Saneiyan and Slater (2021) - Complex conductivity signatures of compressive deformation and shear failure in soils

The mechanical properties of soils play a crucial role in site assessment for construction and infrastructure. Soils with low shear strength can become unstable as a result of natural and/or anthropogenic induced forces. Standard geotechnical methods, such as compressive strength tests, quantify the mechanical properties of soils, but these methods have low spatiotemporal resolution and may involve disruption of existing infrastructure. In contrast, the complex conductivity geophysical method can provide information on spatiotemporal changes in the subsurface in a minimally invasive manner. We investigated complex conductivity signatures resulting from soil deformation and failure during an unconfined compression test. A synthetic soil composed of silica sand (98%) and kaolin powder (2%) was saturated below its liquid limit and packed inside a flexible sample holder custom-equipped with four electrodes under zero confining stress to simulate an unconfined condition. This soil sample underwent a constant and slow rate of compression. Soil stress, strain, effluent volume, along with the frequency dependent real and imaginary parts of the complex conductivity were recorded over distinct time intervals. The first experiment focused on the sensitivity of complex conductivity to soil failure. Imaginary conductivity (equivalent to surface conductivity) abruptly decreased at the failure point (similar to the decrease in stress) compared to the real conductivity signal. The dominant geophysical length scale L2 determined from the complex conductivity spectra (related to pore size) exhibits an inverse linear dependence on compression. The second experiment focused on the complex conductivity during shearing (beyond failure). In this case, the imaginary (or surface) conductivity closely tracked changes in the sample stress. In both experiments, imaginary (or surface) conductivity is highly sensitive to changes caused by rearrangement of soil structure under stress (i.e., deformation and failure). In contrast, the real conductivity is minimally sensitive, and the electrolytic conductivity is insensitive to these changes. Our findings indicate that complex conductivity is capable of tracking mechanical changes of soils under stress and during failure foremost through the surface conductivity.