Seasonal effects on geophysical–geotechnical relationships and their implications for electrical resistivity tomography monitoring of slopes

Abstract

Current assessments of slope stability rely on point sensors, the results of which are often difficult to interpret, have relatively high costs and do not provide large-area coverage. A new system is under development, based on integrated geophysical-geotechnical sensors to monitor groundwater conditions via electrical resistivity tomography. So that this system can provide end users with reliable information, it is essential that the relationships between resistivity, shear strength, suction and water content are fully resolved, particularly where soils undergo significant cycles of drying and wetting, with associated soil fabric changes. This paper presents a study to establish these relationships for a remoulded clay taken from a test site in Northumberland, UK. A rigorous testing programme has been undertaken, integrating the results of multi-scalar laboratory and field experiments, comparing two-point and four-point resistivity testing methods. Shear strength and water content were investigated using standard methods, whilst a soil water retention curve was derived using a WP4 dewpoint potentiometer. To simulate seasonal effects, drying and wetting cycles were imposed on prepared soil specimens. Results indicated an inverse power relationship between resistivity and water content with limited hysteresis between drying and wetting cycles. Soil resistivity at lower water contents was, however, observed to increase with ongoing seasonal cycling. Linear hysteretic relationships were established between undrained shear strength and water content, principally affected by two mechanisms: soil fabric deterioration and soil suction loss between drying and wetting events. These trends were supported by images obtained from scanning electron microscopy.

Analysis of Geotechnical-Assisted 2-D Electrical Resistivity Tomography Monitoring of Slope Instability in Residual Soil of Weathered Granitic Basement

“…It provides a very fast, cheap, and efficient survey technique to characterize landslide-prone slopes (Pasierb et al, 2019). Electrical resistivity is sensitive to changes in the pore fluid resistivity and fluid saturation as the mode of current flow in the subsurface is through electrolytic conduction in the pore fluid (Hen-Jones et al, 2017). ERT is used to differentiate between soil types of differing resistivities as a result of presence of clay minerals (Shevnin et al, 2007).…”

“…ERT is used to differentiate between soil types of differing resistivities as a result of presence of clay minerals (Shevnin et al, 2007). High proportion of clay minerals has a significant influence on electrical resistivity due to surface conduction of electrical ions on the clay mineral that leads to a reduction in electrical resistivity measurement (Yamakawa et al, 2012;Hen-Jones et al, 2017). Recent studies on landslides have shown that ERT is suitable to monitor soil water content variations induced by rainfalls among other methods such as seismic refraction, seismic reflection, and self-potential and ground penetrating radar (Baroň and Supper, 2013;Boyle et al, 2018;Hojat et al, 2019;Whiteley et al, 2019).…”

Geophysical Monitoring of Moisture‐Induced Landslides: A Review

“…The complexities of establishing such relationships in practice are identified by Hen‐Jones et al (), who identified that such geotechnically coupled relationships often display hysteretic behavior in relation to seasonal variations, adding a temporal heterogeneity to the already present spatial heterogeneity present in landslide settings. An additional consideration is that, in a similar way that geophysical responses can be caused by a multitude of factors, geotechnical property variations can have a variety of causes, for example, the reduction of shear strength in soils due to loss of suction in wetting and drying cycles, and soil fabric deterioration across successive seasonal cycles (Hen‐Jones et al, ).…”

“…Materials with larger void spaces, such as sands and gravels, will have smaller capillary zones (the area of saturation above the water table caused by soil suction) than cohesive materials with smaller void spaces such as clays and silts (Craig, ). In some slopes, soil suctions may be the main restraining force preventing failure (Hen‐Jones et al, ). In these settings, understanding of subsurface moisture dynamics relating to increased infiltration is critical for predicting future slope failures.…”

Very‐high‐resolution electrical resistivity imaging of buried foundations of a Roman villa near Nonnweiler, Germany

“…This may impact the water saturation of the subsurface, and hence the electrical conductivity (e.g. Hen‐Jones et al, ; Mojica et al, ). As a result the resistivity contrast between the natural soil and archaeological remnants changes throughout the seasons of the year (Fry, ).…”