| Summary: | If the effects of landslides are to be mitigated and avoided then the causes of landslide activations – and re-activations – must be better understood. The most common subsurface property change in the lead up to rainfall-triggered landslide activation is the moisture content of slope material and associated pore water pressure rises and/or consistency changes. Landslide early warning systems have been developed which observe and monitor characteristic slope properties in advance of activation and give advanced warning of imminent slope displacement. This PhD thesis analyses and presents the results of a four and a half year monitoring campaign of a periodically active inland landslide by – among other methods – a geoelectrical monitoring system called Automated time-Lapse Electrical Resistivity Tomography (ALERT). The ALERT system was trialled on a landslide system located within the Early Lias of North Yorkshire and the suitability of the system for landslide monitoring assessed. The products of the geophysical monitoring campaign range from discrete resistance measurements on the landslide to a four-dimensional, high-temporal resolution dataset which is interpreted in terms of hydrogeological processes. Temperature corrected resistance results of the geoelectrical monitoring system reveal that the system responds very well to rises and falls in piezometric levels and seasonal trends of soil desiccation during warmer, drier months and crack annealing and slope soil moisture accumulation in response to wetter periods. The existence of threshold slope moisture contents, and hence electrical resistances/resistivities, above which the slope activates are not observed in resistance/resistivity results most probably due to the complex nature of the landslide system, the system resolution and a number of physical slope processes taking place. However, trends in precursory soil moisture dynamics during the period leading up to earthflow activation are apparent in temperature corrected resistance results. Time-lapse model resistivity was converted to gravimetric moisture content through laboratory calibration of soil electro-petrophysical properties of each active lithological formation. Seasonal moisture content trends confirm system sensitivity to slope moisture content. However, lower moisture contents than were observed in the field indicate the need for higher resolution, intra-landslide ERT data to make the hydrogeology of landslides more apparent.
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