Isle of Wight Council is on a rescue mission to save a busy cliff-top road from plunging into the sea.
Bernadette Redfern finds out how.
From a distance the majestic, undulating chalk cliffs at Afton on the south coast of the Isle of Wight appear unassailable. Standing over 70m high, it seems impossible that the gentle blue waters below could be eroding the coastline.
But the relentless assault of the waves, combined with the slow but steady exfoliation of the chalk face, means the cliff is retreating, putting a popular cliff top road in peril. At two points, the Military Road is now less than 5m from the cliff edge.
Abandoning the road and leaving it to the sea was never a practicable option. In summer tourist season it is used by up to 5,000 vehicles a day.
Afton Down is a candidate special area of conservation, a site of special scientific interest, an area of outstanding natural beauty, and a piece of heritage coastline. Negotiations over a buildable design in this ultrasensitive environment started in 1997, says Isle of Wight Council principal highways engineer Nick Gallin.
Where the road was under threat a row of 22m deep bored piles would be installed along the centre line of the road. These would be constructed using Ibeam reinforcing and a weak grout to make removal as easy as possible when, eventually, the road passes the point of salvation. To restrain the bored piles as the cliff face fell away, raked tension piles would be installed to the landward side. The seaward carriageway would be supported by a concrete deck cantilevered from the bored piles.
Planning permission for cliff 'The piles must be removed, depending on what comes first: either half of the pile length becomes exposed due to erosion, or the project reaches the end of its 50 year extended life, ' says David Hattersley, supervisor and environmental enforcement officer for project designer High-Point Rendel.
The design was put out to tender in 2002 but all five of the bids received were much higher than the council expected. 'Risk elements had to be incorporated into the tender cost and so we entered into negotiations with contractors to see if we could lower these risks, ' says Gallin.
Contractor and preferred bidder Amec suggested a series of borehole trials along the road to help gauge ground conditions and reduce risk. 'Luckily the trials proved there was very little in the way of voids, ' says Amec site agent Steve Thwaite. However, small fissures running from the cliff edge to the road were thought to be pretty long.
'If you poured in a weak grout you could be pouring for ever, ' says Hattersley. This led to the decision to use standard reinforced concrete piles.
Further design changes swiftly followed. Construction of the piles in the centre of the 6m wide road effectively cut working space down to 3m.
Amec suggested moving the bored piles to the seaward edge of the highway and connecting them with a reinforced concrete capping beam. Steel ties would run from the beam across the road to a reinforced concrete anchor block, which would be retained by steel tension piles.
Following reappraisal by High-Point Rendel, the project was let at £2.7M, considerably less than the original £4.3M originally tendered. Any cost overruns or savings when the project is completed will be split equally between the council and Amec.
Construction began in January along a 120m, lower lying section of road. This spring work on a second, 140m stretch got under way, involving some 260 bored piles.
Forming a first line of defence against the sea, the 22m long, 750mm diameter bored piles are set out at 1.1m centres. Across the road, 50mm diameter steel tension piles are at 3.3m centres.
Excavated chalk has been collected by the local quarry.
Even with the full width of the road to play with, limited space means the piling rig is forced to move back down the hill to make room for the service crane. Two reinforcement cages are used in each pile, the lower consisting of 10 radially arranged T25 bars.
This is bolted to an upper cage of heavier T32 rebar - the top 10m of the cliff is expected to fall away faster than that below, and as it does the heavier rebar will be needed to resist larger bending moments generated by the road and retained soil.
Manoeuvering the concrete wagon is also a tight squeeze.
Each bored pile takes 11m 3ofC35 concrete with 20mm aggregate, which is tremmied straight into the pile from the truck. Average output has been two piles a day.
The capping beam is tied back at 3.3m centres to the anchor block with 26.5mm diameter steel rods. The tie rods are cast in at either end. Meanwhile, the anchor block is in turn bolted to the steel tendons of the tension pile.
In 50 years' time removal of the piles should be quite straight forward. 'You would start by undoing the nut that connects the tension pile to the anchor block, ' says site engineer Dan Squires. The concrete capping beam would then be broken out and the bored piles debonded from the cliff using either overcoring or jet flushing. 'The piles would then just be lifted out.'
The project is due for completion in August, ahead of the scheduled September deadline, and it looks as if both the council and Amec will make a saving.