South West Main Line closed by major landslip
A structure weighing 12,600 tonnes that will carry the high speed route over the M42 motorway has been slid into place.
THE four-track main line from London to Southampton, Bournemouth and Exeter Central was closed from January 15, after a landslip on an embankment at Hook left the down slow line suspended in the air.
The slip also affected the down fast line, and led to a disruption of services between Farnborough (Main) and Basingstoke, leaving the Londonbound
lines unaffected.
The embankment, made of a mixture of London clay and other local soils, became saturated after days of heavy rain and a very wet winter. The slip was 44m long on a 10m high embankment, and is what engineers call a ‘rotational failure’.
Network Rail Wessex route director Mark Killick said at the time: “This is a huge landslip and is having a massive effect on customers. We’re still assessing the damage and it’s difficult to put a detailed timescale in place, but we know it’s going to be at least a week. We will need to stabilise the embankment, essentially stopping it moving.”
In the interim, services from Weymouth, Southampton and Exeter were terminating at Basingstoke, with services from London ceasing at Farnborough. A limited shuttle operation between Basingstoke and Farnborough using the up fast line was instigated, not calling at Hook, Winchfield and Fleet, while Waterloo-Basingstoke stopping trains were cancelled.
Weekend repair work on January 21/22 aimed to permit the reinstatement of limited services in the down direction from January 23 by severing the up fast either side of the slip, and slewing the down fast over to provide a track in each direction.
Network Rail said to complete repairs it would build an access road to bring in 9000 tonnes of rock and then insert steel piles to stabilise the embankment, but was unable to give a timescale for completion of the project.
HAVING been constructed alongside the motorway over the previous six months, HS2’s Marston Box bridge was moved 165m (180 yds) into position over the M42 during the Christmas holidays – a feat that HS2 Ltd believes to be the “world’s longest box slide”.
A team of around 450 people from construction contractor Balfour Beatty VINCI (BBV) were involved in the operation, which took 40 hours to complete, at a speed of four metres (13ft) per hour.
Consisting of a base, three walls and a top slab, and developed by Mott MacDonald and Systra for BBV, the 86m (282ft) bridge, north of junction 9 on the motorway at Lea Marston in Warwickshire, will become part of a structure over double that length, carrying trains between Crewe and Birmingham Curzon Street.
Originally designed for construction in a traditional manner, the box slide technique was adopted in order to minimise disruption to road users. A sliding mechanism designed by Freyssinet pushed the bridge into place on a guiding raft.
Disruption
HS2 Ltd estimates that building a conventional structure would have led to three months of overnight road closures and around two years of narrow lanes and speed restrictions. Planned closures were reduced to around a fortnight in total – in December 2021 between Christmas and New Year for preparation work, and again from December 24, 2022 until the early hours of January 3.
In fact, the bridge was installed and road reinstatement completed 36 hours ahead of schedule, allowing the M42 to be reopened in both directions between junctions 9 and 10 on New Year’s Day.