Last Saturday’s landslip near Harbury Tunnel in Warwickshire will close a section of the railway for several weeks, Network Rail has said.
The collapse of the cutting, described as “extremely significant” by the rail infrastructure operator, means the track will be shut between Banbury and Leamington Spa.
Network Rail engineers are currently assessing the landslide – which is still slowly moving – to determine the scale of the problem.
“Remote sensing techniques are being used to monitor the landslide and remedial works will begin as soon as it is safe to do so,” the organisation said in a statement.
The cutting where the landslip occurred forms the approach to Harbury Tunnel, and is excavated to depths of up to 30m through inter layered mudstones and limestone.
Network Rail had been carrying out stabilisation work at the site after a smaller landslide at the site on 14 February 2014. These works were underway when the slope failed at 14:30 on Saturday 31 January.
No one was injured when the landslide happened.
“We are working with local landowners to gain access and plan the safe removal of materials – initial investigations suggest that up to 350,000 tonnes of earth and rock will need to be removed,” Network Rail said.
It added that it would be able to get a clearer idea on a reopening date for the line “within the next two weeks” and that it would “provide further updates as our investigation and recovery work progresses”.
This section of the railway, engineered by Isambard Kingdom Brunel, has caused problems since it was built 150 years ago, a spokesman for Chiltern Railways said. Brunel’s decision not to build a tunnel meant the cutting was unusually deep with steep sides, he explained.
The line carries long-distance services for Chiltern Railways and Cross-County Trains, and freight services.
Bus replacement services are running between Leamington Spa and Banbury and a revised service is in effect between Banbury and Marylebone, Birmingham and Leamington Spa, and Stratford upon Avon and Leamington Spa.
Freight services are being diverted via the West Coast Main Line, Acton and the Great Western Main line.
Mark Carne, chief executive of Network Rail, said: “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.”