Downhole Magnetic Gradiometry for Estimating the Length of Existing Piles-Paper

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The paper evaluates downhole magnetic gradiometry (DMG) for estimating the depth of existing pile foundations, particularly when pile heads are inaccessible and construction records are unavailable. Reusing foundations can reduce construction costs, time, and embodied carbon, but reliable pile-length estimates are needed to assess their capacity.<br /><br />Field trials used five 3 m-long N24 reinforcing bars as a model pile. A magnetic gradiometer was lowered into cased boreholes positioned 0.5 m, 2 m, and 3 m from the model pile. Researchers tested individual bars and different combinations and orientations of bars, producing 15 data sets.<br /><br />The most commonly used interpretation—the lowermost peak in the magnetic gradient—generally identified the model pile toe, but slightly underestimated its actual 3 m length. For individual bars at a 0.5 m offset, interpreted depths averaged 2.76 m. The combined-bar tests indicated a toe near 2.7 m at 0.5 m offset. Signal strength declined substantially with distance: results at 2 m were weak, while those at 3 m were inconclusive. Bar orientation and magnetic responses varied, and some arrangements did not produce a clear toe signal.<br /><br />The study concludes that the lowermost-peak method is a useful, often conservative estimate, but DMG results are not always unique or reliable from a single test. Boreholes should be placed as close to the pile as safely practicable—ideally around 1 m, and generally no more than 1.5 m under realistic conditions; offsets up to 2 m may work in favourable circumstances. DMG also depends on reinforcement extending to the pile base, so it may indicate only the minimum pile depth where bars are curtailed. The authors recommend testing multiple piles and corroborating results with records or other methods, such as sonic echo or parallel seismic testing.

Keywords

downhole magnetic gradiometry

pile foundation depth

existing pile assessment

magnetic gradient

reinforcing bars

pile toe detection

foundation reuse

borehole offset

sonic echo testing

parallel seismic testing