Void Detection Under Floor Slabs: How GPR Checks Floors

Why does void detection under floor slabs matter in a factory?

Because a factory floor carries point loads that a house floor never sees: forklift wheels, racking legs, press foundations and stacked raw material. A hidden gap under any of them can turn into a sudden failure under a loaded reach truck.

Early cavity detection also saves money over the long term. Grouting a mapped cavity costs a fraction of recasting a bay, and the plant keeps running during the repair. Before construction projects such as a new machine line, a scan under the footprint tells the civil engineering team whether the slab can be trusted.

What causes voids under factory floors?

Most voids come from water carrying soil away, from fill that was never compacted, or from soil settling under heavy, vibrating loads. The slab bridges the gap for a while, so the problem stays hidden until a crack opens or a corner drops.

• Leaking drains and water lines under the slab, which can wash fine sand from the subgrade for years before anyone notices.
• Vibration from presses and gensets, which settles poorly compacted fill under their foundations.
• Collapsed services, such as an abandoned clay pipe or a disused cable trench that has caved in.

How do you know a floor might have a void?

The floor gives warnings: hairline cracks that keep widening, a hollow sound when tapped, joints that rock under a forklift, or water at a crack after rain. Two or three of these together are a good reason to scan before the next heavy load goes on. A hammer tap finds shallow hollows, but it cannot tell a thin air gap from a deep cavity.

How does GPR find a void under a slab?

Ground penetrating radar is a non-destructive method that fires brief electromagnetic pulses into the floor and listens for what comes back from each change in material. Air and water differ sharply from concrete and soil, so the pulse bounces strongly at a gap, and a thick cavity often repeats the echo below the first one.

Slab scanning runs along parallel lines, 0.25 to 0.5 metres apart, then repeats at right angles. Depth slices then show each void as a patch on a plan view, and reflection polarity separates a dry gap from a washed-out zone. For a broader look at the service, see our void detection details.

Which parts of a plant should be scanned first?

Start where water, weight and vibration meet: drain runs, loading docks, heavy machine bases and the aisles used by the heaviest vehicles. Old extension joints, where a new shed meets an older slab on different fill, also deserve an early pass.

What can make a slab scan difficult?

Dense reinforcement, steel fibre concrete, wet or salty fill and metal plates on the floor all reduce what the radar can see below the slab. We scan diagonally across rebar mesh with a higher frequency antenna. Surface features such as steel plates and trench covers block the scan, so the crew records each gap on the plan.

Slab void survey at a glance

The table shows a typical factory floor survey, from the first walk-round to the final marked plan.

Coring through a floor is excavation, however small, so check for drains, cables and gas lines first. Outside the plant fence, read about Call Before You Dig in India for the wider alert system.

How do you scan a running plant without stopping production?

By splitting the floor into zones, scanning aisles and open bays during the shift, and taking machine areas during a weekly off, a planned shutdown or a night slot. The scan is quiet, so operators carry on working without service disruptions.

Before mobilising, we ask for a layout with column grid lines, existing records of drains and the areas where cracks have been seen. If the plant handles chemicals or has classified areas, the safety rules change; for those sites, see getting a SUE survey for refinery done right.

What should you do once a void is found?

Confirm it with a core, find and fix the cause, fill the gap, and only then repair the floor surface. Filling a void without stopping the leak that made it moves the problem a few metres along.

1. Confirm: drill a small core at one mark to measure the slab thickness and the gap.
2. Find the cause: trace and pressure test the drains and water lines nearby.
3. Fill: a structural engineer chooses cement grout or polyurethane foam and the injection pressure.
4. Re-scan: a second GPR pass shows whether the gap has been filled.

Many plants that find one void also realise they hold no utility records for the underground utilities below the shed. A combined slab and utility scan costs less than two separate jobs and helps avoid accidental utility strikes later. To understand what you receive from that work, read: GPR survey deliverables.

What does the scan report contain?

A floor plan tied to your column grid, with each void drawn as an outline, its approximate depth, a confidence note and a recommended action. Radargram snapshots and photos of the painted marks come with it, together with a list of any areas the radar could not reach.

What does a factory floor scan cost?

Floor scans are priced on scan-line length at our standard GPR rate of ₹24 per metre, GST extra. The minimum order is ₹50,000 in Delhi NCR and ₹1,00,000 elsewhere in India. A tight grid over a suspect bay uses more line metres than a wide grid over a sound floor, so the engineer agrees the grid with you before quoting. Send us the floor layout and photos of the cracks, and we will check rates for your site and reply with a written quote.

Frequently asked questions

How deep under a floor can GPR see a void?

For most factory slabs, the slab base and the first half-metre of fill are clear. Depth falls in wet or clay-rich fill and under dense steel.

Can GPR measure the size of a void?

It shows the outline on plan well and gives an indication of thickness. A core at one point confirms the actual gap.

Do you need to stop machines during the scan?

No. We scan around running equipment and take tight areas during a break or shutdown. GPR is quiet and does not affect machines.

Will the scan damage an epoxy floor?

No. The antenna slides on the surface. Marks are chalk or water-based paint.

Can the same visit map pipes and cables under the floor?

Yes, and it is worth doing, because mapped services reduce risks for any later trench or core. For how radar responds to pipes and cables, see GPR scanning at a glance.

Is GPR better than tapping the floor?

Tapping finds shallow hollows quickly but cannot show size or depth. GPR maps the whole floor and gives a plan you can repair from.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026