Tunnel vision

As evening rush hour comes to an end on the east side of London, a fleet of double-decker buses swings onto the Dartford Tunnel slip road. But the passengers are arriving for work rather than leaving it, and are wearing hard hats and safety boots rather than suits and ties. This is road tunnel maintenance 21st-century style, where sustaining traffic flow is imperative and limited time for access has created a new way of working.

The challenge was set when, with its operating contract coming to an end in 2002, Dartford River Crossing (DRC) ordered a maintenance survey. Consultant Babtie Group recommended a change of tunnel lighting and electrical distribution for safety and efficiency. "This may seem like a simple job but it is a logistical nightmare," stresses Robert Owen of contractor Field Systems Designs (FSD).

The Dartford Crossing serves hundreds of thousands of vehicles every week. Two dual-laned tunnels and the four-laned Queen Elizabeth II Bridge take traffic north and south respectively, connecting at both sides with the orbital M25. Securing an uninterrupted flow of traffic 365 days a year is vital: cctv cameras along the roads are linked to banks of monitors in a control centre that ensures any problems are quickly resolved. Any tunnel closures have to be at night and, more importantly, have to be flexible, as during high winds the bridge is closed and southbound traffic is diverted into the east tunnel. With these restrictions immovable, innovative ideas were needed to tackle the necessary maintenance.

FSD won the contract for the east tunnel by tender in April 2001. Its original plan was to use flatbed lorries to transport scissor lifts and materials to and from the site, as the lifts alone would be too slow and physically unable to carry the materials needed within the access time allocated to the contractor – 9 pm to 5 am. But FSD had a masterstroke up its sleeve: double-decker buses.

The idea was to use the top of the buses as a working platform, with HSE- approved safety scaffolding lining the edges. A bus was hired to try for size and, being deemed the perfect height and solution, a fleet of seven was bought. "Use of the seven buses gives a 54 m working platform," explains project manager Nigel Billings, "compared to scissor lifts, which give 5-6 metres." This difference was important in the 1·4 km-long tunnel, where 525, 4·8 m-long, lamps had to be installed in 16 weeks.

The buses had added advantages: they could be used for staff shelter and toilets, and for storage and transportation of materials – removing the need for any other works vehicles. And of course there was the speed at which they could be removed: "There was always the possibility that we could be asked to leave the tunnel at night if there was an emergency, and we always had that in the back of our minds," stressed Billings.

With on-site time tight, close project management was vital. Work and staff were split into day and night shifts. Day staff worked on a production line basis, prefabricating modules comprising the marine-quality luminaires, from Dutch manufacturer Industria, and high temperature junction boxes on lengths of phenolic resin-coated support tray. These were then loaded onto the top of the buses with any additional materials and tools, ready for the night shift's arrival.

The night shift's first task was to drive the buses to the north of the tunnel and turn them to face south, to minimise the exit time. Work was done in a logical and repetitive sequence of removing old lamps and fittings, fixing the new prefabricated units, then making the final connections and switching on the power. Generator-powered halogen lamps around the bus tops provided the light needed to work safely to replace the fittings at an average rate of 27 per night.

Electricity is supplied via marine-quality distribution panels from Roonfoss and Eastern Switchgear by separate sources on each side of the river. Two generators provide security, and for extra peace of mind, two Socomec ups-backed supplies provide a third power source. Photometric sensors added to the tunnels automatically adjust the light level for 250 m at each end so that the change in light levels is graduated for drivers. This connects with the existing control system so that lighting can be adjusted in the east tunnel if the traffic flow reverses due to bridge closure.

FSD won the contract for the west tunnel in July. Schedules here benefited from the experiences learnt in the east tunnel, although a different suspension fitting was needed for the 505 lamps due to the tunnel's older age. This completed on 17 December 2001, just in time for any last minute Christmas shoppers to join the rush hour and enjoy a brighter trip to the two nearby shopping centres.