Soft Sediments: Wave-based Characterization

Abstract

Soft sediments are frequently encountered in geotechnical practice, forming natural deposits or manmade mixtures. These high water content soil-water mixtures involve soils with high specific surface and experience relatively low effective stresses, hence the role of electrical interparticle forces is enhanced and nonstandard soil-like behavior may be observed. Testing such materials presents some unique challenges not faced in soils with a stronger granular skeleton. This experimental study of soft sediments involves kaolinite- electrolyte mixtures. Two test sets are conducted. The first one involves index tests and the second set of tests explores the combined utilization of elastic and electromagnetic waves to assess soft sediments and to monitor their evolution. The kaolinite mixtures tested in both sets of measurements were prepared using fluids of different ionic concentrations to create different soil fabrics. The mixtures change behavior when the ionic concentration of NaCl exceeds ,0.1 mol/ L, indicating internal changes in fabric formation. While the liquid limit is sensitive to fluid-particle interactions, it is less sensitive to fabric effects than viscosity or sedimentation tests. The maximum porosity a soil may attain is determined by the particle slenderness; in the case of kaolinite, a skeleton capable of transmitting a shear perturbation is observed at water content as high as six times the liquid limit. At this incipient skeletal condition, minor differences in shear wave velocity hint to the stiffness of different fabrics. High frequency permittivity is a good indicator of water content and can be effectively used to monitor the evolution of soft sediments. On the other hand, the evolution of the effective conductivity varies with pore fluid concentration: for high ionic concentration fluids, the effective conductivity increases as porosity increases, yet, the opposite is true for low ionic concentration fluids due to the contribution of surface conduction. In a wide range of water contents @0.6 liquid limit ~LL! ,w, 6L Lg, the shear wave velocity is primarily determined by the water content or porosity, and fabric appears to have a limited impact. Test procedures and results can be used to develop parallel field characterization tools and interpretation guidelines.