Linking field electrical resistivity measurements to pore suction and shear strength, for improved understanding of long term landslide stability

Abstract

The mechanical properties and the level of pore water saturation within a slope contribute to its stability; changes in these parameters, like an increase in saturation, can trigger a landslip. Conventional discrete point sampling methods are used to measure crucial mechanical and hydrological properties, but often lack the spatial resolution required to capture the full complexity of the problem. Geophysical methods, such as electrical resistivity tomography (ERT), are spatially sensitive to relevant subsurface properties. Electrical resistivity is related to the moisture content through well understood petrophysical relationships; consequently, recent studies have successfully explored the use of ERT to monitor saturation levels with geoelectrical monitoring. In such applications, repeated surveys (e.g. with a permanently installed measurement system) allow time-lapse tomographic images of the subsurface to be constructed. This is of particular interest given that the majority of landslides are induced by changes in pore pressure due to rainfall infiltration. We present the use of a geoelectrical monitoring system on an active landslide in Lias mudrocks, North Yorkshire, UK. Subsurface resistivity distributions determined from time-lapse ERT are converted into pore suction estimates through direct laboratory calibration between moisture content, electrical resistivity and matrix suction. A custom time-lapse ERT workflow has been developed to account for slope movements during the monitoring period. GPS markers, which cover the extent of the monitoring array, are used to infer slope movements and changing electrode positions. Field samples are recovered from shallow boreholes for mechanical testing and petrophysical relationships are developed, such that shear strength can be estimated from the ERT models. Decreases in ERT-derived pore suction (and hence shear strength) are observed prior and during slope movements measured by infield accelerometers. Given the development of dedicated time-lapse ERT systems, and the use of electrical resistivity as a proxy for suction and saturation, we suggest the use of ERT monitoring as an input for slope-scale stability monitoring.