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On the straight and narrow

Holmesdale Tunnel - How do you improve traffic flow through Europe's busiest road tunnel without widening it? John McKenna meets the men with the answers.

Holmesdale tunnel is the busiest road tunnel in Europe. Situated at junction 25 of the M25, it is used by 120,000 cars per day.

Most of this northern section of the road has three lanes in either direction and is due to go up to four under a £4.5bn widening and maintenance DBFO contract due to be awarded in 2008.

However, on the eastbound carriageway at Holmesdale traffic is squeezed down to two lanes plus a slip lane for junction 25, 300m west of the tunnel.

Improvement works completed on the westbound carriageway in March 2002 allowed three lanes plus a narrow slip lane to pass through the tunnel. However, the continuing high level of accidents and congestion in and around the tunnel and junction saw Holmesdale placed on the government's 92 junction priority action sites in October 2002.

The need to improve Holmesdale was brought to a head by the tunnel's mechanical and electrical equipment, which was reaching the end of its 20 year design life. The tunnel opened in 1984.

'With all the traffic rushing past, tunnels can be quite a corrosive environment for technical equipment, ' says Highways Agency project leader Eamonn Colgan. 'As you approach 20 years, you end up spending more and more money maintaining the equipment.'

In August 2005 Costain was appointed ECI contractor by the Highways Agency, which budgeted the works at £75M. With its designer Capita Symonds, Costain set about devising a solution.

'To create three lanes and an extended slip road, one option was to use the hard shoulder, ' says Mouchel Parkman supervisor Russell Williams, who is monitoring the project on behalf of the Agency. 'But the emergency services said it needed to be retained. We decided the only way to do it was to remove the roadside walkways.' Breaking out walkways that run along both sides of the carriageway in both directions frees up 2.6m for road surface, but removes the central reservation and crash protection for the outer wall.

To provide crash protection, Costain, which began work onsite this May, is stripping out the original asbestos cladding on the outer wall and replacing this with a 1.2m high in situ reinforced concrete wall. Above this, a sprayed concrete fire protection is applied to the 3.5m high steel backing members.

This fire protection, taken from the oil industry and designed to withstand heat of 100MW for two hours, is also applied to the tunnel ceiling and central wall. This wall replaces the original structure of the concrete walkway and concrete pillars extending up to the soffit.

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