Pore pressure generation characteristics of sands and silty sands: a strain approach
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Liquefaction of saturated granular soils during earthquakes has been one of the most important problems in the field of geotechnical earthquake engineering. It is well established that the mechanism for the occurrence of liquefaction under seismic loading conditions is the generation of excess pore water pressure. Most of the previous research efforts have focused on clean sands. However, sand deposits with fines may be as liquefiable as clean sand deposits. Previous laboratory liquefaction studies on the effect of fines on liquefaction susceptibility have not yet reached a consensus. This research presents an effort to find a unified picture regarding the effect of fines content on excess pore water pressure generation. Different from earlier studies that placed an emphasis on characterization of liquefaction in terms of the induced shear stress required to cause liquefaction, this study adopted a strain approach because excess pore water pressure generation is controlled mainly by the level of induced shear strains. This approach was first proposed by Dobry et al. (1982). Multiple series of strain-controlled cyclic direct simple shear and cyclic triaxial tests were used to directly measure the excess pore water pressure generation of sands and silty sands at different strain levels. The soil specimens were tested under three different categories: a) at a constant relative density, b) at a constant sand skeleton void ratio, and c) at a constant overall void ratio. The results from each of these groups were examined. In addition, laboratory measured pore water pressures of clean sands were compared to in situ measured values. The findings from this study were used to develop insight into the behavior of silty sands under undrained cyclic loading conditions. In general, beneficial effects of the fines were observed in the form of a decrease in excess pore water pressure and an increase in the threshold strain. However, pore water pressure appears to increase when enough fines are present to create a sand skeleton void ratio greater than the maximum void ratio of the clean sand. The comparison between laboratory and in situ measurements indicated that larger pore water pressure was generated in situ.