A study of consolidation using mechanical and electromagnetic waves

Consolidation has been a central topic of research in geotechnical engineering. Most studies have been conducted in oedometric cells, and have attempted to assess the influence of multiple material and loading parameters. Yet, the characterization of internal processes and the role of mechanical and electrical interparticle forces still remains elusive. In this study, we monitored the consolidation of bentonite, kaolinite and silica flour with small-perturbation electromagnetic and mechanical waves. The resulting standard consolidation test data, and the simultaneously acquired wave data are presented. Macro-observations are interpreted at the micro-level. Changes in the high-frequency complex dielectric permittivity during the consolidation of kaolinite indicate a decrease in free water inside the specimen, and no changes in double-layer polarizability. The velocity of propagation of shear waves clearly reflects the transfer of load from pore pressure to skeletal stresses, and the corresponding increase in stiffness during consolidation. Velocity-stress relationships reflect contact behaviour, changes in mechanical and electrostatic interparticle forces and modifications in microfabric. The small-strain shear stiffness drops immediately on loading because of spatial variations in pore pressure; the time required to recover the initial stiffness depends on soil permeability. Results indicate the potential for development of innovative wave-based monitoring techniques for field applications.