Geophysics reveals hidden basements

A brownfield site in London was being considered for use as a temporary lorry park but an initial intrusive survey had identified the presence of voids beneath the current ground surface, the majority of which was covered by reinforced concrete. Numerous phases of buildings had previously been present on the site and the voids were thought to be associated with basements; but there were no accurate plans showing the extents or even which buildings may once have had basements. Ideally the client did not want to remove the concrete slabs as these would be needed for the proposed temporary lorry park but he needed to know the extent of the basements that had been identified and also check whether there were any additional ones present in other parts of the site.

Detecting features beneath reinforced concrete is often a problem for non-intrusive surveys as the reinforcement reduces penetration or produces a very strong response which can mask underlying features. Phase SI proposed carrying out a trial survey using ground penetrating radar (GPR) and electromagentics (EM). These techniques can locate voids but both are affected by reinforced concrete. Rather than survey the entire site using techniques that might not be appropriate the trial survey would help determine if either technique was effective at this particular site.

The EM survey provided usable results in areas where there was no reinforcement but where the reinforcement was present (the majority of the site) then the responses from this appeared to mask any underlying variations. This was confirmed when the EM survey did not identify any anomalies suggestive of basements or voids in an area where a known void under the reinforcement was present.

Initial visual inspection of the GPR data on site suggested that this technique was also unable to reliably identify a known void under the reinforcement. However, once the data was fully processed using dedicated software weak but definite responses suggestive of a basement were identified. The GPR survey was extended to cover the entire site and post-processing the data revealed additional ‘basement’ anomalies and other possible sub-surface features.

The results of the geophysical survey meant that the majority of the reinforced concrete slabs could be left in place in areas where no voids were identified, which negated the need for the expensive removal and reinstatement of these concrete slabs. The areas where the basements and voids had been identified could be individually targeted and filled to ensure that these were safe for the proposed land use.