Void detection survey

What the survey finds

When you need a void survey

Voids form when water carries fine material away, when old structures collapse or when ground settles. They often give no warning until the surface cracks or drops. A void survey is used:

• before cranes, piling rigs or heavy plant move onto a site
• where a slab has cracked, rocked or settled
• after a leaking drain or water main has been repaired
• where a road, footway or yard surface has dipped
• to plan grouting or other remedial work
• as part of routine checks on retaining walls and quays

What the survey finds

Air-filled and water-filled voids, washed-out sub-base and loose ground all change the radar signal. The survey shows where these changes are and their approximate extent and depth. Results are drawn up so you can target trial holes, cores or grouting where they are needed.

Voids under and inside concrete

Many voids we find are directly beneath ground-bearing slabs, where the sub-base has settled or been washed away by a leaking drain. The slab can look sound while the ground under it has gone. GPR shows these gaps through the slab, so you can grout or repair before the slab cracks. We also scan walls, columns and suspended slabs for honeycombing and air pockets inside the concrete, where the survey shows changes in the material that can then be checked by coring.

How it is done

We scan the area in a grid, using an antenna chosen for the depth of interest. For voids just beneath a slab, a high-frequency antenna gives fine detail. For deeper features, a lower frequency reaches further down. Suspect areas can often be confirmed on the day and marked on the surface. The full results follow as a CAD drawing and report.

Example: voids found in processing

On a site in Manchester, voids under a reinforced concrete slab had already caused a sinkhole about 4 m deep and were still growing. The voids beneath the reinforcement could not be seen clearly in the data on site. They were found when the data was processed in the office, which is why we don't rely on on-site marking alone where the stakes are high.

Limits you should know about

GPR depth is limited in wet clay and saline ground. A small void deep below the surface may not show. Some features, such as a soft spot in the fill, look similar to a void on the radar. We therefore recommend confirming significant findings with a core or trial hole before remedial work.

Why use GPR

• No damage to the slab, road or wall.
• Small surveys usually take less than a day on site.
• Suspect areas can often be flagged on the day.
• It covers the whole area, not just a few probe holes.

We carry out void surveys for contractors, property owners and local councils, and our senior surveyors have over 20 years' experience with GPR.

Questions we're asked