Mass physical properties, sliding and erodibility of experimentally deposited and differently consolidated clayey muds (Approach, equipment, and first results)

Abstract

On experimentally deposited kaolinite, illite, and Ca‐bentonite consolidated under their own load or by additional vertical pressure, the progress of compaction in relation to excess pore water pressure, mass physical properties, gravitational mass movements in a tilted tank, and erodibility under running water in a flume were studied. The very low consolidated sediments near the mud/water interface do not obey the generally used theory in soil mechanics. They show a different, non‐linear relationship between void ratio or water content and depth below the sedimentary surface on the one hand, or effective overburden pressure and shear strength on the other. The same is true of other physical properties such as permeability, which changes considerably with depth and time of consolidation. High sedimentation rates on slopes induce shallow sediment flow, whereas at low rates and critical slope angles different types of slope failures including the breaking up of water‐rich sediment into sharp boundered blocks are observed. Flume studies on soft clay muds show three different types of erosion: continuously suspending, discontinuous erosion of crumbs or shreds, and wavy deformation of the clay surface with disintegration of particles from the crests. The critical tractive stress depends not only on clay type, void ratio, and shear strength, but significantly also on the ‘geologic history’of the clay (i.e. deposition from thin suspension or dense slurry, fabric, consolidation and swelling generating minute inhomogeneities etc.). The experiments may lead to a better understanding of all mechanical processe's including pore‐water flow taking place near or not far below the sediment/ water interface.

Physical problems of the benthic boundary layer

“…The minimum shear stress for erosion was also independent of the shear strength of the bed. The properties of artificially sedimented and compacted sediments were investigated by Einsele et al (1974), who demonstrated the changing types of erosion which occurred as the shear stress was increased above the threshold value. As the surface layer of a weekly consolidated-sediment was brought into suspension the critical stress often increased with depth, an effect attributed to the greater shear strength of the compacted sediment.…”

Erosion of mud/sand mixtures

“…Therefore, Einsele et al (1974) suggest that those soil-mechanical parameters, which are strongly influenced by inhomogeneities and the local distribution of voids (e.g., tensile strength or permeability) are better related to the erosion resistance of cohesive sediments than grain size distribution, shear strength and void…”

Deschutes Estuary feasibility study: Hydrodynamics and sediment transport modeling

“…Sediment erodibility has been the subject of intense research in the recent past. Early work suggested that the physical properties of sediments such as particle size and water content were the primary factors controlling erodibility (Hjulstrom, 1939;Postma, 1967;Einsele et al, 1974). The physical characteristics of the bed continue to be recognized as important determinants of erodibility (Amos et al, 1997), but alone cannot be used to make accurate predictions (Dade et al, 1992).…”