Underground Void Detection

A void detection survey using Ground Penetrating Radar (GPR) is a low risk method to scan an area of ground and determine the location and extents of underground voids, sinkholes and subsidence. This can form part of a risk reduction process to check the integrity of the ground before moving or using heavy equipment and cranes, or to determine the extents of potential underground voids and subsidence to aid in remediation planning.


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Causes of underground voids

Shallow voiding can be caused by several reasons including:

  • Sinkholes (the causes of which can be geological or man-made such as broken water pipes, old sewers, buried basements, and disused mine shafts which have been compromised).
  • Material washout due to underground water movement which carries some material with it (typical in coastal promenades and car parks, masonry arch structures and docks & retaining walls).
  • Natural subsidence in which the ground itself is moving and settling over time.

Subsidence will be visible from the surface in the form of dipping ground in soft layers and cracks in harder materials. However, in cases when the surface is firm and supports its own weight the material beneath can subside until a significant void has developed underneath the surface layer with no visible warning signs. The surface may then break when heavy loads are applied or, if the void reaches a size in which it can no longer support its own weight, it will collapse on its own.

Voiding can usually be remedied by first exposing and fixing the source (if that is possible), and then pumping material into the void until it is filled. A GPR void detection survey is be the ideal method to determine the extents of the problem.

Why use GPR for underground void detection surveys?

GPR is a fully non-destructive and non-intrusive technology (NDT), it is relatively light weight equipment which can be used to scan areas of any size and it does not cause any damage to the surface being surveyed. A void detection survey using GPR is a cost-effective way to scan large areas safely.

Detecting sinkholes, underground voids and subsidence using GPR

Ground Penetrating Radar works by transmitting an electromagnetic signal into the ground and recording reflections. In GPR, reflections are caused by any change in the below surface environment. GPR is therefor able to detect most types of underground features including sinkholes, voids and subsidence (provided the voids are large enough to be detected within the resolution and penetration capabilities of the GPR being used).

How a GPR survey to detect sinkholes, voids and subsidence is performed

How a GPR survey to detect sinkholes, voids and subsidence is performed

A GPR survey to detect underground voids is usually performed using handheld equipment which is mounted on a small trolley. Parallel lines of GPR data are collected in two axis to form an orthogonal grid. Each line represents a vertical cross section through the ground at that specific location, these lines of data are saved for office-based processing and interpretation.

In the office the data is loaded into our advanced GPR post processing and imaging software, by compiling the vertical cross sections together a 3D picture is formed. Using this 3D image underground features such as voids, sinkholes and subsidence can be detected and tracked from scan to scan to produce a CAD drawing.

Examples of GPR void, subsidence and sinkhole detection surveys

Examples of GPR void, subsidence and sinkhole detection surveys

Some examples of void detection surveys include:

This GPR survey to detect underground sinkholes in Manchester. In this case large voids and areas of subsidence were present beneath a surface layer of reinforced concrete, they were caused by a sinkhole 4m deep. We were able to successfully detect and map them in our post processed GPR survey even though they were not clearly visible in the data on site.

We also have experience detecting shallow voids and sinkholes in concrete and road structures. As well as badger tunnels and chambers, and man-made voids such as underground tanks, basements, culverts, mine shafts, and gas pipes etc.

In this example, KB GPR Surveys was asked to survey the floor in a church and detect the location, depth and extents of an underground void which was identified as a disused crypt.

In these two examples, retaining walls were surveyed to detect potential voids caused by material washout. In one case voids were detected – in the other, the retaining wall was confirmed to be sound.

Sinkhole, underground void detection and subsidence survey deliverables

The results from a GPR survey of underground voids and subsidence will usually take the form of CAD drawings and/or a report. The final output may vary depending on the best way to represent the information.

If you require a void detection survey or a survey to detect sink holes and subsidence, please contact KB GPR Surveys for a quotation at info@kbgprsurveys.co.uk

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