Network Rail recently used soil nailing to stabilise a section of embankment on the South Wales and Bristol Line at Westerleigh near Bristol. One of the main high-speed rail routes from London to Bristol and Wales, it was originally built as a bypass to the Great Western Line, which goes through Bath.
The railway's history dates back to the Industrial Revolution, and the section of embankment earmarked for the works was probably built about 1897.
In 2000 a survey concluded that the slopes should be stabilised to avoid extensive earthworks in the future and to address stability problems before they escalated.
The area is underlain by steeply bedded Upper Carboniferous Coal Measures with predominantly undivided mudstones with interbeds of Pennant Sandstones. The site is centred on the Coalpit Heath Syncline.
The embankment generally consists of about 2-4m of made ground over reworked natural material, with a total height of 8-10m. Although there was little or no ground water, there was seepage 2m below ground level in the mudstones on some borehole and window sampler logs.
Consultant Arup chose soil nailing for the steeper slopes at the locations of the railway underbridges.
Slope geometry here meant special measures were needed to ensure permanent stability and to provide a cess walkway at rail level. Arup chose soil nailing because it was the least disruptive stabilisation method, allowing the line to stay open.
Reinforcement is provided by Dywidag MAI hollow bar, 4m to 9m soil nails declined between 10° to 20° from the horizontal, using a combination of R38N and R32N hollow bars. The top row of nails features the stronger R38N bars to give extra stiffness for a track ballast retention system, as well as ensuring straight drilling for the nails at the level nearest the track.
Keller Geotechnical Engineering drilled the soil nails and meshed the face for main contractor Laing O'Rourke, which won the £2M earthworks package.
The hollow bars are manufactured from thick wall steel tubing - 9.5mm for the R38Ns and 6.75mm for the R32Ns. They are supplied as a rope thread system (with a coarse heavy thread on the outside of the bar) to improve the bar/grout bond.
Drill bit choice had to incorporate the drilling requirements of mixed ll, overlying the Mercia Mudstone and Coal Measures. The 130mm diameter R38CRC retroush drill bit was chosen for its versatility in coping with this mixed ground.
Keller used simultaneous drill and grouting for the soil nails to ensure grout is placed at all points as the drill string advances.
The soil nails' 60-year working life is ensured by a sacricial corrosion allowance with supplementary galvanising.
Limited access meant works were carried out from the bottom of the slope using a 42t long reach excavator with a linkage-mounted Klemm drifter rig.
This set-up also featured a mobile elevated working platform to provide the driller's assistant with access to the drill location for coupling and rod changes.
The soil nail slope was faced using Fortrac 3D-30 mesh and then plated with a 460 x 460mm x 14mm thick galvanised plate. After meshing, the embankment was hydraseeded.
Soil nail testing at four positions con med design values. Keller installed the test nails in pairs; one short, drilled to the slip plane, and the other to full length. Three working nails were also tested.
Laing O'Rourke built a small reinforced retaining wall at the crest of the slope to retain track ballast and support the cess walkway adjacent to the underbridges. It also installed concrete-lled shear trenches at 5m intervals to intersect the potential deep-seated failure plane.
The main contractor benched and regraded the slope between the trenches, ted rabbit netting and hydroseeded the embankment.
Keller finished soil nailing in April, while Laing O'Rourke completed the project in May.