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Double trouble

PILING & DEEP FOUNDATIONS

Stabilisation of a scenic but treacherous section of the A36 carriageway effectively became two piling operations. Gareth Beazant reports.

Steep unstable slopes alongside the A36 near Bath meant preliminary piling had to be installed to support the rig working to stabilise the carriageway.

The route, winding through scenic hills around Limpley Stoke, has been the subject of previous piecemeal schemes aimed at halting slides which have seen the road edge falling away.

Now the whole 2km section is being shored up in a £2.5M Highways Agency framework agreement. Atkins is framework manager and Lafarge is framework contractor. May Gurney is working for Lafarge in a £620,000 12 week contract which involves closing the road.

The first task was to remove kerbs and fences and cut roadside trees - except those used by roosting bats, which had been identified and marked for protection.

More than 400 piles varying from 350mm-900mm in diameter are being installed at depths ranging from 4m to 12m.

The route has been divided into seven sections. Section three has been the most difficult, with 127, 750mm diameter piles driven to depths of 8m to 12m through 2m of limestone.

On sections four and five the land drops away significantly and the latter includes a layby. A cottage lies near section six where 25 of the biggest piles, 900mm diameter, are being driven to 8m.

An almost vertical drop on section seven meant May Gurney had to use a temporary platform for the rig to install the 64, 900mm piles safely.

Most of the piling is being carried out by a 75t Llamada rig and challenging though the project is, it is only half the story.

'Before we started piling the client asked Symonds to carry out slope stabilisation tests on where our rigs would be working, ' explains Steve Longdon, southern area manager at May Gurney.

'This determined that the factor of safety was not suitable for our rig. This meant that before we could start on the main piling works we had to work ahead with a lightweight rig - installing piles to shore up the ground to support the bigger rig. The work doubled, effectively.' May Gurney opted for a 28t rotary rig for this part of the project. It formed holes for universal 152mm by 152mm, 37kg steel piles that made the ground suitable for the bigger machine.

'Working on slopes like this problems occur with soil falling down the side when you are working, ' says Longdon. 'Also the steepness of the slope meant that man access would be dangerous.

We suggested creating some sort of shield that could be attached to an excavator and we consulted MJ Church Plant on a possible solution.' The result was a steel attachment to the excavator which rests on the ground and arcs inwards at each end. This surrounds the auger and catches any spoil that would otherwise hurtle down the hill. It also acts as a safety barrier for workers who need to get close to the piling area.

Once the piling work is finished, concrete capping beams are cast on top of the piles to provide support for the carriageway as well as supporting safety fencing or parapets.

The whole project is due for completion at the end of this month but it is unlikely to be the final chapter at Limpley Stoke.

'The hill is on the move so it'll never be a permanent solution, ' says Longdon.

'When I say move I mean 50 to 60 years for a notable change. But holes have been left in the capping beams, possibly to add ground anchors at a later date.'

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