Stabilising steep slopes above a live railway line meant Nuttall had to tackle access and safety issues as well as technical challenges at the Hooley Cutting. GE reports
Anyone who still doubts the quality and ingenuity of Victorian railway engineers should take a trip south from London to Brighton and marvel at the railway cutting at the village of Hooley. This small slither of Surrey countryside marks a pass in the North Downs and also the point that, in the 1840s, two rival railway companies recognised would be perfect for running their London - Brighton lines.
These two railway lines run parallel to one another and pass through deep cuttings separated by just a few metres of land that forms a central spine between them.
Housing what are now known as the “Slow” or “Redhill” lines and the “Fast” or “Quarry” lines, these cuttings have been plagued by slope failure over recent years. The 30m deep slow line cutting has been particularly badly affected, with resulting disruption to train services.
In a bid to stabilise the slow line cutting, Bam Nuttall, with the inhouse ground engineering specialist Bam Ritchies, is working on a £7.5M scheme to reprofile the central spine and install a combination of soil nails with concrete beam and grillage stabilisation.
The procedure is complicated, even on cuttings with easy access. But at Hooley, Ritchies contracts manager Andrew O’Donovan and the project team are working above a live railway on a cutting that boasts slopes with angles varying from 70ºto 90º.
But it is not just the slope of the cutting that is causing the instability in the 650m long section between the Forge Lane Bridge and the portal where the line disappears into a tunnel. It is the lie of the land itself. The location on the chalky North Downs means the underlying chalk strata dips from east to west across the site.
“It’s not so much the inclination of the chalk strata that affects the stability, but the layer above it”
Andrew O’Donovan, Bam Ritchies
This is topped by a layer of dry valley gravels that thickens across the site from just a few metres deep on the eastern side of the Fast Line cutting to almost two thirds of the 30m slope depth on the western side of the Slow Line cutting.
And it is this dry valley gravel layer that causes problems for trains running along the line. “It’s not so much the inclination of the chalk strata that affects the slope stability but the layer above it. Dry valley gravel is just a geological term for the material. It is anything but dry and can slip circularly into the cutting,” says O’Donovan.
Which is precisely why the stabilisation scheme is being carried out on behalf of client Network Rail.
The Nuttall team first began its work at the Hooley Cutting site during a nine day blockade over the Christmas period in 2011 but since then it has been busy working on the stabilisation work itself.
The cutting has been broken down into four separate sections by the team which is installing the three stabilisation solutions.
On the western cutting there are two sections behind 24 houses and a couple of commercial premises, as well as a new concrete beam and grillage section. The remaining work is on the eastern cutting of the central spine.
In these two sections, the dry valley gravels are being stabilised through the use of 100mm diameter soil nails installed at depths of up to 14m at nominal 1.5m centres. The highest line of nails is encapsulated in a 430m crest beam which is being installed along most of the 620m length of the cutting. This beam will form part of the access and inspection walkway once the project has been completed.
“The bores are incapable of staying open after drilling, which is why we are simultaneously grouting”
Andrew O’Donovan, Bam Ritchies
Where the cutting passes behind a petrol station on the main road the nail design has been tailored to ensure there are no clashes with its fuel storage tanks.
Drilling and grouting for the 6,200 soil nails is being carried out using three Rippamonte hydraulic slope rigs that have been built for the Ritchies team. These lightweight slope rigs are supported from temporary mechanical anchors that are installed at the crest and place the nails through a simultaneous drill and grout system.
“The bores are incapable of staying open after drilling, which is why we are simultaneously grouting,” explains O’Donovan.
Toward the northern end of the cutting, the team is installing 21 new concrete columns along the face at 5m centres.
These are crossed with a crest beam and another beam at the lowest edge of the dry valley gravel plus an intermediate concrete beam placed between the two. Access is difficult and the slope is very steep, so reinforced concrete columns are being placed using spray applied concrete techniques.
The columns are sprayed in 2m lifts with plywood shuttering attached to the reinforcement cage and 4m3 of the 40kN concrete sprayed across six columns each day. They are launched from a series of 130mm diameter micropiles to 8m depths.
“The sprayed concrete is easier to use because of the access issue. It’s more efficient, the material stays in line and it’s only really the nozzles that need to be washed,” says O’Donovan.
Across the railway line and in the central spine major earthworks are being carried out to reprofile the dry valley gravel layer.
Approximately 14,000m3 of this layer is being scraped from the cutting face and the top of the central spine using ultra-long reach excavators more normally associated with demolition projects.
The plan is to reduce the angle of the face itself from the point where the valley gravels layer sits above the chalk, reducing the amount of material that sits above it and minimising the possibility of slip failure. The finished face will also be nailed and netted to prevent any falls.
It may be a complicated scheme but when the work is finally completed later this year, the cutting will be stable once again. And those Victorian railway engineers will themselves have something to admire.
Christmas start secured access
While the rest of the country was settling down to listen to the Queen’s speech on Christmas Day 2011, engineers at the Hooley Cutting were working away at the foot of the 30m deep cutting installing around £1M of enabling works in advance of the major stabilisation project.
With a main scheme involving the installation of more than 6,000 soil nails and the excavation of more than 14,000m3 of material, all to be carried out without affecting the day-to-day running of the London to Brighton “Slow” line.
The enabling work for the Hooley Cutting project was carried out during a nine day rail blockade during the Christmas slowdown.
During that time around 1km of rope netting catch fence was installed at 5m above the running line level and a further 260m of scaffolding supported on micropiled foundations.
The catch fence is installed using a 2m length bar anchored into a shorter 100mm socket drilled perpendicular to the slope. A further upslope socket is drilled to provide an anchor point for the fencing posts.
Approximately 200 post and anchor positions have been installed for the catch fencing to be slung between them. The micropiles for the scaffolding foundations are just 100mm diameter and 3m deep and have been placed straight into the chalk.