A school site in the Midlands was in an area of known mine workings. Mineshafts were known to be located nearby but the exact layout of the mine workings associated with these shafts was not known. The client wanted confirmation that there were no sub-surface voids that might impact on a proposed school extension.
Rather than rely only on an intrusive investigation strategy the client wanted to carry out a geophysical survey followed by targeted boreholes. This approach provides more reliable results than just carrying out a borehole survey as the geophysical survey covers the entire site. Boreholes are then targeted to investigate anomalies identified by the geophysical survey.
The best geophysical technique for locating mine workings is usually microgravity. This technique measures variations in sub-surface density and is very reliable but time-consuming. The survey area at this site was only 0.15 ha but this still took 3 days on site to survey.
The survey was able to map the density variations across the entire survey area and a broad, linear area of low density was detected crossing the site. It could not be determined from the available evidence if this was caused by a geological change / feature or whether it was associated with deeper mine workings. However, by targeting one borehole over this low density anomaly and another in the surrounding higher density material it will be possible to determine the cause of the anomaly; the extents of which have been shown by the geophysical survey.
Even with the cost of targeted boreholes the use of geophysics was still more cost-effective than peppering the site with boreholes and was far less disruptive to the school.