A Soilmec CM48 rig is doing most of the piling work and is part of contractor Van Elle's fleet that has recently undergone an upgrade to install the NDT-manufactured PL3000 on-board monitoring system for CFA piling.
"The old system helped with managing a project because it kept a record of what work had been done," says Van Elle managing director Andy Sneddon. "But we wanted to upgrade to a system that didn't just allow the rig to talk to our server, but also allowed the server to talk to the rig."
The new system means that project information can be sent directly to the rig so its operator can make sure parameters (particularly depths and concrete requirements) for each pile are accurate. Sneddon says revisions to pile schedules previously meant work would stop while he or a colleague made a visit to site. Now information is received and sent directly to and from the rig, and operators know immediately if any changes have been made.
The 170m by 220m site is bordered by a railway line on one side and a road opposite. The ground conditions – up to 2m of made ground overlies chalk – require 1500 CFA piles with a 300mm diameter to support the 150 units of three and four-storey apartment and town house buildings.
The piles are designed to a safety factor of three, which removed the need to do any pile load testing – a decision developer Bellway took to save time. The rig is installing these piles at depths of between 16m and 19m at a rate of 25 per day.
Sneddon says the £300,000 piling subcontract is typical of the kind of work the company has found itself doing recently. Immediately prior to starting work on the Bellway site in mid-August, it completed work on a similar brownfield housing development about 13km down the road in Kings Langley.
That site was over a 150m by 200m area and a Soilmec R312 (also fitted with the upgraded monitoring computer) again installed CFA piles, this time with a 350mm diameter down to a maximum depth of 19.5m.
Client Nicholas King Homes planned the piling contract at Kings Langley to last eight weeks using two rigs. Van Elle quoted the programme on the same schedule, but using just one of the upgraded rigs. However, with more efficient project times, Van Elle managed to complete the £250,000 contract in six weeks.
Sneddon says that CFA piling is becoming an ever more popular choice on brownfield developments because, as with the Bellway development, driven piles are ruled out due to the close proximity of neighbouring buildings.
The process involves drilling with a long auger down to the pile depth. At this point the auger is withdrawn enough to begin concreting. The pile is formed by extracting the auger and replacing the void with concrete pumped through its hollow stem. Once this is complete, spoil is removed from the top of the pile and a reinforcing cage comprising four 7m long T16 bars (16mm in diameter) is installed by hand into the concrete.
With the railway line along one edge of the site, Sneddon says it was fortunate that the ground in that area was deemed strong enough to cope with the new development without needing piling. If it had been necessary, a different rig would have had to come on site to comply with Network Rail restrictions on mast heights allowed close to railway lines, which would have added to the project's cost.
Van Elle is due to complete work on the Bellway site by the middle of this month when ground works subcontractor Keane will follow on with finishing pile caps and ground beams ready for the new buildings.