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Double agent at work in Croydon

CONTRACTS

ONE RIG is installing both auger displacement piles and vibro stone columns on a housing development at the end of a dismantled railway line at the old Addiscombe railway station in Croydon, south London.

Pennine Vibropiling says its Stratamax is the first UK piling rig that can both drive augers and use a vibroflot.

The 45t rig is based on a Samsung excavator chassis and is driven by a Caterpillar engine. It can install bottom or top feed stone and concrete columns to 12m with a maximum pull-down force of up to 30t. It can drive 250mm to 400mm piles up to 14m deep.

Changing the head and putting a flot on allows the rig to install vibro stone columns.

'It takes about four hours to change the rig over, ' said Pennine southern area manager Derek Taylor.'However, it is a lot cheaper to mobilise one rig than two.'

The site was once owned by the East India Company and was the site of its military college.

During expansion of the railway network in the 19th century, the South East Railway company forged the line into Croydon but it terminated abruptly at Addiscombe Station because the East India Company would not grant permission for a railway across its land.

The station, which opened in 1864, was closed in May 1997 as a result of the Tramlink Act, to make way for construction of Croydon's tram system. The station building and engine sheds were demolished, allowing Pennine to start work in midJanuary this year.

Site geology comprises 2- 3m of fill over a thin band of gravel followed by about 6m of stiff London Clay and then the dense sands of the Lambeth Group.

'Demolishing the engine sheds disturbed the ground down to a depth of about 2m and there were also some gravel workings on the site. We therefore decided to pile these areas, ' said Taylor.

'Auger displacement piles were chosen to minimise the amount of spoil which has to be taken off site, reducing congestion on the road outside the site, ' he said. The 350mm diameter piles are installed to a depth of 10m to 12m, toeing into the dense sand and will carry loads of between 250kN and 440kN.

Between the piled blocks of houses, the ground beneath the proposed roads is being improved using top feed vibro stone columns. These are between 2.5m and 3m long, installed on a 2.5m grid.

Bellway Homes, the main contractor and client, is building 70 units on the site, ranging from town houses to flats. About 360 piles and 1000 stone columns will be installed on the £100,000 contract.

When GE visited site in February more than 600 stone columns and half of the piling had been completed.

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