Partially saturated tailings sand below the phreatic surface

An opportunity to re-examine the liquefaction potential of hydraulically placed tailings sand has been provided by the Canadian Liquefaction Experiment (CANLEX). As part of this experiment, undisturbed samples of tailings sand were recovered after freezing the tailings in situ. Examination of undisturbed cores of frozen tailings sand clearly showed that the specimens were not fully saturated. This was confirmed by both physical measurements and laboratory tests in which gas was recovered from thawing specimens and analysed by gas chromatography. The gas was mainly air although a small amount of microbial gas was also present. Cryogenic scanning electron microscopy and confocal laser scanning microscopy were used to further confirm the existence of occluded gas bubbles. Triaxial undrained compression tests were carried out on undisturbed tailings specimens that were not back-saturated prior to shearing, in order to preserve the in-situ degree of saturation. Occluded air bubbles within the tailings sand, even if only in very small percentages by volume, are shown to have a marked effect on the response to undrained loading of the pore pressure within the tailings specimens. It is suggested that the liquefaction potential of tailings sand that could be expected to be contractive under undrained loading may be reduced by the occurrence of occluded gas bubbles within the voids. While it is not possible to quantify these effects accurately until further laboratory testing of both unsaturated and saturated loose tailings specimens has been conducted, this attribute could modify present engineering design. In this respect, obtaining undisturbed samples of granular soil is an important component of evaluating the liquefaction susceptibility of a specific deposit.