Combining pile testing to prove project
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In the UK capital, waste from many London boroughs is routinely transported by barge down the Thames to landfill in Essex. However, a waste-to-energy power station is being constructed at Belvedere on Halfway Reach in east London to reduce landfill volumes and supplement national power supply.
Consultant Mott MacDonald was appointed by Costain Ltd to undertake detailed design of the marine works associated with the power station, for which it developed and specified a novel partially sleeved onshore pile test to prove the capacity of offshore piles.
The process involved an innovative use of strain gauges fixed to the outside of the pile to enable interpretation of mobilised skin friction in each of the soil layers. The gauges were protected within a steel shroud, with the majority of them remaining intact during driving. Gauge data recorded during the pile test enabled the design to be optimised, adopting lower than normal factors of safety, reducing pile length and offering substantial cost savings compared with more conventional pile test methods.
"Sleeved on-shore pile tests have been used before, as have instrumented pile tests, but we feel that the combination on this project was novel," says Mott MacDonald's project manager Peter Mallin. "In fact, additional cost savings were obtained as we were able to reduce the required number of static pile tests to one onshore from two offshore."
In addition, alternative torsion arm type fenders were proposed by the fender supplier for the relatively low energy requirements of the waste barges. This design reduced loads on to the structure compared with a more conventional arrangement, providing cost savings on the fender face panels.
Asked about the success of the project, Mott MacDonald's project director Richard Stephens noted there was a direct saving in time and cost by carrying out a single on shore pile test in lieu of multiple offshore tests. Indeed, the instrumentation enabled the pile design to be optimised with savings in required pile embedment lengths.
He adds that the cost saving of the on-shore pile test was significant and that Mott MacDonald would expect to adopt this strategy for port infrastructure projects with consistent ground conditions and the structures close to the existing shore line.
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