Downhole Magnetic Gradiometry for Estimating the Length of Existing Piles-PPT Presentation

PPT Presentation

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The presentation examines downhole magnetic gradiometry (DMG) as a method for estimating the length of existing bored piles, particularly when the pile head is inaccessible. A geophysical tool is lowered into a PVC-cased borehole drilled near the pile to detect the base of its steel reinforcement cage. The method assumes the reinforcement extends to the pile toe, so reinforcement curtailment can lead to an inaccurate estimate.<br /><br />A November 2024 field trial tested up to five 3 m-long N24 reinforcing bars in an ungrouted model pile. Boreholes were positioned 0.5 m, 2 m and 3 m from the pile. Tests showed that individual bars produced complex magnetic fields; using the commonly applied “lowest peak” interpretation, the estimated bar depth averaged 2.74 m against an actual length of 3 m. Tests with multiple bars showed that bar arrangement and orientation can substantially affect the signal, sometimes making the estimated toe unclear or incorrect.<br /><br />The trial suggests that, even under favourable conditions, DMG is unlikely to be useful beyond an offset of about 2 m; in typical practice, the borehole should be within 1–1.5 m of the pile. The lowest-peak interpretation was the most consistent approach, though it tended to be slightly conservative. Because reinforcing bars generate complex magnetic fields that combine, some configurations yield inconclusive results. DMG can therefore support pile-length investigations, but its estimates should be interpreted with awareness of these limitations and the assumption that reinforcement reaches the pile base.

Keywords

downhole magnetic gradiometry

DMG

bored piles

pile length estimation

reinforcement cage

steel reinforcement

borehole testing

magnetic fields

field trial

pile toe