Restricted headroom and poor ground conditions on a heritage foundation project proved to be a major headache for the piling team at historic Chatham Dockyard. Dave Parker reports.
Pile design is usually dictated by the loads and location the structural engineer specifies and the ground conditions on the site. Not so at Chatham Dockyard, where main contractor Galliford Midlands is engaged in restoring the Grade II listed Boilershop and constructing an accompanying retail factory outlet. Here, the headroom within the cast and wrought iron skeleton of the 1845 structure dictates both pile type and maximum permissible load, as piling subcontractor May Gurney engineering manager John Chick explains.
'In the centre of the Boilershop there is 16.9m headroom, but this drops to only 5m under the mezzanines at each side.
Basically, the limit is the power of the rigs we can get into each location.'
Erected first in Woolwich as weather protection for a naval slipway, and moved to Chatham in 1876, the Boilershop has an elegant and relatively lightweight cast and wrought iron structure - so lightweight by early Victorian standards, that Woolwich Dockyard workmen are reputed to have refused to work in it when it first opened. Its future is to form the reclad centrepiece of the £21M development, with new construction outside, new retail units inside and a tented structure at one end.
But no extra loads can be put into the original structure - hence the need for additional piling.
From a foundations point of view, the Boilershop could hardly have been moved to a worse location - directly above a buried river channel, with up to 18m of made ground and soft silty alluviums overlying dense sand and gravel and weathered chalk.
Worst of all, the eastern end of the 100m by 30m building sits above the steeply sloping bank of the buried channel, where the sands and gravels drop suddenly from only 3m below ground level to 18m below. The Victorian engineers tackled this problem by founding each of the structure's 100 columns on concrete pilecaps sitting on four timber piles each. These could not be disturbed, which pointed towards a bored pile solution.
The final factor was the site's long history as a naval base.
'An enormous amount of concrete has been added to the ground over the years, including long abandoned air raid shelters, ' explains Galliford project manager Guy Shepherd. 'And there is possible asbestos insulation to underground ducts, and high levels of copper in the made ground, so any solution which minimised arisings would be preferable.'
With driven piles ruled out and founding the piles in the gravels the most economic option, May Gurney would have preferred to use displacement auger piling across the whole site, had the option been available.
Outside the Boilershop, there were no restrictions, so a high powered rig was able to install 800, 350mm diameter piles at 1,050mm centres up to 20m deep at a rate of more than 20 a day.
But the rig was 23m high, far too tall to sit inside the iron frame, so a different solution was needed.
'The only rig we could fit in was only about one third as powerful as the outside rig, ' reports Chick. 'This meant it couldn't penetrate as far into the dense sand and gravels, so we had to limit maximum load to 450kN, as opposed to 600kN outside.'
Under the mezzanines there was no room for a rig big enough and heavy enough to install any form of displacement piles. Continuous flight auger (CFA) piles were the only option, with the piles taken down up to 22m into the chalk. Even then, the 40t crawler rig used had to be specially adapted with a cut down mast. And there was a further complication.
As piling progressed from east to west and the gravel suddenly dipped downwards, it thinned dramatically. 'So we had to punch through and form displacement auger piles in the chalk, ' Chick explains.
'We have to do a lot of probing to monitor changes in ground conditions as piling progresses - we really need to know the exact depth to chalk and the depth to gravel at each pile position before we can proceed.'
Against all expectations, the probing also failed to find many obstructions. Four weeks into the £350,000 contract, May Gurney is on track to finish on time in the third week of January, by which time, it should have installed 1,000 displacement auger piles and a further 50 CFA piles. May Gurney's inhouse design team will calculate the load capacity of each pile and pass the data on to Guildfordbased structural engineer Mellis & Partners.
This will be used to modify the design of individual retail units whose supporting columns will eventually sit on the final concrete pilecaps.
Shepherd says he is 'very pleased' with the displacement auger piling system employed on his project, particularly the reduction in arisings.
Chick says: 'This is one of those once a year projects where you have to monitor every single pile design individually.'