Patterns of movement in the Ventnor landslide complex, Isle of Wight, southern England

The patterns of ground movement were monitored within a large, deep-seated landslide complex at Ventnor in southern England, between May 1998 and June 2002 using automated crackmeters, settlement cells and vibrating wire piezometers. It was found that the landslide maintains a state of marginal instability, such that it is subject to continual very slow deformation. Movement is primarily on a low-angled basal shear surface at > 90 m depth. The movement record shows a series of distinct deformation patterns that vary as groundwater conditions change. Continuous slow deformation occurs across the landslide complex at rates of between 5 and 10 mm/year. The background pattern of movement does not appear to correlate with local porewater pressure. Periods of more rapid movement (reaching up to c. 34 mm/year during the monitoring period) were associated with a period of elevated groundwater, although the relationship between movement rate and porewater pressure was complex. The patterns of movement and the landslide geometry suggest that the likelihood of a rapid, catastrophic failure is low. Future episodes of faster movement are likely during periods when porewater pressures at the basal shear surface are elevated above a critical threshold. Whilst the resulting surface deformation damages the town, it is unlikely to occur rapidly without significant changes to the landslide hydrogeology or the stress state within the landslide.