19 December 2014
The Ventnor landslide
One of the most interesting landslides in the UK lies under the town of Ventnor, which is located on the southern side of the Isle of Wight in southern England. This is a large, slow moving landslide that nonetheless causes considerable damage, particularly in periods of accelerated movement. Over quite a long period, the Isle of Wight Council worked with Halcrow (now CH2M HILL) to understand the landslide by monitoring its movement patterns. In recent years I have worked with a part time PhD student, Jon Carey (who is now at GNS Science in New Zealand), and Roger Moore at Halcrow, to understand the relationships between the movement of the landslide and pore water pressures. That work is a part of Jon’s PhD thesis (which is available online), and has just been published in the journal Landslides (Carey et al. 2014).
Perhaps the most interesting aspect of this landslide is a large graben structure that is opening up at the crest of the landslide. This is one of the figures from the paper, showing a geomorphological map of the landslide together with, bottom left, the graben structure:
In the paper we examined some very detailed monitoring data that was collected in a study led by our co-author, Roger Moore, between 1998 and 2002. During this time, Halcrow monitored the pore water pressure at depth using piezometers located in deep boreholes and the opening of the graben using crackmeters (to measure horizontal movements) and settlement cells (to measure vertical movements). We found that the movement of landslide can be divided into two key components. In the background is long-term creep of the landslide, at rates of about 5 to 10 mm per year, regardless of the groundwater conditions. This indicates that the landslide is in a condition of marginal stability / instability. However, when the groundwater level rises due to periods of heavy rainfall the landslide moves more rapidly – up to about 35 mm per year – but in a complex manner. This is captured in the diagram below, which is part of Fig. 6 from the paper:
The upper panel of the diagram shows the movement of the landslide as indicated by extension of the graben sides and subsidence of the graben floor. The lower panel shows the same data for the crackmeter, but expressed as a displacement rate, and the measured groundwater level. It is clear that as the groundwater level increases the landslide movement rate goes up, and vice versa. However, across the entire dataset we found that the relationship between the pore water pressure and movement rate was not simple, and in particular that sometimes the movement rate remained elevated even as pore water pressures reduced.
At Ventnor there has long been discussion of the likelihood that the landslide might transition into a more rapid movement event. Our data suggests that because the landslide is occurring on a very well-developed basal shear plane, and because the toe of the landslide is buttressed by large landslide blocks, this is unlikely without some fundamental shift in material behaviour.
Carey, J.M., Moore, R. and Petley, D.N. 2014. Patterns of movement in the Ventnor landslide complex, Isle of Wight, southern England. Landslides. doi: http://dx.doi.org/10.1007/s10346-014-0538-1