Network Rail has said that repairs to the large landslip near the Harbury Tunnel will call for the rail line between Banbury and Leamington Spa to be closed for several weeks.
The rail infrastructure organisation has confirmed that the landslide on Saturday 31 January occurred in an area that was undergoing stabilisation work following a smaller landslide in another nearby location on 14 February 2014. The cause of the new failure has yet to be established.
Network Rail has said that the failed cutting forms the approaches to Harbury Tunnel, and is excavated to depths of up to 30m through inter layered mudstones and limestone.
Assessment of the landslide is currently underway but continued slow movement of the failure is hampering work to determine the scale of the problem. Remote sensing is being used to aid the assessment work but Network Rail has said that remedial work will not start until it is safe to do so.
Landlside specialist Dave Petley who is pro-vice-chancellor (research and nterprise) at the University of East Anglia has assessed the images released by Network Rail, and said: “The Harbury Tunnel landslide is complex – the big (4m) drop of material at the top of the slope appears to support a rotational mechanism, but the very complex deformation in the area of the gravel drains is more difficult to understand and requires proper mapping.
“The image of the whole site suggests that the slope is bulging in the toe area. Weather conditions are currently cold and mostly dry, and fortunately there is no heavy rainfall in the forecast for the coming days. Once the slope stops moving then it might be possible to start to carefully remove material from the crest, which will improve the stability of the site, allowing proper mitigation.
“An important question is centred on how this failure occurred during the stabilisation works. It will be interesting to find out in due course what unanticipated factor led to this slope failure. At present no structures at the top of the slope have been affected, and the tunnel itself looks to be outside the boundaries of the slide. From those perspectives the Harbury Tunnel landslide could have been far worse.”
Network Rail chief executive Mark Carne added: “This is a massive incident and I am making sure we have all the resources we need to tackle this problem. Our safety precautions ensured the railway was shut before there was any risk to passengers. Looking forward, our focus is on making sure the site is safe before our engineers move any of the affected earthworks.
“We know just how important this key route between Birmingham and London is, for both passengers and freight. This is a very deep cutting built back in Brunel’s days which we have been repairing for some time. I can assure passengers whose journeys are being disrupted that we will do our utmost to restore their services as soon as possible. However, it may take several weeks to stabilise it before we can start some services.”