Stabilising a 180 year old railway cutting has brought Bam Ritchies’ engineers face to face with the same issues Brunel dealt with when the route was built
Dipping strata and fault lines within the lias limestone, siltstones and overlying clays and red marls did not put Isambard Kingdom Brunel off when it came to constructing a cutting near Weston-super-Mare for his Bristol to Exeter railway. The local name for the 35m single arch masonry bridge that crosses the cutting - Devil’s Bridge - gives an indication as to the problems this route presented during the construction phase.
Almost 180 years later and the cutting at Uphill still proved to be a devil of a problem when the steep slopes needed reinforcement to bring them in line with current design standards. Several shallow surface slips in recent years led to Network Rail calling on Bam Ritchies to find a solution to prevent future failures and upgrade the cutting.
Ritchies carried out the design and construction of the soil nailing and netting solution after submitting a winning tender on a joint venture basis with parent company Bam Nuttall with Tony Gee & Partners as designer. Experience gained by Ritchies from its work on the Hooley Cutting on the London to Brighton line stood the company in good stead with Network Rail and allowed the company to transfer some of the solutions used
on that scheme.
One idea taken from the Hooley project was the installation of a robust catch net at the foot of the cutting. According to Ritchies project manager Phil Howard, this allowed the drill teams to continue their nailing work throughout operating hours without interfering with the day-to-day running of the railway or impacting on the travelling public.
With 2,700 nails and 14,000m² of steel mesh netting to install, maximising working time was seen as vital to the success of the project. The cutting varies in depth from around 8m at the northern end of the scheme to a maximum depth of 20m on the eastern side and, for the purposes of the project, has been broken down between the bridge and railway line into four sectors.
The team only benefited from good access at the crest of the slope in part of one of the four sections. Here, the site team were able to use a long reach excavator with drilling attachment to reach down the cutting and install 90% of the soil nails.
Across the rest of the project Ritchies is using four of its in-house designed and built lightweight, rope-supported Terrapin drill rigs because access is so
“This is a residential area and in places there is very little access at the crest of the cutting,” explains Howard. “We are using the Terrapin rigs because that’s the only way we can get in.”
These rigs require anchors at the top of the cutting, and 450 of these were drilled to 3m depth by hand auger in three of the four sectors because of access issues. In the remaining sector the 110 anchors needed were machine drilled to 5m. All the anchors were tested to 150% of the working load, and when the Terrapin rigs no longer require
them they will become part of the permanent solution.
The Terrapin rigs use two anchor points while installing the main pattern of 25mm diameter Dywidag Gewi-steel nails. Th ese are spaced on a staggered grid at 2.5m centres to a depth of 5m and angle of 70° from horizontal across the top of the cutting. Th e nail length and angle of installation changes to 6m for the next two rows at 60° and 50°, while further rows are installed to 8m depths at angles of 30°.
“The top nails are placed at a steeper angle so that we can avoid needing to impinge on anyone’s property,” says Howard.
The team is using down-the-hole hammers to drill 90mm to 100mm diameter holes through the variable rock conditions before the soil nails are installed in 2m and 3m sections and grouted into place using tremmie pipes. The nails are broken down into smaller sections to ease handling on site and linked using couplers.
The nails themselves are left to protrude 100mm or so from the cutting surface which enables the high-tensile steel Dywidag Deltax mesh facing to be installed over the top of them. This is cut to length from a 30m long, 3.5m wide roll in the site compound. The 3.5m wide strips are then rolled down the face of the cutting before being locked into
position by a 250mm diameter, 15mm thick circular steel plate installed over the nails. The mesh is also held in position thanks to a series of 13mm diameter steel core vertical cables set at 20m intervals along the length of the site. Similar horizontal cables will feature at the top and bottom of the slope.
Ritchies is now in the final stages of the £3.6M stabilisation and so far the work has not impacted on passengers’ journeys. Successful completion means the cutting will not pose a risk to disruption in the future either.
It is not just the proximity of the gardens at the top of the cutting that have provided access challenges for the site team. With the Devil’s Bridge (pictured below) spanning over the cutting being a listed structure, the team has been unable to fi x any of its grout or air lines to the bridge itself. To overcome this, grout lines have been installed underneath the tracks in a number of locations at the site to ensure materials can be supplied to all the nailing points.
Two site-based grout mixing plants feed the scheme with the water/cement grout that is required to reach a strength of 30N at 28 days. “We did think we might have to locate a grout station on a public footpath close
to the houses in one of the sectors on the eastern side of the cutting, but we have been trying to limit the
impact of our work on local residents and the travelling public,” says Ritchies project manager Phil
Howard. “The grout line crossing beneath the track helps us do just that.”
Some of the work on the scheme has been carried out overnight during track possessions so minimising
the impact on local residents was a key factor in the choice of laying under track grout pipes, as well as the access.
“To keep noise to a minimum we have also used super-silenced machines and boarded around them to provide additional sound proofi ng,” adds Howard. The approach appears to be paying off with the site scoring positive feedback from interim inspections under the Considerate Constructors Scheme.