01.04.13
Stabilising the Hooley Cutting
Source: Rail Technology Magazine April/May 2013
At Hooley near Croydon, a team of engineers is using its expertise to stabilise a railway cutting hit by land slips. Paul Thompson, writing on behalf of BAM Ritchies and BAM Nuttall, explains more.
Victorian railway engineers knew a thing or two about construction.
There is a small slither of Surrey countryside that encompasses all of their knowledge and now our site team is helping to ensure the railway cutting at the village of Hooley, cut more than 150 years ago, maintains the level of performance those engineers first envisaged.
This village 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 to Brighton lines.
These two railway lines run parallel with one another and pass through deep cuttings separated by just a few metres of land, which forms a central spine between them.
Now known as the ‘slow’ line and the ‘fast’ or ‘quarry’ line, these cuttings have been plagued by slope failure over recent years, particularly that of the 30m-deep slow line, which has seen several trains derailed by landslips.
In a bid to stabilise the slow line cutting, our team of engineers from BAM Ritchies and BAM Nuttall is installing soil nails, reprofiling the central spine, installing concrete beam and grillage stabilisation and placing a concrete beam along the crest of one side of the cutting.
It’s a complicated procedure even on the shallowest of cuttings, but at Hooley our contracts manager Andrew O’Donovan and the project team are working above a live railway on a cutting that theoretically should be a 70-degree slope but in practice is vertical in many places. It is very steep sided indeed.
The cliff-like fall of the cutting isn’t the only reason it has been prone to failure over recent years.
The lie of the land between the Forge Lane Bridge to the north and the tunnel that marks the southern end of our 650m section of work is also contributing.
A layer of gravel, stones and boulders sits on top of the chalk strata which make up this section of the North Downs. At the Hooley Cutting that layer of gravels thickens, becoming less stable where the Victorian engineers cut through it and prone to landslips which have caused derailments.
O’Donovan said: “It’s not so much the inclination of the chalk strata that affects the slope stability, but the layer of dry valley gravel above it. Dry valley gravel is just a geological term for the material.
“It’s anything but dry and can slip into the cutting.”
Which is precisely why the stabilisation scheme is being carried out on behalf of client Network Rail.
The BAM Ritchies team first began its work at the Hooley Cutting site during a nine-day blockade over the Christmas period last year (see overleaf), but since then has been busy 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.
These are on the western slope from the tunnel portal through to the fi rst of the row of houses that teeter at the top of the cutting, the ‘Esso houses’ section a little further down the cutting, the new beam and grillage section and then work on the central spine.
In the two ‘houses’ 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 being installed along most of the 620m length of the cutting, which will form part of the access and inspection walkway once the project has been completed.
Where the cutting passes behind the fi lling 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 BAM Ritchies team.
These lightweight slope rigs weigh just 850kg each and are supported from temporary mechanical anchors installed at the crest, and place the nails through a simultaneous drill and grout system.
O’Donovan explained: “Despite its name, the dry valley gravel material we are drilling into is neither dry nor particularly granular. It’s a real mix, with the bore incapable of staying open after drilling. That’s why we grout them up immediately.”
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 the crest beam and a further beam at the lowest edge of the dry valley gravel and an intermediate concrete beam placed between the two.
But such is the difficulty in access and the steep gradient of the slope that the reinforced concrete columns are being placed using sprayapplied concrete techniques.
The columns are sprayed in 2m lifts with plywood shuttering attached to the reinforcement cage with 4cu m of the 40Kn concrete sprayed across six columns each day.
They are launched from a series of 130mm diameter micropiles to 8m depths.
O’Donovan explained: “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.”
Across the railway line and in the central spin there are major earthworks being carried out to reprofile the dry valley gravel layer.
Approximately 14,000cu m 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 any 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 back to its original, safe best.
And those Victorian railway engineers will themselves have something to admire.
Christmas break enables BAM Ritchies to make site start
Scheme: Hooley Cutting Stabilisation
Client: Network Rail
Contract Value: £7.5m
While the rest of the country was settling down to listen to the Queen’s speech on Christmas Day last year, 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 14,000cu m 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 plastic mesh 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 between. point for the fencing posts. Approximately 200 post and anchor positions have been installed for the catch fencing to be slung.
The micropiles for the scaffolding foundations are just 100mm diameter and 3m deep and have been placed straight into the chalk
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