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Vibro stone columns will support new slip road embankments as part of M25 widening at junction 14. Damon Schünmann reports.

Coping with badgers, aeroplanes and an electronic traffic regulation system was part of the package for Pennine Vibropiling during its contribution to the widening project on the M25 near Heathrow.

Pennine's £1M job, which finishes this month, is part of Balfour Beatty Civil Engineering's £147.5M design and build contract for the Highways Agency and Heathrow Airport to widen the motorway between junctions 12 and 15 and to build a spur road to Terminal 5, which includes a viaduct over the M25. The works are due for completion by December 2005.

The project is now in phase two; advance earthworks were completed during phase one by Laing O'Rourke. Pennine's role is to install vibro stone columns to accelerate settlement, enabling the ground to support embankments for new slip roads for junction 14.

The old slip roads will be used to widen this busy stretch of motorway to up to six lanes in each direction. The London Clay embankments are being built to a maximum height of 8.5m and have 1:3 slopes.

Gifford WSP is the consultant for Pennine's work and the construction of the embankments, two of which will each contain about 50,000m3 of material. Vibropiling under these has covered areas up to 600m long by 20m wide.

Badgers living at the site were not impressed by such feats of digging.'There were certain times when we couldn't work close to the badger sett, ' says Paul Hewlett, production team leader for Balfour Beatty. 'In the afternoons we couldn't get within 50m to comply with a licence issued by English Nature.'

Because the site is on the take-off and landing path of the nearby airport, height restrictions apply. This means Pennine has to stay aware of the prevailing wind direction in case the drillers need to stand the two 15m high Stratacaster rigs down.

Midas, the system used to display temporary speed limits during heavy traffic between junctions 10 and 15, is another complication.

Pennine project manager Richard McKenna says: 'On the southbound on-slip for junction 14 we could only do certain sections [of vibropiling] at a time as the Midas cabling had to be moved around us.There was also a gas main we could only get within 6m of.'

Pennine expected to pre-bore 20% of the stone columns but using the Stratacasters meant this was unnecessary.'We've only had to pre-bore 138 columns out of 10,000 as the rigs have very powerful pull down, ' McKenna says.

Pre-boring was needed where a 300mm thick dense band of fused ash and clinker was encountered.

A second benefit of using the lighter Stratacaster rigs is the reduced cost for Balfour Beatty of constructing the piling platform. The 35.5t rigs were supported by a 700mm thick platform of recycled material, less than heavier rigs such as Pennine's 49t Terrafirmers would have required.

'We used our lighter rigs because we didn't want Balfour Beatty to compact the stone [to take heavier machines], ' McKenna adds. This would have meant more pre-boring for the stone columns.

The gravel for the bottom feed vibropiling rigs is fed directly from the front bucket of a JCB as the poker drives into the ground. Compressed air from the tip of the poker forms a sheath around it to help it enter and withdraw from the ground, and this pressure also supports the sides of the hole.

The stone columns are either 450mm or 600mm diameter and are installed on a triangular grid, at centres of between 1.8m to 2.7m, in an area of former gravel pits.These have been backfilled. Gravel is found in places sitting on the underlying London Clay.

Because of the variations in strata thickness, the stone columns vary in depth from 1m to 7.5m, with 5m being the average length needed to reach into the clay.

An added benefit is the 40mm locally quarried gravel which means the columns act like French drains, drawing water out of the surrounding ground.

Some of the zone and plate tests at the site are being done at night to prevent readings being affected by site traffic, which could not be rerouted.Tests are being carried out on four stone columns at a time. The triangular layout of the columns means that the 3m by 3m plate sits over a diamond formation.

Four zone tests are being conducted over the four main areas of vibropiling with a maximum load of 60kN/m2 applied in two of them and 170kN/m2 in the other two. The higher load tests are being carried out on areas of ground which will support greater embankment loads (ie the highest embankments).

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