Electrical resistivity monitoring of a collapsing meta-stable soil

A model of electrical conduction through clay-coated, silt-sized quartz-grains inter-connected by clay-bridges (e.g. brickearth) is developed. Underpinned by SEM studies of brickearth, the model predicts resistivity to be proportional to the size of the quartz-grains, where the resistance afforded by clay grain-coatings and clay-bridges is comparable. The model accommodates resistivity that increases through bridge breakage and decreases through bridge compression. The resistivity of in-situ undisturbed brickearth was found to be in the range 15 to 35 ohm-m. At such low values we demonstrate that electrical flow is dominated by conduction within clay-coatings and their interconnecting clay-bridges, rather than in mobile pore-water. A small electrode array, buried at shallow depth beneath the load plate (1.0m by 1.0m) of a field collapse experiment, monitored resistivity to a depth of 1.5m over a 260 hour period. While the water level beneath the load plate remained below 1.0m depth, the resulting 3D inverted resistivity models detected water injected immediately beneath the plate; recording rapid increases, in stages over 90 minutes, in the depth interval 0.45 to 0.75m directly under the plate, during what appears to be collapse. These increases are attributed to breaking of clay-bridges weakened by injected water.