Comparison of imaging technologies for geotechnical investigation of abandoned coal mine workings: a case study

Terrestrial laser scanners have revolutionised the mapping of rock exposures, in both above and below ground environments, and they are an ideal tool to explore old abandoned underground spaces where records of mining activity are unreliable, contradictory or simply non-existent. This paper describes the deployment of a laser scanner in holes drilled into abandoned coal mine workings, to obtain data to create a model of voids and caverns left behind. From the combination of scans taken from three boreholes, with underground line-of-sight connectivity between pairs of boreholes, a three dimensional model of the workings was produced that was sufficiently detailed to make a reliable assessment of the extent of extraction, and the viability of remediation by backfilling. The model produced contained enough detail to not only give a reliable estimate of the volume of backfill that would be needed, but also information on the incremental quantities required for a progressive backfill. The laser scanner was demonstrated to give superior outcomes to a simple horizontal laser measurement tool, however, downhole photography was found to be a beneficial complement to the data from the scanner. In the case study presented, the laser scanner was able to achieve outcomes that a direct inspection of the workings could not.