge tests were designed and conducted to investigate the liquefaction mitigation capacities and seismic behavior of RCDWs with one and two chambers in a gently sloping liquefiable deposit,obtaining abundant response data of different locations and of RCDWs,such as data of acceleration,excess pore pressure,and displacement,under weak,moderate,and strong earthquake motions.Specific consideration was given to the performance effects of two integral parts of the RCDW foundation,namely,the RCDW frame and cap,as well as the influence of the different properties on the system performance in a liquefiable deposit.The mechanisms of mitigating liquefaction of RCDWs were discussed on the basis of the tests results and theoretical analysis.Furthermore,the advantages of RCDWs used as bridge foundations in sloping liquefiable deposit under different types of earthquake motions were evaluated based on the discussions on liquefaction mitigation capacities and displacement responses of RCDW,and seismic demand to superstructure from RCDW foundations.The main results in this paper can be drawn as follows:(1)An RCDW foundation was verified to be effective for mitigating liquefaction in the soil core by comparing the dynamic responses of the soils in the far field and in the soil core under three different earthquake motions.(2)The displacement responses of RCDW foundations in nonliquefied and liquefied deposits were investigated by applying a series of earthquake motions with different PBAs.For RCDWs with single chamber,the displacement mode is settlement with slight downslope rotation under the action of the unbalanced lateral wall pressure and the inertial force.However,for RCDWs with mutiple chambers,the displacement mode is rotation with slight settlement under the action of the unbalanced lateral wall pressure,the inertial force,and the “chamber effect.”(3)A comparison of the results of Test-1 and Test-2 shows that the presence of the cap alters the dissipation mode of the excess pore pressure and the stress field in the soil core,increasing the mitigating liquefaction capacity of the soil in the shallower layer in the soil core and shorting the duration of liquefaction in the soil core.However,the cap has negligible influence on changing the displacement mode of RCDWs despite of increasing the inertial force.(4)A comparison of the results of Test-3 with two-chamber RCDW and Test-1/2 with one-chamber RCDW shows that increasing the stiffness of the RCDW foundation enhances its ability to mitigate liquefaction in the soil core,and the “chamber effect” changes the displacement mode of RCDWs,from settlement mode for one-chamber RCDW to rotation mode for multichamber RCDW.(5)A comparison of the results under Taft motion and Kobe motion shows that different motion characteristics have insignificant influence on the mitigating liquefaction capacities of RCDWs,so RCDWs would mitigate liquefaction in the soil core effectively under different types of ground motions.Moreover,the comparison also indicates that different motion characteristics have negligible influence on changing the displacement mode of RCDWs.(6)The RCDW frame restrains reduction of the soil stiffness and alters vertical hydraulic gradient in the soil core owing to its high stiffness.The RCDW frame also terminates the horizontal migration of the excess pore pressure from far field to soil core owing to its impermeability.The cap alters the distribution of stress field in the soil core,increasing the initial effective stress and enhancing the mitigating liquefaction capacity of the soil in the shallower layer in the soil core.(7)There are three mechanisms of mitigating liquefaction of RCDWs.First,mechanism of confinement,i.e.,RCDWs confine the soil in the soil core effectively owing to their special structure form,and RCDWs restrain the shearing strains in the soil core under earthquake motions owing to their high stiffness,and thus suppress reduction of the soil stiffness and the development of excess pore pressure.Second,mechanism of reinforcement,i.e.,the cap provides additional vertical effective stress on the soil in the soil core,and thus increases the initial effective stress and enhances the mitigating liquefaction capacity of the soil in the soil core.Third,mechanism of cutoff,i.e.,RCDWs work as cutoff wall effectively owing to their impermeability,and thus terminate the horizontal migration of the excess pore pressure from far field to soil core and protect the soil from early liquefaction due to the inflow of high excess pore pressure.(8)There will be acceptable displacement of RCDW foundation and small dynamic load translating to superstructure,if the structure form of RCDW and the depth embedded to bearing layer of RCDW are reasonably designed.Therefore,RCDW foundation can support the bridge effectively and reduce the seismic demand on the superstructure under weak,moderate,and strong earthquake motions in the deposits whether liquefied or not.