Such was the importance of the piling aspect of the job that traditional pile testing methods seemed a little too rudimentary.
Monitoring the effects of a large vertical load applied to the head of the test pile would not have given enough detailed information this time due to the complex founding conditions of the layers of silty sands.
The remedy came in the form of the Double Osterberg Cell test, which was initially developed in the US. This technique involves embedding two rings of fully instrumented hydraulic jacks in the pile at around mid depth and close to the base. These full diameter cells were able to apply tensile and compressive forces to the pile and provide the information sought by the project team.
'The test is a very intelligent idea which yields extremely detailed results, but immense care is needed while constructing the piles' says Halcrow project manager, David Mizon. Testing dummy piles this way is expensive as the high-tech cells are lost forever and specialist contractors need to be drafted in to perform the tests. And Swift explains that pouring the concrete through the small hole in the centre of the cell needs special consideration and extra effort. 'But on a job where the performance of the piles is so crucial having confidence in the quality of our pile designs is very reassuring.'