Experiments on the complex conductivity signatures of pore pressure variations in soils

SUMMARY Understanding the effects of pore pressure changes on soil stability is important in geohazards and geotechnical studies. In situ measurements of pore pressure (PP) are difficult at large scales. Geophysical methods can offer an indirect approach in understanding the effects of PP in soils. In this laboratory study, we investigate the complex conductivity (CC) signatures of soils undergoing increasing pore pressure inside a rigid cylinder. We experimented on different synthetic soil mixtures (with various clay percentages) as well as a natural soil sample collected from central Oklahoma, United States. We measured the CC response of the soil as we increased the pore pressure in small increments starting from atmospheric pressure up to 200 kPa. Our results show that the CC method is sensitive to changes in pore pressure values with imaginary conductivity magnitude increasing with increasing pore pressure for the samples containing clay minerals. The pure sand soil sample showed a less pronounced yet similar trend to clayey mixtures. The natural soil sample and samples with montmorillonite showed a direct relationship between imaginary conductivity and PP while real conductivity and PP showed an inverse relationship. In the samples without montmorillonite, we observed no changes in the characteristic relaxation time (τpeak) indicating no pore geometry changes in these samples. However, the samples with montmorillonite showed a direct linear relationship between PP and τpeak. Our findings indicate that under our controlled conditions, the CC measurements are sensitive to PP changes in clayey natural and synthetic soils, and although further research in the field with site-specific calibration, a wider spectrum of natural soil types and larger PP increments, is needed to validate our results; this is a starting point to evaluate the possible sensitivities of CC measurements to PP changes in earth materials.