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Not a cloud in sight

CONTRACTS

A MIX of stone and cement is forming vibro foundation columns to stabilise ground for a large distribution centre near Doncaster.

The Nimbus Park development will consist of a 70,000m 2 warehouse alongside 1800m 2 of office space.

The 20ha site is former farmland, sandwiched between the M18 motorway and a railway on the outskirts of Thorne.

Main contractor Tolent Construction is leading the project on behalf of Helio Slough Developments.

When Roger Bullivant began installing 20,650 vibro stone columns to improve the poor ground in May, it had schedule of just nine weeks, so it decided to use three rigs to ensure the works were finished on time.

The rig crews install a daily average of 460 columns, each measuring 5-6m. For the floor slab with 60kN/ m 2 loads, columns are placed on a 2m grid. Under the warehouse's steel frame foundations they are at 1.3m centres.

Vibro compaction usually involves tipping stone into the ground while using a vibrating mandrill to create a column. But on this site weak soils overlay hard rock to a 6m depth.

'Due to the nature of ground, cement stabilised stone columns were used to reduce long term settlement, ' said Bullivant national operations manager for commercial foundations, Seamus McNulty.

'Use of standard stone columns could have resulted in bulging, leading to excessive settlement of the building. We decided to mix cement into the stone on site, which will not only help reduce bulging but strengthen the columns.' The stone and cement mix is poured into a hopper attached to the front of a rig. Stone is released into the ground and the rig's mandril vibrates it down, compacting the column along with the surrounding soil.

Site supervisor Jack Spence says: 'The integrity of each column is enhanced and the columns help to remove elasticity from the soil. Each column is being installed in just four to five minutes.' Two of the company's 4000 series rigs have a vibrating unit positioned on top of the mandrill. A third, larger 8000 series rig, has two vibrating units, making it powerful enough to penetrate hard ground. All three rigs have air bags fi tted around the units to help to prevent vibrations travelling right through the rig.

Warehouse construction is due to finish by November this year.

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