Void Location Surveys

A void doesn’t always mean a hole you can see. Within a structure or beneath a slab, a void can be a hidden gap between materials, a pocket left by settlement, or space where fill or grout has washed away over time, none of which is visible until the structure above starts to show signs of stress.

Structural Voids vs Ground Voids

Voids generally fall into two categories, and knowing which you’re dealing with affects how a survey is approached. Structural voids occur within a built element itself, such as a gap within a concrete slab, wall or column, often caused by poor compaction during construction or material degradation over time. Ground voids occur beneath a structure, in the soil or fill material supporting it, typically caused by erosion, water ingress or settlement. Our surveyors identify which type of void is present and adjust interpretation accordingly, since the cause, and the appropriate remedial response, differs between the two.

How We Locate and Measure Voids

Our surveyors use Ground Penetrating Radar to scan the structure or ground on a systematic grid, identifying the presence of a void and mapping its approximate size and extent. This gives more than a simple yes or no on whether a void exists, it tells you how significant it is, which matters when deciding whether a void needs immediate remedial action or can be monitored over time.

Why Void Location Matters

An undetected void can lead to structural failure under load, whether that’s a road surface collapsing under traffic, a slab cracking under racking, or a wall losing its structural capacity over time. Where voiding is suspected within a retaining wall specifically, our dedicated Retaining Wall Surveys combine void detection with a wider structural assessment tailored to that type of structure. Identifying voids early allows targeted repair, such as grout injection or localised reinforcement, rather than reactive repair once a failure has already occurred.

Subvision Surveys is SSIP and ISO accredited, and our team carries out void location surveys for engineers, contractors and building owners across London, the South East and nationwide.

Contact Subvision Surveys today to discuss your void location survey requirements, or speak to one of our experts on 0333 920 942.

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CABLE DETECTION ESSEX FAQ’S

Structural voids occur within a built element itself, such as a gap within a concrete slab, wall or column, often caused by poor compaction during construction or material degradation over time. Ground voids occur beneath a structure, in the soil or fill material supporting it, typically caused by erosion, water ingress or settlement. Both can ultimately affect the stability of a structure, but the cause and the appropriate remedial response differ between the two. Our surveyors identify which type of void is present, since accurately diagnosing the cause is the first step towards planning the right repair.

Our surveyors use Ground Penetrating Radar to scan the structure or surrounding ground on a systematic grid, sending radar signals in and recording how they reflect back. A void produces a distinctive signal response compared to solid, consistent material, allowing our team to identify its presence, approximate location and extent. This process is entirely non-intrusive, requiring no drilling or excavation, meaning a survey can be carried out without disturbing the structure or ground being investigated, giving you accurate results without adding disruption or cost to the process.

Yes. A GPR survey doesn’t just confirm whether a void exists, it also maps its approximate extent across the surveyed area. This distinction matters because a small, isolated void might only need monitoring, while a larger or more extensive void could represent a genuine structural risk requiring immediate repair. By scanning on a regular grid rather than taking isolated spot checks, we can show how a void’s extent changes across a structure, giving a much clearer basis for deciding what level of remedial action, if any, is actually needed.

Voids can form for a range of reasons depending on whether they’re structural or ground-based. Structural voids often result from poor compaction or workmanship during original construction, or from material breaking down over time. Ground voids are more commonly caused by water washing out fine material, erosion, poorly compacted fill settling under load, or natural ground movement. In both cases, voids tend to develop gradually and aren’t usually visible from the surface until the structure above begins to show signs of stress, such as cracking or uneven settlement.

An undetected void can lead to structural failure under load, whether that’s a road surface collapsing under traffic, a slab cracking under racking, or a wall losing its structural capacity over time. Voids tend to worsen gradually rather than appear suddenly, which means early detection gives you the opportunity to plan targeted remedial work, such as grout injection or localised reinforcement, before the problem escalates into a more serious and costly structural failure requiring extensive repair or replacement.

You’ll receive a report and CAD drawing showing the location and approximate extent of any voids identified, whether within a structural element or beneath it. This gives engineers and building owners a clear, evidence-based reference for deciding on next steps, whether that’s ongoing monitoring, targeted repair, or further investigation. Rather than working from visible surface symptoms alone, such as cracking or settlement, you’ll have accurate data showing what’s actually happening beneath the surface that’s causing those symptoms to appear.

Yes. Subvision Surveys often combines void location surveys with slab thickness measurement and wider GPR services, giving engineers and building owners a fuller structural picture from a single site visit. This is particularly useful where reduced slab thickness and voiding are linked, since one can be a contributing factor to the other. Because we carry out all these services in-house, this doesn’t require coordinating between separate providers, our own team applies whichever combination of surveys best suits the structure and concerns you’re investigating.

Have Any Questions?

If you have any questions regarding our services feel free to contact the Subvision team.