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Photos | Stepnell and Tarmac build ‘super-smooth’ underground test track
Neil Gerrard Associate editor
Tarmac laid the super-smooth track inside a Victorian railway tunnel (Images courtesy of Tarmac)
Contractor Stepnell and Tarmac have built a ‘super-smooth’ 2.7km stretch of automotive test track in Northamptonshire, almost all of which is underground.
The track, which Tarmac claims is “completely flat” is dead straight and protected from the elements to allow repeatable tests in all weather conditions.
The track has been designed to test everything from supercars to HGVs.
The Aero Research Partners’ (ARP) Catesby Tunnel, which sits inside a former Victorian railway tunnel, enables testing of every facet of vehicles, including aerodynamics, performance – such as speed, acceleration, braking and ride comfort – aeroacoustics, and engine emissions.
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A dumper transports material through the tunnel (Image courtesy of Tarmac)
Tarmac laid a specially-designed SMA asphalt surface, with specialist polished stone value (PSV) 65, 10mm gritstone aggregate, which exceeds the paving tolerances of less than one millimetre down to 0.3mm , which Tarmac claimed is better than many Formula One race circuits.
Standard tipper trucks could not be used, so the asphalt material had to be transferred using feeders and dumper trucks, forming a giant conveyor.
Robert Lewis, director of Aero Research Partners, said: “We have worked closely with Stepnell, which led a professional team of great subcontractors, to successfully complete the civil engineering and building works. The team converted the tunnel from a wet hole in the ground to a pristine running surface that allows sophisticated vehicle testing.
A feeder inside the tunnel (Image courtesy of Tarmac)
Rob Doody, managing director (Midlands region) for Tarmac, said: “The seamless way this project was planned and delivered resulted in a truly world-class finish that is amongst the highest-known paving standards in the world today.
“The level of paving accuracy has delivered a surface with the exceptional consistency and uniformity needed to meet ARP’s aspiration of producing a world-leading aerodynamic test site. It enables automotive engineers to take any surface irregularity out of the equation.
“Achieving the specific tolerances required continuous paving along the full length of the tunnel, so we had to guarantee a continuous supply of asphalt to the paver from Tarmac plants at Mountsorrel, in Leicestershire, and the Elstow plant, near Bedford.
Doody added: “In two shifts, the paving teams worked from 7am to 7pm, laying 1,340 tonnes of asphalt with a seamless changeover without stopping the paver.”
Richard Wakeford, director at Stepnell, said: “We are proud to hand over the Catesby Aero Research Facility to ARP. This has been an amazing and exemplary project bringing together the best of the industry to deliver a modern test facility that will drive transportation efficiencies for years to come. Overcoming several engineering challenges, this project has been a truly collaborative experience and my thanks to all those involved.”
The November/December 2025 issue of Construction Management magazine is now available to read in digital format.
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