FOUNDATION CONTRACTOR Expanded Piling has introduced a laser-based method of verifying the quality of underreamed piles which ensures consistent depth irrespective of whether single or multiple kelly bars are used to form the pile.
Remould has been the 'scourge of the underreaming industry', says Expanded Piling managing director Graham Bell.
He blames a lack of understanding rather than the ground conditions, saying remould is a product of poor workmanship, inexperience, poor tooling or a failure to maintain a consistent pile depth.
Instead of looking for ways of improving underreaming, the industry has tried to develop equipment to measure the extent of remould, 'Bell says.
'We have never accepted this as a satisfactory way forward and have gone to great lengths to come up with a positive solution to the problem.'
The firm devised a way of ensuring consistent depth to verify the quality of the underream. Previously, the only accurate way of verifying the underream quality was by using CCTV or by manned descent, says Bell.
'But CCTV pictures were not always acceptable or convincing and manned descent has been outlawed by certain organisations for some time.'
The laser is accurate to 1mm and can measure up to 30m depth. After every full cut by the underreaming tool, the laser, fixed to a surveyed-in point, beams down to the top of the tool.
The piling rig is then adjusted to match the tool's depth with the original position. This process is repeated throughout underreaming with final diameter verified using a mechanical indicator which is checked when the tool is withdrawn.
After tests at the firm's Grimsby depot, the system was recently used to construct a 750mm diameter pile with a 1800mm diameter underream on a site in London. CCTV inspection was still required by the contract and showed the trial pile to be 'perfectly formed', says Bell.
The pile was then load tested, with a deflection of less than 5mm recorded at the design verification load of 370t. Even at 1,000t, the pile only deflected a total of 55mm, indicating that its ultimate capacity had not been reached.
'The results demonstrate that remould could not be present in the underream, ' says Bell.
Bell believes this new tool will help convince doubters of the underreaming technique, but admits lasers alone are not an answer to all of the problems.
'Years of experience and a clear understanding of the product are very important if remould is to be avoided, ' he says.