17 June 2009

Cayton Bay / Knipe Point landslide – report on options

Posted by Dave Petley

A year or so ago I posted on an ongoing landslide at Cayton Bay in North Yorkshire, just an hour or so down the road from my base Durham. This landslide, which is no commonly called the Knipe Point landslide, was threatening 50 or so houses (see image below) – in the end three were demolished, although the rest are still under threat from the slide. Since my post the BGS have created a nice summary website here, from which this picture is taken:

The local council, Scarborough Borough, managed to find from a range of sources about £300,000 to pay for an investigation of the site, which was undertaken by Halcrow. This investigation was completed this week and will be discussed by the Council in a few days time. The Council has put out a press release here, although the report is not available online. The key findings are as follows:

a. The landslide is a deep seated, ancient landslide system. consisting of:
1. A main deep seated failure for which ground movement is controlled by the residual strength of the clay and a deep confined natural groundwater table;
2. Shallow mudslides in the overlying glacial tills for which the ground movement is controlled by an upper natural groundwater table. This slide is highly sensitive to small changes in the groundwater conditions.

I find the above quite surprising as the failures affecting the houses do not move that often – so this apparent high level of sensitivity is a little odd. I will need to read the report (I will try to get a copy).

The press release then discusses stabilisation issues – which is I am sure what the householders are worried about. It briefly mentions the constraints (minimal impact on the environment as the landslide contains a Site of Special Scientific Interest (SSSI), but a 50 year design life).

Three key actions are recommended:

  1. “Maintain and continue monitoring of the surface and subsurface ground movement, groundwater and weather station network.”
  2. “Liase with stakeholders to review the findings of the report and discuss the way forward for managing the cliff instability risk at the site in the short and long term.”
  3. “Review funding options for promotion of the preferred engineering stabilisation options, and prepare an application for funding under the relevant and most appropriate legislation.”

The preferred stabilisation option is unfortunately large and complex:

  • Installation of deep drainage to reduce and control groundwater levels in the deeper water table.
  • Construction of bored piles at Knipe Point to isolate the lower Cayton Cliff landslide system from the land above the cliff top.

The press suggest that this would cost £12 million (some reports suggest £20 million!). I would think that finding this sum of money is going to be tricky given the limited number of houses involved, although maybe the threat to the main road will help here.