Showing papers by "Jacques Locat published in 2012"

The Effects of Salinity and Shear History on The Rheological Characteristics of Illite-Rich and Na-Montmorillonite-Rich Clays

Abstract: Particle–particle interactions in natural clays can be evaluated by their rheological behavior, but the results are often affected by the physicochemical properties of the clays. The behaviors of two fundamentally different types of clays (low-activity and high-activity) differ with respect to salinity and a time factor (duration of shearing at a given shear rate): illite-rich Jonquiere clay (low-activity clay, Canada) and montmorillonite-rich Wyoming bentonite (high-activity clay, USA). The purpose of the present study was to investigate these different behaviors. Most natural clays exhibit shear-thinning and thixotropic behavior with respect to salinity and the volumetric concentration of the solids. Natural clays also exhibit time-dependent non-Newtonian behavior. In terms of index value and shear strength, lowactivity and high-activity clays are known to exhibit contrasting responses to salinity. The geotechnical and rheological characteristics as a function of salinity and the shearing time for the given materials are compared here. The clay minerals were compared to estimate the inherent shear strengths, such as remolded shear strength (which is similar to the yield strength). Low-activity clay exhibits thixotropic behavior in a time-dependent manner. High-activity clay is also thixotropic for a short period of shearing, although rare cases of rheopectic behavior have been measured for long periods of shearing at high shear rates. The change from thixotropic to rheopectic behavior by bentonite clay has little effect at low shearing speeds, but appears to have a significant effect at higher speeds.

Upstream migration of knickpoints: Geotechnical considerations

Abstract: Multiple knickpoints have been observed in recent years on the lakefloor of Wabush Lake, Labrador, Canada Two or more sequential high-resolution geophysical surveys have revealed that these knickpoints tend to migrate updip, ie upstream relative to the turbidity currents that overflow them In order to understand the different processes accountable for the migration of these knickpoints, laboratory studies have been performed to reproduce the sedimentological setting and hydrodynamics of Wabush Lake Knickpoints are topographic depressions, characterized by a steep slope and a flatter profile downslope and upslope In the present case they are initiated by morphodynamic interaction between an erodible bed and an overriding turbidity current A set of 22 experiments have been performed in a facility able to run continuous turbidity currents Knickpoints created in laboratory are up to 1 cm deep and migrate upstream at variable speed Experimentation results show that migration of the knickpoints is controlled by two factors: erosion by the turbidity current and a landsliding process in the knickpoint head scarp Knickpoint migration has also been observed when no turbidity current was present These results show that this phenomenon is governed by not only hydraulics, as previously described in the literature, but must also be studied from a geotechnical point of view

Development and Potential Triggering Mechanisms for a Large Holocene Landslide in the Lower St. Lawrence Estuary

Abstract: The Betsiamites 7,250 cal BP submarine landslide mobilized a volume of 1.3 km3 in the St. Lawrence Estuary, Eastern Canada. The failure was initiated in shallow water between 10 and 140 mbsl and most likely developed in a unit stratified clayey silt deposits following a very strong earthquake. Most of the failed mass appears dislocated and evacuated the failure source area. This paper concludes that gas hydrates dissociation could not have influenced slope stability on the shelves of the Lower St. Lawrence Estuary in the Early Holocene and that despite the high sedimentation rates prior to the failure this condition did not act independently as a significant trigger for the Betsiamites failure.