20 March 2014

Dawlish: inducing landslide failure using firehoses

Posted by Dave Petley

The Dawlish landslide

The Dawlish landslide is one of the many problems that arose on the mainline railway line that links the southwest of England with London.  This is a historic line that takes a very challenging route along the coastline.  It has a long history of landslides, as my earlier Landslides in Art posts showed.  In the recent winter storms a large section of track washed out, and a series of landslides have also caused significant problems.  Clearly Network Rail (the track managers) would like to reinstate the track quickly and to have a long term solution, both of which are difficult.  One key problem is a large (reportedly 30,000 tonnes, although some sources have suggested it is even larger) slide above the line.  The landslide is shown in a very nice drone / UAV video collected by the BBC and available via the Eyes of Dawlish Facebook page.  This is a screenshot of the landslide from that video:

Dawlish landslide


Mitigating the Dawlish landslide

On first inspection the slide looks to be a slump – there is certainly a large displaced block.  Network Rail have started an attempt to deal with this landslide that can best described as being impressive and perhaps even audacious.  Since the weekend they have been using high pressure fire pumps to push water into the landslide in order to induce failure – if you look carefully you can see this ongoing operation in the image, with several streams of water being pumped onto the slide.  Presumably the intention is to fail the landslide onto the track, and then to remove the debris.

This type of approach is occasionally proposed for landslides, but is rarely attempted.  There are several reasons for this.  First, actually it is rather difficult to get a natural slope to behave as you want it to.  So, for example, there is a risk that the slide might partially fail and then stop, leaving an unstable mass that is dangerous to clear.  Or the failure might be larger than expected, though that is unlikely here. The greatest danger is that the landslide does not move at all, leaving a now even less stable lump of rock above the asset in question.

I am not sure what is going on at Dawlish.  Today the Herald Express reports that the army are involved in the operation, according to the BBC perhaps even using armoured excavators to clear the debris.  It will certainly be interesting to see how this pans out over the next few days, and I am in admiration of Network Rail for what they are doing at that site.