On the straight and narrow

Holmesdale tunnel at junction 25 of the M25 around London, UK, is the busiest road tunnel in Europe. Most of the northern section of the motorway has three lanes and is due to be widened to four lanes. But on the eastbound carriageway at Holmesdale, traffic is squeezed into two lanes plus a slip lane for junction 25. Improvements on the westbound carriageway in 2002 allowed three lanes and a narrow slip lane to pass through the tunnel. The tunnel's mechanical and electrical equipment is reaching the end of its 20 year design life. Costain was awarded a contract to improve traffic flow. The solution involved removing walkways on both sides of the carriageways in both directions. This removed the central reservation and crash protection for the outer wall. To provide crash protection, the asbestos cladding on the outer walls will be replaced with a 1.2m high in situ reinforced concrete wall with a sprayed concrete fire protection on the upper wall and ceiling. The inner wall will consist of a reinforced concrete wall. The new ventilation system will be the Saccardo ventilation system, housed in portals at the entrance to the tunnel. Progress with the project is described. (A)