A study of the dynamics of saturated granular assemblages and its implications for transport, stress propagation and failure in marine sediments

The subject of small-scale marine sediment mechanics is an area of active research, motivated by its key role in beach and shelf processes. Much of the existing small-scale work has focused on two-phase flows and particle behavior at the sediment interface, and their role in sediment transport and shaping seabed morphology. The study presented here considers the behavior of unconsolidated sediments below the seabed, 100 to 1000 grain diameters deep. Motivated by field observations off La Jolla Shores Beach that suggest dynamical behavior at such depths, a series of laboratory experiments were conducted to study the behavior of unconsolidated, saturated granular assemblies. Careful measurements of the apparent mass at the base of dry and saturated, glass beads pilings were acquired to examine stress redirection effects. The experiment protocol included slowly lowering the piling base to achieve reproducible piling configurations through wall friction mobilization. Results are analyzed using the Janssen and Oriented Stress Linearity (OSL) models, the latter allowing interpretation of the arching behavior in terms of arch direction and friction angle along arches. The data show qualitative similarities between dry and saturated granular assemblies, suggesting a commonality in their underlying physics, but quantitative differences in the intensity of the arching mechanism, probably associated with lubrication effects. Analysis of the piling quasistatic descent shows evidence of a progressive upward decompression punctuated with dynamic piling reconfigurations interpreted as relaxation oscillations. Both features are associated with spatial and temporal stress heterogeneities and have implications for modeling the mechanics of granular assemblies. Granular matter theories are expected to reproduce such observations occurring under minute deformations in these laboratory experiments. This work also describes a study of nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, using new seismic data and bathymetry. Isopach maps of the sedimentary sequences overlying the bedrock reveal the critical role of the tectonic landscape on the spatial distribution of modern stratigraphic units. A detailed examination of the morphological characteristics of the Scripps and La Jolla canyons refines the relationship between canyon location and development and regional tectonic deformation, and provides insight on the stability of unconsolidated, saturated marine sediments.