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Small, but perfectly formed

PILING AND GROUND IMPROVEMENT

A displacement pile system is reducing spoil and lorry movements on a tiny site in south-east London.

Gareth Beazant reports.

Borough, the area south-east of London Bridge, is an up and coming part of the capital. Piling rigs protrude above site hoardings as new apartments, offices and mixed-use developments begin to rise. Lorries line the main roads, ready to remove vast amounts of spoil from the numerous developments.

Yet among the hustle and bustle, one site appears to have fewer wagons waiting outside, despite its large piling rig.

This is thanks to a technique that creates a tiny amount of spoil compared with conventional piling methods. The system is the Pennine Penpile, which uses a patented boring tool to displace soil laterally as it bores into the ground.

Stan Mimms, northern area sales manager for Pennine Vibropiling, says: 'The client [Buxton Homes] knew it wanted piles for the project and it was fully expecting the muck that comes with a CFA method.

'When we tendered, Buxton was keen for the benefits of reducing the amount of excavated material on such a congested site and area. There is also the issue of congestion charging in London, which could have had cost implications [for lorry movements].' The site, which will house a seven-storey block of work units and apartments, is only 50m 2. It has residential and commercial buildings on three sides and the one access point is also the entrance for apartments behind the site. Organisation was crucial to minimise disruption.

Pennine Vibropiling contracts manager Gary Kime says: 'Having occupied buildings right beside this project ruled out any driven method.

'Our method is similar to CFA in that it creates less vibration than driven piles, but it creates less spoil because of the soil displacement. The piles have an expanded base so we haven't had to go as deep as other methods, so less concrete is required - again limiting disruption because fewer deliveries are needed.' The piling rig almost filled the site, so there probably would not have been space for an excavator to remove spoil in any case. Over the three-week contract last month, Pennine installed 325, 300mm diameter piles to an average depth of 5.5m. These have a working load of 400kN.

Penpiles are created by rotating a former into the ground to the required depth. Concrete is pumped through the former while it is withdrawn.

The former can also be reinserted into a partially concreted pile to create an enlarged base.

This is a key part of the Penpile system, especially for work in granular soils. Enlarged bases can be created by using a stiff, low slump (60-75mm) concrete combined with high pumping pressures.

Only a modest enlargement of the base diameter is needed to increase bearing capacity.

Pennine installed an average of 30 piles a day through the ground, which was up to 3m of made ground then 5m of dense Terrace Gravels underlain by very stiff London Clay.

The rig used was a Pennine Strata Max 12-30, designed and built at Pennine's Lancashire base especially for this type of work. Pennine claims it is the first piling machine in the UK that can drive augers and take a vibroflot.

Weighing 45t, it is based on a Samsung excavator chassis driven by a 225kW (300HP) Caterpillar engine. It can install bottom- or top-feed stone and concrete columns to a depth of 12m with a pull-down force of up to 30t. In displacement auger configuration, its 15t/m torque can drive piles from 250mm to 400mm in diameter, up to 14m long.

The machine has a Pennine Data Logger that displays and records depth and inclination of the flot, rig pressure and weight of stone in the hopper for bottom feed. It is the first in what could be a lucrative market for Pennine.

'There are plans to expand the fleet, ' says Mimms. 'We're looking at a rig that will install bigger piles and go deeper, maybe to around 22m, and a smaller rig for restricted access work.' While the Borough site was not contaminated, Pennine believes the Penpile system could help projects reduce contaminated soil disposal costs by reducing the amount of spoil produced.

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