Abstract:
The re-use of existing piled footings in the absence of as-built documentation requires investigation and geotechnical assessment. To assess the geotechnical capacity of existing piles, an understanding of the length (i.e. depth) is necessary. One method of investigating the pile toe depth is via the downhole magnetic gradiometry (DMG) method.
This research has built on the work of prominent authors (Packer et al. 2022 and Carlos et al. 2022) to investigate the magnetic fields of steel N24 reinforcing bars (i.e. a pile cage), on the basis of a specific signal pattern (traditionally assumed to be the ‘ lower most peak’ ). The bars were 3m in length and were placed in an ungrouted 100 mm diameter hole (the ‘ model’ pile). Cased boreholes were installed at 0.5m, 2m and 3m offset from the edge of the model pile. This research presents 15 data sets, involving variations in offset distance and bar orientation.
The findings demonstrated that the lower most gradient peak is an acceptable method of estimating the base of the steel bars, albeit slightly conservative (e.g. 2.7 m vs 3.0 m actual length). The findings also demonstrated a maximum interpretable borehole offset of around 2 m for ‘ best-case’ conditions, although this should be reduced to 1.0 m or 1 5 m for more realistic conditions. The data indicated strong dipole anomalies throughout the length of the bar, thought to be high amplitude remanence left from the steel mill (e.g. high-powered electromagnetic lifter/s). Some combinations of bars led to inconclusive results, typical of many non-unique geophysical problems. The results are generally consistent with the findings of previous authors, although challenge some conventional assumptions.