Our non-intrusive GPR void detection surveys can detect voids beneath slabs. Scanning on a regular orthogonal grid allows the void location and extent to be determined and plotted.

Sub-surface voiding

Voiding can occur for a number of reasons. Natural voiding can be caused by settlement, washout (e.g. due to a leaking water pipe), and sinkholes. Voids can also be man-made: basements, culverts, underground tanks, tunnels, pipes, chimney flues, etc.

Voiding in the sub-surface can cause failures of the overlying strata, particularly if the area is subjected to abnormally heavy loads. The collapse of a void under a carriageway is likely to cause structural damage and can result in accidents. Voids beneath a warehouse slab or loading bay can result in slab deterioration and, ultimately, a failure, particularly if trafficked by forklifts or lorries. Therefore, the detection and location of voids are highly desirable and enable repairs to be undertaken before structural problems occur, improving safety and saving money.

Void detection using GPR

Shallow voids can be detected using Ground Penetrating Radar (GPR). Scanning on a regular orthogonal grid allows the location and extent of voids to be determined and plotted. The survey is nonintrusive, cost-effective and can cover large areas quickly.

GPR can be used with antennas with different central frequencies. A higher frequency provides better resolution, while a lower frequency will provide greater penetration depth, albeit at a lower resolution. Voids at greater depths, therefore, need to be larger in order to be detected by GPR.

Because GPR detects changes within the subsurface, it will detect not only open voids but also infilled voids, provided the infill material has sufficiently different properties from the surrounding material.

Detection of voids below concrete slabs

Voids under concrete slabs eventually lead to cracking and potential slab failure. Therefore, it is desirable to detect any voids as early as possible to minimise risk to users and expensive repairs.

Settlement due to poorly compacted fill, erosion, burrowing rodents, decaying organic material and even a loss of moisture (e.g. due to tree roots) can result in voids.

By scanning the concrete slab on a regular grid, GPR can detect voids below the slab and determine their extent. Appropriate remedial measures can then be undertaken.

Detection of voids in stone walls

Stone walls are generally very durable and perform well. Some stone walls, notably old churches, are built with two skins of stonework and a rubble core infill. Over time, voids can develop within the walls’ core, resulting in water’s conduction through the wall.

Determining the wall core’s condition and the voids’ position and depth is difficult. The traditional approach is to drill into the wall and “feel” for the voids. When a void is encountered, the drill penetrates deeper with little resistance. By plotting the results on a grid, a picture and extent of the voiding can be built up. This method is very slow, and results can be variable.

GPR offers a much faster and potentially more reliable method of detecting voids. By scanning on a regular orthogonal grid, large areas of a wall can be scanned relatively quickly, and a detailed picture of voiding can be built up. The information can then be used to plan the most effective remedial action; most often, this would involve pumping grout into the wall core and filling any voids.

GPR radargram showing detected voiding below a reinforced concrete slab
GPR radargram showing voiding below reinforced concrete slab; – the yellow dashed line indicates underside of the slab, the red dashed line indicates voiding beneath the slab

Survey deliverables

Void location survey results are typically plotted on CAD drawings showing the location and extent of detected voids.

Need more information?

Contact us

Contact us to find out more about GPR void location surveys and how we can help you:

Tel: 0207 565 7056
Email: gpr@sandberg.co.uk

Information sheet

Download our information sheet about GPR void location below concrete slabs.

Are you wondering what else GPR can be used for? GPR surveys can investigate the subsurface and detect unseen features. Learn more about our most popular GPR surveys.