Systematic field test of non-destructive techniques for driven cast in situ pile lengths

The need for reliable non-destructive techniques (NDTs) to determine pile toe depths is highly relevant to the reuse of piles; this is particularly relevant in congested brownfield urban sites. A systematic full-scale ‘blind’ site trial of four geophysical techniques by five independent specialist contractors was conducted to assess the most appropriate sensing tool/s for the detection of driven cast in situ (DCIS) pile toe depths. There is dearth of published studies detailing direct comparative pile length test programmes. This investigation provided a unique opportunity to evaluate widely used NDTs in a controlled and methodical manner. Magnetometry, ground penetrating radar, parallel seismic (PS) testing and transient dynamic response (TDR) were carried out on four specifically constructed end-bearing 380 mm dia. DCIS piles up to 8.5 m long, with a range of embedment lengths in the founding sand stratum. Boreholes at 500, 800 and 1300 mm offsets from each pile were installed to examine the influence of sensor lateral distance from the pile. These trials yielded variable results, with typical accuracy within ±500 mm. It was considered that TDR (frequency domain) when used in tandem with PS test was the most successful geophysical NDT. However, the results were not comparably repeatable between individual contractors thereby illustrating the influence of test methodology and data interpretation.