Groundwater and geotechnical controls on landslide mechanisms of coastal cliffs formed in glacial till

Abstract

Landslides behave in a complex manner especially in terms of the relationship between groundwater variation and the mechanisms of movement; a relationship that remains poorly understood. The aim of this research is to improve the understanding of landslide movement patterns for coastal cliffs. The objectives are (1) to understand controls on landslide initiation; (2) to evaluate landslide movement patterns so as to explain the development to failure and their mechanisms; and (3) to investigate the hysteresis relationship between groundwater and displacement rate.

Field monitoring has been undertaken at Upgang, a cliff formed in glacial till in North Yorkshire, UK (54.29° N and 0.38° W) for 30 months between 2010 and 2012. The monitored section extends 250 m along the coast and is 30 m high. The characteristics of the cliff face and its deformation over time were monitored monthly by terrestrial laser scanning. The landslide complex displays seasonal movements, associated with rotational failures, sliding and deterioration of the failed mass into a complex of mudslides. Movement is dominated both by groundwater variations resulting from precipitation, and marine cutting of the cliff toe. Data from a reactivating landslide, situated at the upper till section of the cliff, was selected to characterise post-failure change. The high spatial and temporal resolutions of the monitoring undertaken in this research allow these processes to be better understood. Uniquely, this study combines three detailed datasets including terrestrial laser scanning, in-situ monitoring and rigorous laboratory testing to explore the controls on the nature and style of landslide movement.

The results demonstrate that reactivation of the landslide occurred in phases of accelerated (0.2 – 6.27 mm/hr), slow (< 0.2 mm/hr) and negligible movements, which were found to be influenced by the fluctuation of groundwater. A complex relationship has been found at the reactive shallow landslide, where the onset of accelerated movements corresponds to an increase in groundwater at the landslide base. Two types of hysteresis patterns, both clockwise (Strain hardening) and anti-clockwise (Strain softening), relating groundwater and displacement rate, were observed. The key controlling mechanism appears to be plastic deformation. The varied velocities are dependent on the stress state controlled by both rate of change in groundwater and absolute groundwater level. The intensity of rainfall has a significant effect on the style of movement. The results have wider implications for understanding the controls on landslide movement in coastal cliffs and reactivation landslides.