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The archeology of a station platform

Design consultant, Corus Railway Infrastructure Services, has used data captured by ground penetrating radar (GPR) and remote visual inspection (RVI) to assist in the design of a new canopy renewal on platform one at Derby railway station for client, Network Rail.

Unravelling complex detail of the platform’s construction, the survey data has also helped with designs for other upgrades as part of general refurbishment of all six of the station’s platforms.

The refurbishment is currently at detailed design stage with site work scheduled to start in early 2008.
Historical drawings showed that voids, in the form of a series of parallel service tunnels with intermittent walls, existed under the 330m long platform one.

Although some of the tunnels are accessible, many are bricked up or have been back filled. As well as confirming their presence, detail was needed about their condition, dimensional arrangement and location in relation to above-ground structures to help guide designs for the platform refurbishment. A secondary objective was to determine the structure of foundations to the columns supporting the roof canopy.

Martin Thackeray, senior engineer at Corus Railway Infrastructure Services explains, “The station has undergone significant expansion over the years, and historical records often give conflicting information about construction. Fugro Aperio’s survey data confirmed much of the information we had on historical drawings, but also pinpointed some unrecorded voids.”

Completed in one night under a Tii protection, the GPR survey involved the collection of data along five transverse profiles spaced at 250mm centres in 18 locations. Two longitudinal profiles were also collected along the entire length of the platform. Intrusive works, combining RVI and coring, were undertaken over two nights.

The GPR confirmed the presence of six tunnels: four running the full length of the platform north of the pedestrian subway, and two of indeterminate length running south of the subway. Only two were fully open; sections of backfilling were present in the remaining four.

“The data showed the spatial arrangement of the voids, the location of dividing walls and the extent of backfill so we could tie it in with above-ground data mapped out by our topographical survey,” Mr Thackeray says, adding, “We were also interested in data on the construction of the top deck of the platform in planning new tactile paving. We knew that it generally consists of stone slab and concrete, but construction varies a lot along the platform, as confirmed by the GPR.”

I spy with RVI - what the remote cameras revealed

Fugro Aperio recommended the use of RVI to determine whether the original column foundations to the canopies, which pass down into the platform and underground voids, might be reused in constructing new ones.

Using specialised endoscope equipment, Fugro Aperio carried out the RVI survey via drill points in thirteen locations to confirm the foundation type of the columns.

The RVI targeted locations where the GPR had identified sections of tunnel, as well as areas of apparently solid ground. It found that one of the 455mm square concrete encased columns was supported on a foundation block - 440mm above the tunnel floor – while other columns pass straight down through the tunnel floor.

The results provided sufficient information for Corus to determine that it would be unviable to reuse the existing column foundations.