When ground engineering specialist Fondedile was faced with the near-impossible task of stabilising a 180-year-old railway cutting with limited access, without disrupting train services, it approached the problem logically and rose to the challenge, writes Richard Thompson
HERE is your challenge. There is an old chalk railway cutting whose sides keep falling onto one of Britain's fastest lines. Your job is to stop the houses at the top of the cutting falling onto the railway.
Oh, by the way, the railway must be kept open at all times. And another thing - there is no access through the gardens at the top. What do you do? It is like one of those seemingly impossible situations used to advertise careers in the army. But this challenge was real. And the Johnny-on-the-spot in this situation was Fondedile director Mike Durkan.
The essence of the problem was how to stop a five to eight metre deep layer of gravel perched on top of a 20 m high chalk cutting at Hooley in Surrey from falling onto the main London to Brighton line. There had been a number of major slips over the past 50 years, but following the removal of trees from the 53 degree slope the problem had increased.
The network manager had removed trees from the cutting after a large tree had fallen on the line, causing it to close. Additional fears about a lack of topsoil to support the mature trees in the 180-year-old cutting led Railtrack to remove them from a 0.5 km stretch. An unexpected consequence of their removal was an increased number of rockfalls.
This phenomenon was not new. The scarred slope bore testimony to many rockfalls over the years, along with the remains of previous attempts to stop the danger - a king post retaining wall at the foot of the slope, and red brick buttresses running the height of the slope to prop up brick retaining walls at the top.
Understandably, given the importance of the railway line in question, the issue of how to prevent any future landslides was given a top priority by Railtrack, and this time it wanted to stop the problem for good.
A number of solutions were considered including rock filled gabions, sheet piles at the top of the slope, shotcreting the slope and even adding matting with special seeds that would grow to stabilise the soil. The winning idea, as usual, was the simplest.
The chosen solution was a 0.5 km long reinforced concrete upstand beam, cast in front of the unstable gravel layer at the gravel/chalk interface, to prevent the gravel from falling. The retaining wall was supported by a giant grillage of reinforced concrete columns and beams - all cast in situ - that now sprawls across the slope like a giant hair net.
Forty 600 mm x 500 mm columns at 9 m intervals were placed to run vertically up the full height of the cutting face, with 32 shorter intermediate columns at 4.5 m intervals. Beams measuring 750 mm x 500 mm were placed between the columns. Steel netting was fixed to the concrete in the spaces between the columns and beams to catch the loose material. A mixture of tension and compression piles were used to keep the superstructure in place.
The solution was developed between principal contractor Laing, Railtrack southern region's slope stabilisation engineer Graham Birch, and Fondedile, the Italian ground engineering specialist which is 49 per cent owned by contractor Sir Robert McAlpine and had the £4 million pound contract for the giant grillage.
'This was the only practical solution, ' says Fondedile director Mike Durkan. 'When we arrived we thought 'my God', what do we have that can retain properties, protect trains and stabilise loose gravels and sands?
'We viewed the systems we had available as a geotechnical firm, such as our reticulated piling system, but to get piling systems at the top of the slope was impossible because of the weight of rigs and access.
Getting a suitable platform was impossible because of the tight site boundaries. So all those approaches were ruled out and we were left with the grillage solution.'
Where scars from previous rock falls had left unstable gravel exposed, bags of grout were stacked like sand bags from a wartime gun emplacement to retain it.
'It was a very positive solution, ' says Mr Durkan.
'The grillage was flexible enough to deal with large tree trunks and scars. If there was a major trunk in the way we could move a column or beam. And where there were scars, we could put in an extra beam to the support grout bags.'
The remains of previous landslides affected the positioning of the column bases. 'The king post retaining wall at the foot of the slope has a good 50 years' worth of landslip spoil, which would have cost too much to remove. So the base of the columns is 4 m above track level, ' explains Mr Durkan.
A combination of tension and compression piles was used to hold the grillage in place (see substructure box).
