Dynamic Behavior Of Rectangular Closed Diaphragm Wall Used As Bridge Foundation In Liquefiable Deposit

Liquefaction has caused severe damage to bridge foundations,specially in sloping liquefiable deposits.Rectangular closed diaphragm wall(RCDW)foundations,as a new type of bridge foundation,can mitigate liquefaction to some extents,but their seismic behavior is not clear.Therefore,the study of dynamic behavior of RCDWs used as bridge foundations in sloping liquefiable deposits is of great significance.In this thesis,a dynamic centrifuge test of 50 g centrifugal acceleration was carried out to investigate the seismic behavior and capacity of mitigating liquefaction of RCDWs used as bridge foundations under different shaking events with PBAs of 0.13 g and 0.16 g,respectively.On the basis of the recorded data of acceleration,pore water pressure,earth pressure in the far field and the soil core,as well as the displacements of RCDWs,the main conclusions were obtained as follows:(1)The accelerations in the soil core and the far field were amplified under slight white noise motions.The amplification in the soil core is much smaller than that in the far field,indicating that the disturbance of soil in the soil core was prevented by the RCDW foundation under slight white noise motions.(2)The predominant frequencies of accelerations in the far field decreased from the deeper layer to the shallower layer,with the high frequency contents being seriously attenuated under the shaking events with PBAs of 0.13 g and 0.16 g.The accelerations in the far field were attenuated obviously,while the accelerations in the soil core were only attenuated slightly with amplification in a depth of 4 m before 15 s.These phenomena indicate that the soil stiffness in the far field decreased seriously,while soil stiffness in the core soil only decreased slightly.The acceleration responses under different shaking events were similar with slight difference.The attenuation of amplitudes and predominant frequencies of the accelerations was much more significantly and the reduction of soil stiffness was higher in the far field under the shaking events with PBA of 0.16 g.(3)Liquefaction both occurred in the far field and the soil core under the shaking events with PBAs of 0.13 g and 0.16 g.Liquefaction occurred earlier in the far field than in the soil core,and the onset of liquefaction varied in different depths,as the shallower layer liquefied earlier than the deeper layer.However,liquefaction occurred simultaneously at different depths in the soil core.In addition,the liquefaction duration in the far field was different from that in the soil core.The duration of liquefaction in the far field was much longer,which indicated that the RCDW foundations have capacities to mitigate liquefaction in the soil core.(4)The effective earth pressure on the outside of RCDW decreased more severely than that on the inside of the RCDW at the same depth under the shaking events with PBAs of 0.13 g and 0.16 g.Furthermore,the effective earth pressure on the inside and outside of the RCDW changed at different rates,i.e.,the effective earth pressure on the outside of the RCDW decreased rapidly,and remained at constant after reaching its peak,while that on the inside of the RCDW decreased slowly and recovered after peaking.This indicates that RCDWs can restrain the shear deformation effectively in the soil core which is confined by them.Moreover,the changes of the effective earth pressure on the inside of the RCDW was different under different shaking events.The response of the effective earth pressure corresponded to that of the excess pore water pressure,indicating again that RCDWs have capacities to mitigate liquefaction in the soil core.(5)RCDW foundation settled unevenly and rotated slightly with settlement and rotation being smaller than 20 mm and 0.002,respectively,under the shaking events with PBAs of 0.13 g and 0.16 g.The uneven settlement of foundation increased as the PBA of the input increased,resulting in larger rotation.Permanent horizontal displacements of the RCDW foundation occurred with a value less than 50 mm,and influenced by the PBAs of the input.The surface morphology of the deposit after the test and the down-slope deformation of initially vertical colored sand columns after excavation indicated the lateral deformation of the liquefied soil surrounding the RCDW foundation.However,the uneven settlement,rotation and horizontal displacement of the RCDW foundation were insignificant in this liquefied deposit,indicating that RCDW foundation can still provide sufficient support for the superstructure in liquefied deposit.