Rapid observations to guide the design of systems for long-term monitoring of a complex landslide in the Upper Lias clays of North Yorkshire, UK

Abstract

The Whitby Mudstone Formation has one of the highest landslide densities in the UK with 42 landslides per 100 km 2 . Landsliding at Hollin Hill in North Yorkshire, UK is complex and continuing, and includes shallow, retrogressive rotational failure on the upper slope, translation, and flow from the base of the Whitby Mudstone Formation over the scarp slope of the Staithes Sandstone Formation. Surface observations augmented by information relating to lithological, moisture and strength variation with depth allowed rapid initial interpretation of the masses affected by movement. These were provided by a single person operating portable probes providing depth logs of cone penetration resistance and soil moisture based upon dielectric property measurements in conjunction with a sampling auger. The gathered information was used to guide the design of further invasive site investigation and the configuration of permanent systems to monitor changes in dynamic moisture distribution and direct movement. At Hollin Hill, the near-surface materials in the upper 5 m interval are distinctly weathered or destructured, predominantly comprising silty clay in the Whitby Mudstone Formation, and fine silty, clayey sand and silty clay in the Staithes Sandstone Formation. Direct and secondary evidence was observed showing high moisture variation to be related to narrow intervals within the upper 5 m. Cyclic variation in moisture has played a key role in the movement and break-up of sliding materials, especially within the prograding lobes resulting from flow over the Staithes Sandstone Formation. Since these observations, permanent monitoring systems have been installed, including electrical resistivity tomography (ERT) arrays, which have successfully mapped the distribution of the Whitby Mudstone and the Staithes Sandstone, but will also be used in time lapse mode to image the near-surface moisture movement driving the landsliding processes. ERT array installations included a large area, low spatial resolution grid designed to investigate the potential coupling between the upper and lower slope hydrogeological processes and a small area, high spatial resolution grid designed to investigate the hydrogeological processes driving the earth flow.

Landslide activation behaviour illuminated by electrical resistance monitoring

“…This is supported by Gunn et al . (), who reported water and washed out fine material emerging from the slip surfaces at the earthflow toes during periods of activation. Likewise this is also consistent with observations on other slow moving landslide where the role of fissuring, including the dynamic opening and closing of flow pathways, is seen to significantly influence ground water movement (Van Asch et al ., ; Bièvre et al, ; Krzeminska et al ., ).…”

Brief communication: Landslide motion from cross correlation of UAV-derived morphological attributes

“…The site has been monitored since 2009 by the British Geological Survey (BGS) using various methods, including terrestrial and airborne laser scanning, as well as ground-based geotechnical and geophysical investigations. BGS investigations have revealed that the landslide has a complex behaviour with seasonal surface variations and episodic failures mostly triggered by intensive rainfall and increased pore-water pressures within the constituent geological materials (Gunn et al, 2013;Uhlemann et al, 2017).…”

“…Monitoring of these structures can provide a valuable insight into a landslide's subsurface dynamic failure mechanism and thereby help mitigate hazards (Gunn et al, 2013). Conventionally, in addition to geotechnical and geophysical monitoring of the subsurface, survey markers are often used to quantify surface displacement by monitoring discrete locations through periodic observations.…”

Acoustic emission monitoring of a soil slope: Comparisons with continuous deformation measurements

“…having no threat to infrastructure) make Hollin Hill an ideal site to trial new landslide monitoring techniques; the British Geological Survey has investigated and monitored landslides at Hollin Hill for several years (e.g. Chambers et al, 2008Chambers et al, , 2011Gunn et al, 2013;Merritt et al, 2013). Two of the lobes of slumped material were instrumented with three clusters of active waveguides (as described by Dixon et al (2014)); the instrumentation cluster locations are shown in Fig.…”