Providing access for the piling rig was crucial for installing the substructure. As there was no room for piling machinery from either the top down or bottom up, a giant scaffold covering the full face of the cutting was erected (see scaffold box). This provided access for the hydraulic mini piling rig by creating three levels of 3.6 m wide platforms running the full length of the site.
The same piling rig was used for compression and tension piles. This had its own logistical problems. One of these was how to lift the rig from one level to the next.
'You cannot use a crane to lift over the live tracks, ' says Mr Durkan. 'So we had to develop a system of scaffold towers designed with consultant engineer Andrew Burgess Partnership.'
The scaffold towers were the full height of the cutting support and each carried a 2 tonne capacity crane, with an 18 m travel that reached out over all three deck levels to enable access to all platform levels. When the piling rig had to be moved it was driven along the scaffold platform, dismantled and hoisted to the next level.
The cranes also lifted formwork for the beams and columns of the grillage superstructure.
'Concrete was the main logistical problem, ' says Fondedile project manager Kanti Patel. 'We pumped it through a 100 mm diameter static line from above. At one point we had to pump a 300 m distance.'
Fondedile's work was made more difficult by the very thing they were trying to prevent. Work had to stop three times for landslides. Surprisingly, that was not the worst thing about the job.
'Uppermost in your mind, ' says Mr Patel, 'is the wind that blows down the cutting.'
THE BEAM/column grillage was held to the face of the cutting by two rows of tension piles at 3 m intervals up the columns - one at approximately mid-height and the other just below the chalk/gravel interface. The tension piles were designed to resist 1 KN/sq m of uplift forces.
Each column was founded on two 170 mm diameter compression piles, auger drilled 12 m to 16 m into good chalk. The chalk was heavily laden with layers of flint up to 300 mm thick.
'The slope was already moving so we did not want to introduce water. Auger drilling was the best way, ' says Fondedile director Mike Durkan (below).
The piles were filled with a sand/cement grout and had a single 25 mm diameter reinforcement bar running their full length. A 3 m long, 140 mm diameter tube was fitted to the top of the pile to carry horizontal moment.
A 3.6 m wide platform on the massive three-level scaffold was erected to allow the rubber-tracked hydraulic mini piling rig to run the full length of the job to install the piles.
The same rig was used to install the 133 mm-diameter tension piles using the pali radice technique patented by Fondedile. These were reinforced by a 32 mm-diameter Macalloy bar. The first 6 m of the bar was double corrosion protected.
Pali radice, meaning 'root pile', was developed by Fondedile in Italy to underpin ancient monuments.
It involves drilling through the structure to act like a root.
The giant scaffold
ONE OF the biggest challenges for Fondedile, which had the £4 million contract to design and build the slope stabilisation system, was access.
Railtrack wanted to keep the line open, so, apart from a period at the beginning of the project when principal contractor Laing had a month of nighttime possessions to replace a drain between the two high-speed lines, there was no access from below.
Access from above was restricted to a narrow path because of the location of local residents' gardens.
The solution was to construct an enormous £1 million three-tier scaffold covering the full face of the cutting. Each tier had a 3.6 m-wide deck, wide enough to allow a skid steer and piling rig to operate. The bottom tier also had a 2.5 m-high timber parapet to catch anything that fell.
The scaffolding contractor, Loughlin Scaffolding, was able to piggyback on possessions by another contractor which was renewing drains. Two massive scaffold towers were built to carry the beam crane needed to lift the piling rig and concrete form boxes between platforms. It had to be carefully designed to ensure there were no clashes between the scaffold elements and the grillage beams and columns.
The scaffold looks most impressive at night, when the lights are on. 'It's like Blackpool on the railway, ' smiles Fondedile project manager Kandi Patel.
Project details Project: 34-week contract to stabilise chalk railway cutting in Surrey Value: £4 million Client: Railtrack southern zone Principal contractor: Laing Ground engineering: Fondedile Scaffolding: Loughlin Scaffolding, London Scaffold design: Andrew Burgess Concrete & formwork: SMD Formwork Concrete supplier: London Concrete, Purley nNetting: HRS Sheffield Fencing: Stenoak