Holmesdale tunnel is the busiest road tunnel in Europe. Situated at junction 25 of the M25 around London, 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 $8.5bn widening and maintenance DBFO contract due to be awarded in 2008. But 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 list of 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 $143M. 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 re 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 soft.
Like the outer wall, the new wall is made from reinforced concrete but is 3m high. Rather than leave a 1.7m gap between the top of the wall and the tunnel ceiling, Costain's choice of Saccardo ventilation forced the contractor to block up the space with breeze blocks, as the system only pushes air in one direction and would otherwise blow smoke into oncoming traffic in the event of an accident (see box).
The work on the central section was the first phase of the project to be completed, in August. The second phase, which is underway, constitutes the complete refurbishment of the eastbound tunnel bore and construction of its third running lane. The tunnel is under possession for the 28 week duration of the second phase, with all traffic passing through the westbound section.
After this, the westbound bore will receive the same refurbishment over 28 weeks, enabling a wider hard shoulder and getting the tunnel ready for the 2008 widening. The project is due to complete December 2007.
The real innovation at Holmesdale comes from Costain's solution to the problem of ventilation. Its fans are already at the end of their design life and by removing the walkways, they would also become an impact hazard for tall vehicles. Unable to leave the fans where they were, Costain opted for a system never before used on the UK's roads.
Saccardo nozzles have been used in ventilation systems for uni-directional vehicular tunnels in the US and Japan since 1924.
However, their only application in this country to date has been on the Channel Tunnel Rail Link, where it was last year praised for preventing a fatal re on the line from escalating into a major incident.
'But the question was, ' says Costain project manager Steve Meadowcroft, 'where would we put it?' By extending the tunnel entrance on either side, Costain could create a portal to house five fans and five Saccardo nozzles (see diagram).
On the eastbound bore, piles have been drilled and last week 18 prestressed concrete beams were craned into position, ready for an insitu concrete deck to be poured, with a 4m wide gap for the Saccardo nozzles.
Using the nozzles and housing them in the portals has the combined benet of allowing clearance for tall vehicles and ease of maintenance in the future.
'Previously to do any maintenance the Agency would have to close off a lane of trafc to get up to the fans, ' says Meadowcroft.
'Now they can just come in on to the top of the portal from the road above, avoiding any