Ground Penetrating Radar (GPR) can be used to detect the presence and extents of sinkholes, underground voids and subsidence.
During the construction of a new tower block in Manchester (UK), the road collapsed underneath a lorry delivering construction materials for a new block, virtually swallowing the lorry into a sink hole 4m deep and several meters across; fortunately, no one was injured during the incident. The cause was determined to be the prior collapse of a sewer dating from the Victorian era, over time material had been progressively washed into the sewer until the sinkhole grew to reach the surface and then spread underneath the stable reinforced concrete layer.
Due to nature of the reinforced concrete road, there was no surface evidence that the sinkhole had existed and therefor no warning before the lorry fell through the road. The reinforced concrete beneath the road also dated to the Victorian era and the rebars were significantly corroded, reducing its load capacity.
Prior to the commissioning of a crane on the same road, KB GPR Surveys were asked to attend site and perform a survey using our advanced Ground Penetrating Radar to detect evidence of any other potential underground voids or subsidence, which may cause the surface to collapse under load. This would have been potentially catastrophic if it occurred during lifting operations.
Using our custom built UTSI Electronics triple-frequency, multi-antenna GPR, we started to perform a survey of the road. We quickly identified the presence of reinforced concrete covering most of the site, which blocked the signal from the lower frequency antennas completely, but our high frequency 1000MHz channels could penetrate to some extent between the rebars.
We performed a survey of all the accessible surfaces of the road, it was not possible to see anything in the data on-site, nevertheless, we completed the data collection and returned to the office to process and interpret the data to see what we could find.
Using GPR-Slice to post process and interpret the GPR data we were able to identify several significant voids located just below the reinforced concrete layer. This information was immediately provided to the customer and the site was closed until remedial works could be completed.