Study On Mechanism And Its Influencing Factors Of Sand Reliquefaction In Shaking Table Test

Cases of recent and early liquefaction show that sand deposits can liquefy yet again by a succeeding earthquake afterward primarily liquefying in seismic shaking.The occurring sand re-liquefaction can cause significant damage to the surrounding environment and structures.A good understanding of the mechanism underlying reliquefaction of sand is necessary to take appropriate measures but the outcomes of reliquefaction performances of sand examined by many scholars are uneven.This thesis aimed to improve the understanding of the sand liquefaction and re-liquefaction mechanism in case of earthquake-induced shaking and to investigate the effect of shaking duration and PGA of input motion on reliquefaction of sand.This was done by subjecting one degree of freedom(1-DOF)shaking table tests under undrained conditions.In every test,the saturated sand model of different heights was shaken numerous times till it wouldn't reliquefy.In the shaking table experiment with varying PGA and shaking duration of input motion,the settlement,the excess pore water pressure(EPWP)and the acceleration response of the sand in the process of reliquefaction were measured.The first part of this thesis focused on the basic reliquefaction phenomenon.Based on the EPWP,acceleration and settlement response obtained,it was found that: the liquefaction resistance of sand was the lowest in the second liquefaction,not in the first liquefaction.The result indicates that:the phenomenon of re-liquefaction of sand induced by the earthquake is not the same as that of the first liquefaction.The larger relative density of the sand layer in the second liquefaction does not seem to improve the reliquefaction resistance.Therefore,the traditional theory for liquefaction resistance needs to be improved before it can be used for the analysis of reliquefaction.In the model test,the attenuation of seismic shear wave acceleration is not only related to distance but also the vibration frequency and the liquefied layer.Also,the attenuation of seismic shear waves in the liquefied soil layer has some properties different from the normal soil layer.Hence,the traditional attenuation calculation method needs improvement in the evaluation of site liquefaction.Furthermore,the soil layer near the surface of the soil is most likely to be liquefied.Based on the findings of the first part,the second part of the thesis investigated the reasons for the shallow sand layers to be more susceptible to re-liquefaction.Sand model of varying height and thickness subjected to the earthquake were used and it was found that: shear wave propagation mechanism in a layered deep sand model is different from that with layer interconnected deep sand model and shallow model.Thus,the re-liquefaction phenomenon is depth-dependent,and the soil layer near the surface of the soil is most likely to be liquefied.Furthermore,a comprehensive parametric study was carried out to investigate the influence of the PGA and the shaking duration of an earthquake.Liquefaction and reliquefaction of sand are observed to be duration dependent and sand model subjected to shorter duration have a greater tendency to liquefy in multiple numbers of events.Thus the seismic zones which are subjected to short-duration earthquake-liquefaction in the near past should be well studied for reliquefaction hazard as it has a greater susceptibility for liquefaction again.Finally,based on the findings of parametric studies,there exists a Critical Liquefaction Void Ratio(CLVR)within a zone with which we can forecast the presence or absence of liquefaction in the next earthquake.However,CLVR is PGA and depth-dependent.