A3 Hindhead Road Tunnel, United Kingdom

The A3 Hindhead project is a major UK road improvement scheme that will complete a dual carriageway road system between London and Portsmouth along the south coast. The road section in question will be four miles (6.5km) long and will include a 1.1 mile (1.8km) section of twin-bored tunnel.

The route will run from Bramshott Common (Surrey-Hampshire border), north past the village of Hindhead, into new tunnels and on to rejoin the A3 at Thursley. The project was designed to relieve the congestion around the A3/A287 crossroads and the small country roads that are subject to rat running (the A3 carries over 28,400 vehicles per day). One advantage of the project will be the closure of the A3 section through Hindhead Village (freeing the village from grid-lock every morning) and around the Devil's Punch Bowl hill (an area of special scientific interest). The new tunnels will carry the A3 underneath this special region at a depth of around 65m.

Pedestrians and motorcycles below 50cc will not be allowed through the tunnel and diverted via Thursley and the Tilford Road.

The tunnel

Balfour Beatty proposed the sprayed concrete lining method (SCL) of tunneling for the Hindhead project.

Designed to have two separate bores, the tunnel is 1.83km long with approximately 1.77km of bored tunnel and 30m of cut and cover at either end. Maximum depth of the tunnel is 65m below ground level.

Each bore includes a 7.3m-wide two-lane carriageway with 1.2m-wide verges on both side. Headroom of 5.03m and further clearance of 250mm to the underside of any plant suspended above the vehicle gauge is maintained in each bore. It acts as a safeguard against flapping ropes and tarpaulins.

The tunnel bores are linked by pedestrian cross-passages at every 100m throughout the tunnel.

Design safety measures incorporated

The tunnel design includes important safety measures:

• Continuous traffic monitoring and CCTV surveillance
• Early automatic detection of vehicle breakdowns, accidents and fires
• Automatic variable message signing, radio interrupt systems and loudspeakers to give appropriate information to drivers
• Well equipped emergency points with fire extinguishers, a manual call button and emergency telephones that can be used by disabled persons
• Easily identifiable safe escape routes

The Tunnel Design and Safety Consultative Group (TDSC) and the Highways Agency will together develop these issues. Representatives of emergency services, local authorities and the Environment Agency comprise the TDSC group.

Controlling the tunnel alignment

The Highways Agency established a network of survey stations in the planned A3 improvements area approximately 16 years ago and linked these to the Ordnance Survey grid. A global positioning system (GPS) checks the existing stations and establishes new stations necessary for the works. The GPS receivers set on the survey stations record their position for at least once an hour after which the results are processed to give at least centimetre accuracy.

At each end of the tunnel four permanent survey ‘monuments’ are established. The most advanced station (the one nearest the tunnel face) helps control the tunnel excavation.

The tunnel guidance system includes a robotic theodolite which is orientated by sighting back down the tunnel, recording readings on the previous survey stations and then tracking targets on the tunnel excavator. The guidance system indicates to the excavator operator where to dig.

Contractors and timelines

The project began in January 2007 and tunnelling work started in February 2008. The tunnel is expected to be open to traffic by mid-2011. The two tunnels were joined at the breakthrough point in February 2009. The project design-build is being carried out under the Highways Agency’s Early Contractor Involvement (ECI) programme to speed up the project.

The designer of the £371m road and tunnel scheme is Mott Macdonald, while the general contractor is Balfour Beatty. Specialist consultants for the project include Wessex Archaeology, RPS (landscape and ecology consultant) and Persona Associates. Sandvik Mining and Construction supplied the tunnel jumbos to Balfour Beatty.

Tunnel construction

The tunnel will be a twin bore construction, each of which will carry two lanes through the sandstone of the Hythe Beds (the 10m bore tunnels will be lined by spray concrete and some cast in-situ elements). The southern end of the tunnel will be excavated through sand and will require the use of a crown canopy support system. This will be the longest non-estuarial tunnel in the UK and will carry state-of-the-art safety specifications. There will be pedestrian access cross-passages every 100m (escape tunnels) along with communications and radio interrupt systems to provide drivers with information.

In addition, the tunnel will have CCTV, a high lumina lighting system and ventilation systems. The supply of electricity to the tunnel will be from both south and north power networks. If one fails the other can be put to use, and in case if both power networks fail, there is back up power supply.

The project will involve the handling of over one million cubic metres of earth, which will be reused in the scheme. Prior to opening, the landscape will have to be fully reinstated along with green slope and cutting finishes as well as a green footbridge.

Techniques to minimise land take during the project include soil nailed cut slopes and reinforced soil embankments to provide steepened slopes. A portion of the land involved around the Devils Punch Bowl belongs to the National Trust, which has been involved in the consultation process.

Construction of Canadian Memorial Underpass in the southern part of the tunnel was completed in 2007. The underpass in the northern part of the tunnel was completed in 2008, along with Blackhanger Underpass and Hazel Grove overbridge. Hammer Lane Underpass was opened in the same year. The Greensand way through which the local traffic can pass, was also completed in 2008.