Highways Agency

• A3 Hindhead Improvement

A3 Hindhead Improvement

The Tunnel

The tunnel is 1.83km long comprising about 1.77km of bored tunnel and approximately 30 metres of cut and cover at either end. The maximum depth of tunnel below ground is about 65m at Gibbet Hill, (measured to the top of the tunnel).

The tunnel will have two separate bores. Each bore will include a 7.3m wide 2 lane carriageway with 1.2m wide verges on each side. The verges are wide enough for emergency use by a disabled person in a wheel chair. Each bore will have a maintained headroom of 5.03m, together with a further clearance of 250mm to the underside of any plant suspended above the vehicle gauge as a safeguard against flapping tarpaulins and ropes.
 
The tunnel bores will be approximately parallel and linked by pedestrian cross-passages at approximately every 100m throughout the tunnel. The tunnelling method proposed by Balfour Beatty is the Sprayed Concrete Lining method (SCL).

The tunnel alignment is in the middle of the Upper Hythe beds at the southern portal. These beds consist of silt sand with rock bands. Further north, it passes through the Upper Hythe beds where the percentage of rock starts to increase and then in the Lower Hythe beds there is rock with minor sand bands.

A larger image of the cross section is available to view here.

General Questions about the Scheme

Q. Will there be a services tunnel between the two main bores?

A. No, we do not need to provide a separate tunnel to accommodate services.

Q. What happens if the electricity supply fails?

A. The tunnel will receive its power supplies from two power networks, one towards the north and one towards the south. So if one supply fails the other one will take over. There will be back-up power supplies, which will come into use should both supplies fail or should the supply within the tunnel fail for some reason.

Q. What happens if the ventilation fans break down?

A. Under normal weather and operating conditions, the movement of traffic itself would be sufficient to create the draft through each tunnel necessary to ventilate exhaust fumes. The ventilation system is designed to operate only when weather conditions (such as high winds) or traffic congestion means that exhaust fumes start to build up. Additionally the ventilation system is designed to operate successfully even if some of the fans break down.

Q. Will any road users be banned from the tunnel?

A. Yes, in the interest of safety, cyclists, pedestrians, equestrians and motorcycles below 50cc will be banned from the tunnel. Cyclists, pedestrians and equestrians will be provided with an alternative route across Hindhead Common and motocycles below 50cc will be diverted via Thursley and the Tilford Road.

Q. Where have similar road tunnels been built in the UK?

A. Similar tunnels constructed in the UK in recent years are at Roundhill on the A20 and at Southwick on the A27.

Q. How much earth will be excavated from the tunnels and what will you do with it?

A. There will be around 300,000 cubic metres of spoil generated which will all be used to build the highway embankments and earthbunds on the northern side of the tunnel. No spoil will be taken away from the site.

Q. Will there be any settlement above the tunnel?

A. Our calculations predict that any settlement is unlikely to be noticeable; however we will monitor this during all tunnelling operations

Q. What safety features are you incorporating into the design?

A. The tunnel will have:

• high quality lighting and ventilation
• continuous traffic monitoring and CCTV surveillance
• early automatic detection of vehicle breakdowns, accidents and fires
• quick appropriate information to drivers using automatic variable message signing, radio interrupt systems and loudspeakers
• 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.

In addition we will develop plans to:

• ensure emergency vehicles can get to the tunnel quickly
• route traffic away from the tunnel if blocked by an incident
• assess the risks from all credible hazards
• manage incidents.

These issues will be developed in association with the Tunnel Design and Safety Consultative Group, which comprises representatives of emergency services, local authorities and the Environment Agency.