Scanning a suspended slab so a data centre could build up

The brief

Custodian Data Centres were fitting out additional space at their Dartford facility and wanted to add a mezzanine to win back floor area. Straightforward enough — until an earlier intrusive investigation returned an awkward finding: the ground-floor slab was not ground-bearing. It was a suspended slab, carried on piles.

That changes the job entirely. A mezzanine’s loads have to be taken down through the structure, and on a suspended slab you cannot simply set base plates and drill fixings wherever the layout suggests. The structural engineer needed to know where the reinforcement ran and how deep it sat before the mezzanine could be designed and the base plates positioned. Drilling first and finding out later was not an option — not into a structurally critical slab over piles.

Custodian came to us with exactly the right question and asked us to guide the rest: how much of the floor to scan, and how, to give their engineer what the calculations required.

On site

We specified a combined GPR and Ferroscan survey across the proposed mezzanine footprint and attended on 20 May with a Proceq GP8100. Conditions were good; the slab surface was dry. The unit was mid-construction, so the survey was planned around live works and the areas were set out to capture the runs the engineer needed — four scan areas in total, including large-format grids up to around fourteen metres long.

Every scan area was tied back to the engineer’s floor plan, so the data could be read straight against the drawing rather than re-interpreted. This is the part that matters on a job like this: the deliverable has to drop into the engineer’s workflow without translation.

What the scan showed

The GPR data resolved two distinguishable layers of reinforcement in an apparently orthogonal arrangement, with indicative cover of roughly 22–30 mm to the upper layer and around 53–75 mm to the lower. One area also showed a pronounced linear, high-amplitude feature running through the centre of the scan — consistent with embedded metalwork of a different geometry to the surrounding steel, and flagged for caution rather than guessed at.

We were equally clear about the limits. Bar spacing and diameter could not be reliably confirmed from the processed image exports and were flagged for verification from the native scan files. Stating that up front is the point: a survey that hides its limitations loses its value the moment an engineer queries it. Ours doesn’t.

The deliverable

Custodian’s engineer received three things:

• A provisional survey report documenting method, conditions, and indicative findings, area by area.
• A marked-up floor plan showing exactly where each scan sat, so the data could be located against the design.
• A live link to the full GPR dataset in the Screening Eagle cloud workspace, so the engineer could interrogate the depth slices directly rather than rely on a static PDF.

The outcome

The timing was tight. Custodian’s engineer needed to finalise his calculations and pass them to the mezzanine contractor to hold the delivery date. We scanned on the Wednesday and had the provisional report and data link with him the following Wednesday — the day before his deadline.

With the reinforcement layout, indicative cover, and flagged features in hand, the engineer could design the mezzanine and locate its fixings against real data instead of opening up a suspended slab to find out the hard way. Custodian got to build up, on programme, without putting a single hole in the floor to do it.

“Real pleasure working with the Corvus team. They came to site, walked us through how they generate the sub-floor scans, and turned the results around in time for our engineer to finalise his calculations and keep the mezzanine on programme.”

— Brindley Kinch, Operations Manager, Custodian Data Centres

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Planning a mezzanine, a fit-out, or any work over a slab you can’t see into? Get a quote or call 020 3886 3660. We scan before you cut.