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Location of voids below slabs


Due to the nature of reinforced concrete slabs voids can form below them and go undetected for years. Until the void is large and the area of unsupported slab is enough for the slab to crack, or even collapse. These voids can be a result of subsidence, ground shrinkage or the subbase being washed away from a water leak.

It is imperative to repair these voids, often with grout injection, before they cause a problem to the slab. However, to be able to repair voids they first have to be located and their extents determined.

Ground penetrating radar is an ideal method for determining the presence, location and extents of voids below slabs. Our surveyors conduct a number of scans on a grid pattern across the whole of the slab. The radar penetrates the slab (upto 500mm) and by analysis of the reflected signal voids can be mapped.

The location and extents of the detected voids are then plotted onto CAD drawings to provide accurate information to the client.


Note that GPR will only be able to provide information on the extents of any voids detected in plan and the depth to the top of the voids. GPR is very unlikely to be able to determine the internal depth of the void.