Much of the railway line linking London with the popular south coast resort of Brighton is nestled in sleepy Sussex countryside. But in recent years, a 200m long and up to 30m high embankment flanking one section at Copyhold Cutting near Haywards Heath has started to slip.
This slope comprises Lower Grinstead Clay, Ardingly Sandstone and Lower Tunbridge Wells Sand and has historically suffered from landslips. But Network Rail south east territory geotechnical engineer Derek Butcher says in recent years failures have started to happen more frequently. Between May and November 2006 these were severe enough that he had to impose speed restrictions of 20mph (32kmh), which translated into delays for passengers.
The railway operator called on contractor Edmund Nuttall for an emergency response to remove slip material from the slope and provide toe containment following each of these movements at the cutting. Although this restored normal line speeds of up to 90mph (145km/h), the operator needed a permanent slope stabilisation scheme.
Geotechnical contractor Ritchies revealed that the worst degradation appeared in the upper half of the slope. The team passed the information to consultant Tony Gee & Partners to come up with the stabilisation design.
As a result, since October last year, Nuttall has been carrying out works according to this plan. This began with re-grading the top half of the cutting with Ritchies' rope access workers removing 6000t of material in tandem with long reach excavators – taking the slope angle down from 50˚ from the horizontal to 30˚.
One of the recurring problems for the team was how to minimise further disruption to passengers while carrying out the work.
Ritchies came up with a plan that involves a mid-slope barrier, built prior to main works starting. This spans the slope laterally and is designed to catch any debris generated during the works that could otherwise have fallen on to the tracks.
Vertical Gewi steel bars acted as fence posts and support the 2m high blast netting that was used to create the barrier. Workers secured the top of the bars with wire rope attached to upslope anchors and attached the netting to horizontal cables. Rock ballast bags and king posts help with containing any smaller loose material at the base of the slope.
"We considered a toe wall [instead of] dealing with the problematic slope," says Butcher. "However, it would have proved costly and difficult to install without disruption to travellers, and would not have dealt with the problem slope areas. This idea [for the mid-slope barrier] will certainly be used on other similar projects across the country."
In addition to the barrier, one of the rig operators came up with an idea to improve communication while excavating with the long-reach rigs.
The operator's new plan removed reliance on hand signals for directing work by mounting a CCTV camera and microphone near the end of the rig's dipper arm.
These work in conjunction with 200 soil nails installed in the upper slope using long-reach excavators equipped with an air-powered drill mast. Each of these 25mm diameter, solid galvanised steel Gewi bars was installed perpendicular to the slope down to a depth of 9m.
Workers temporarily attached a tremi pipe to the nail as each was installed to allow the 40Nmm2 grout to flow out from the base of the nail towards the slope face.
Although typically not as weathered than the less steep upper slope, sections of the lower section – which reach an angle of up to 70˚ from the horizontal – also showed significant weathering of the weak sandstone. To counteract any potential problems with these areas in the future, the design here also included soil nails.
The long-reach excavators were unable to install nails in the lower slope, so Ritchies switched to using its own designed and built A-framed Terrapin rigs. Workers installed additional temporary anchors to provide support for these rigs, which allowed operators to winch them around the face of the lower slope.
The original design had specified using Gewi bars throughout, but because the space is so narrow at the toe of the slope the team was concerned that the masts of the Terrapin rigs could stretch over the railway tracks. This was not an option when trying to maintain optimum train speeds on the open line.
To avoid this, Ritchies and Nuttall asked Tony Gee and Partners to help with an alternative. This meant using the flexible anchors at a shallower angle of 65˚ from the face allowing the masts to remain more upright. The wire in these anchors is more capable of dealing with the bending moment at the new angle than the rigid steel bars.
On completion of the soil nailing, the long-reach excavator was due to treat the 2500m2 surface of the re-grade section with topsoil and grass seeding to improve the appearance of the embankment.
Site workers follow on by laying down North American Green-manufactured biodegradable matting NAG C350. This will act to stop erosion of the topsoil while encouraging plant growth.