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Quarterly Journal of Engineering Geology & Hydrogeology

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Electrical resistivity monitoring of a collapsing meta-stable soil

^{1 1}British Geological Survey, Keyworth, Nottingham NG12 5GG UK (e-mail: pdj@bgs.ac.uk)
^{2 2}Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT UK
^{3 3}Department of Civil and Building Engineering, Loughborough University, Loughborough,Leicestershire LE11 3TU UK

Amodel 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 (1.0 m by 1.0 m) of a field collapse experiment, monitored resistivity to a depth of 1.5 m over a 260 hour period. While the water level beneath the load plate remained below 1.0 m 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 0.75 m directly under the plate, during what appears to be collapse. These increases are attributed to breaking of clay-bridges weakened by injected water.

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