LAUSR

Abstract Dry and moist tamping methods are widely used for the investigation of sandy soil liquefaction. However, the effect of these two tamping-based reconstitution methods on composite soil liquefaction behavior have been rarely assessed. In this study, the dry and moist tamping methods were assessed using loose silica sand matrix (HN31) containing various amounts of non-plastic silt particles (C500), and by performing a series of undrained triaxial compression tests. Microstructural observations using a high-resolution optical microscope were also performed on loose silty sand specimens that had the similar initial states to those for the undrained triaxial tests. Triaxial results show that for silty sand specimens prepared by dry tamping method, a global dilatant behavior was observed and the undrained shear strength decreased as the fraction of silt particles increased. By contrast, a total reversed phenomenon was observed for those prepared by moist tamping method: all specimens showed more or less limited liquefaction behavior and silt particles mitigated the contractive tendency, which provided beneficial contribution to the undrained shear strength. Microstructural observations show that in the dry tamping method, overall structure formed by the host sand and silt particles was regularly deposited, promoting a more stable strain-hardening response. Moreover, some silt particles were found to be located near sand-sand contact points, significantly loosening the sand matrix. On the contrary, by using moist tamping method, the sand was uniformly covered by bulked silt particles, and the aggregated sand formed the metastable structure, facilitating the occurrence of static liquefaction. Moreover, with the increase of fines fraction, this metastable structure was weakened, characterized by the filling of large inter-sand voids, leading to the increase of undrained shear strength. Thus, a critical value of total fines fraction FcTot equalling to 21% was determined, which theoretically verified the insignificant effect of specimen preparation methods on the undrained shear strength. In that case, the metastable structure in specimen prepared by moist tamping method was expected to disappear and the overall response of specimen changed from the dominance of host sand to that of fines.