In this paper we describe two case studies where a combination of geophysics and ‘conventional’ geotechnical techniques was used to characterise the sites for risk associated with voids in the near surface soils that could potentially collapse. One is a former mine site where the collapse of mineshaft backfill has occurred. Before intrusive investigations were carried out desk studies highlighted the possibility of other shafts on the site and voids below demolition rubble. In order to assess the potential risk from these an initial Ground Penetrating Radar (GPR) survey was carried out and possible voids identified. This enabled targeted ground investigation and safe siting of plant and workers. The second case is an electricity substation where ‘tomos’, (shallow and potentially large voids created by piping erosion in pumice rich sands) were suspected. To improve the likelihood of striking a tomo in a borehole, reduce the risk of provoking an unexpected collapse and of striking buried services, a GPR survey was first carried out. In both cases further intrusive investigation is necessary to locate voids but this has potential for collapse, threatening the health and safety of site users and those carrying out investigations. By firstly characterising these sites through desk studies and GPR, we could carry out the future works more safely and optimise our intrusive investigations.
This paper was originally presented at the Geotechnical and Geophysical Site Characterisation 5 conference and is reproduced here with permission of the Australian Geomechanics Society. For additional papers from this conference see http://australiangeomechanics.org/public-resources/downloads/#dlISC5 Proceedings