Slope Engineering

Seaside Special
A few clever tricks have helped keep trains running on time beside a slippery slope flanking a section of the London to Brighton railway. Alexandra Wynne keeps her footing to take a closer look.

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 near Haywards Heath has started to stir causing a number of landslips along its face.

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 became 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 wanted to mitigate the risk of future occurrences and so looked to its Network Rail Civils Framework (South) to come up with a permanent slope stabilisation scheme.

In January 2007, geotechnical contractor Ritchies (a subsidiary of Nuttall) came on board with the framework to carry out Panda probing work to determine the extent of weathering of the slope.

This involves using a hammer to hit a rod mounted with a cone at the end. The head is spring loaded and measurements are taken to check how hard the hammer hits the rod and how much of the cone and rod penetrates the ground with each probe. The contractor carried out 222 of these to 3.5m depths, which revealed that the worst degradation appeared in the upper half of the slope. The team passed the information to consultant Tony Gee and 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. Nuttall construction manager Guy Keep says this kind of project typically involves speed restrictions and / or night-time possessions. These allow only limited windows to work in than normal daytime operations and would have meant a much longer period would have been needed to complete the project.

Instead, Ritchies came up with a plan that enables work to be carried out while the trains continue to run at top speeds through the affected Copyhold Cutting. This came in the form of 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.

Ridgway Holdings supplied the Komatsu Super Long Reach rigs - one a PC340-7 with a 22m reach and the other a PC2IO-8 with a 16m reach. This enabled rig operators and slope workers to negotiate the working area more effectively, providing real time images in the cabin of what was being excavated.

The job was made even more delicate by the need to remove large tree root balls from this part of the embankment while ensuring none tumbled down the slope and on to the tracks.

Following on from the re-grade, rig operators were able to carry out the principal stabilisations work beginning with installing pairs of raking anchors and micro-piles - 54 of each - along the embankment crest.

These work in conjunction with 200 soil nails installed in the upper slope using long-reach excavators mounted with an air-powered drill mast. Each of these 25mm diameter, solid galvanised steel Gewi bars is installed perpendicular to the slope down to a depth of 9m. Workers temporarily attached a tremi pipe to the nail as each is 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 lower slope nails comprised a combination of more Gewi bars supplemented by Geobrugg wire rope anchors for the bottom row - all of which go in at the same depths and with the same grout used on the upper slope nails.

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.

To cap off the re-graded area, Maccaferri manufactured rock fall netting is hung and held in place with face plates. At the same time Maccaferri's Macmat-R rock netting, combined with soil erosion matting, is installed across 1000m2 for the soil-nailed sections of the lower slope. The team is finishing off the job with rock netting for the less weathered areas to act as containment for any loose material.

Ritchies project manager Andrew O'Donovan says: "Working on the bottom of the face so close to such a busy line, while managing to design a whole system that allows you to move the rigs about the site, has been challenging. But being able to come up with this system and have Network Rail approve the ideas the team came up with has been the most rewarding part. "

The contracts are worth over £2M for Edmund Nuttall and £750,000 for Ritchies and all the work was due to be completed by late March.

Article courtesy of Ground Engineering - April 2008