عباس قدیمی

Liquefaction resistance of sands is one of the most important topics in seismic geotechnical engineering and has been the subject of numerous researches in this field. Although these studies has resulted in increasing the general knowledge about the issue, on the other hand, they have shown the great complexity the affecting factors having on the behavior. One of these complicated subjects is to estimate the liquefaction resistance of sand against aftershocks or other subsequent earthquake events. Observations show that liquefaction resistance of sands may increase or decrease by applying different cyclic stress histories. In this research several tests were performed using cyclic triaxial apparatus to investigate the effect of cyclic stress history on the dynamic behavior of Anzali shore sand. All the samples were subjected to isotropic consolidation. In order to examine the effect of various cyclic stress histories on the behavior, the first stage of cycling on samples was stopped at different levels of normalized excess pore water pressure, before triggering the liquefaction. The samples were cycled from different initial void ratios and consolidation stresses. The results show that the liquefaction resistance increases by applying the stress history which stops before triggering the liquefaction. Furthermore, the highest increase in liquefaction resistance is observed in samples having a normalized excess pore water pressure of around 0.4 in their first stage of cyclic loading. Based on analyses of the tests data, a series of equations were suggested to predict the liquefaction resistance of sand as a factor of intensity of cyclic stress history (represented by the value of normalized excess pore water pressure reached at the end of the previous stage of cycling) and the state parameter at the beginning of the second stage of cyclic loading. Some other equations were also suggested for predicting the procedure of excess pore water pressure generation in the seco