On the Cornish south coast on the Lizard peninsula, contractor Bam Nuttall is putting its latest piling rig through its paces.
It is a hammer-driven device to set to slash the time needed to set piles under water. The machine is being used to help the Royal National Lifeboat Institution replace an existing lifeboat station so the site can accommodate new Tamar class lifeboats, which are much larger than those currently in use.
The Lizard lifeboat station is located on a “tight cove on the Lizard peninsula”, says Bam Nuttall framework manager Richard Hayman.
“It will be constructed on existing foundations. The piling is for the slipway, with 27 piles in total. Once in, we will erect concrete beams spanning the piles and then install the steel-framed slipway,” says Hayman.
The new £7.5M Lizard station is being constructed by Bam Nuttall, using its new down-the-hole hammer technique. Consultant Royal Haskoning project manager Jonathan Kinkaid says the rock at this spot is “a highly fractured schist. It is hard, but with large zones of fracture and produces pebbles,” he says.
“We are using 200m³/minute of air to drive the hammer and flush rock, sand and water”
Pile depths of 10m are required, “to get a good 5m, because of the indeterminate levels of fracture,” he says.
Piling subcontractor Bam Ritchies geotechnical manager Chris Wade says there are really only two ways to pile into underwater rock - the hammered solution, or a grinding solution. “Compared to grinding methods, we think this takes around one-quarter the time to drive each pile,” he explains.
Where time is always money this is an obvious cost saving, especially at this site, where the rig, grouting equipment and crane are perched on a 30m by 18m jack-up barge.
According to Wade, the hammer technique also gives more control over tolerances.
Bam Ritchies is using a 43t rig to drive the hole, flushing the debris and seawater away with compressed air. Housed in a steel casing, the hammer pounds the undersea rock. “We are using 200m³/minute of air to drive the hammer and flush rock, sand and water,” says Wade.
The 27 piles are set in a single row of three piles closest to land followed by 12 pairs moving out to sea. Setting each pile takes a full 12 hour day.
The 864mm diameter hammer is lowered through the casing and begins to operate on contact with the rock. The first task is to make a seal with the bedrock. The hammer is left to run for around 30 seconds and is then flushed with compressed air. “This continues until the seal is made,” says Wade. “Here the rock is such that we can drive anything up to 3m before a seal is made.”
Once the seal is confirmed, the hammer is removed and a casing plug attached to the top of the casing, which is then rotated, reaming it tightly into the new hole.
The hammer can then be replaced, and drilling continues at a significantly increased rate. Once the required depth is reached, 762mm diameter, 20.6mm thick circular hollow sections are then positioned in the shaft. This is then braced with aggregate to steady it. The pile is then grouted from the bottom up, filling voids around the aggregate. “Doing this we make instant concrete,” says Wade.
“Because the hammer-driven pile shaft is far from smooth, the grout enters fault lines to leave a skin friction pile,” he adds.
The piling programme is expected to be complete in November, with the whole project due for completion in September 2011.