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Buried Secrets: Dartford Tunnel

In early January, with one more possession of the tunnel still to go, drivers of vehicles using Kent’s Dartford crossing have been blissfully unaware that a major project has been underway which could have impacted mightily on their travel plans.

That it has not is testament to the professionalism and care shown by participating companies working hard and safely in onerous conditions. Not one M25 user has been stuck in traffic as a result of the resurfacing works.

“Dartford is a very sensitive part of the road network we look after and we know that causing congestion there can create chaos in the south east of England,” says Beverley Waugh, M25 programme manager for Connect Plus, the DBFO consortium responsible for maintaining and upgrading the M25 motorway.

“This has driven the way we – as client – have developed the scheme, working with our supply chain to develop the methods and culture that would deliver the project to time,” she says.

Disruption to traffic has not been an option. More than 150,000 vehicles a day use the Dartford crossing’s northbound twin tunnels and its southbound bridge. Traffic build-up is rapid and intense if any part of the crossing has to close, particularly at peak times.

“Dartford is a very sensitive part of the road network we look after and we know that causing congestion there can create chaos in the south east”

Beverley Waugh

Connect Plus got consent from the Highways Agency to close the tunnel in question only at weekends, between the hours of 8pm and 10am the following day, during a limited number of Friday and Saturday night possessions.

“Shopping centres at Lakeside and Bluewater came into the equation, as did Christmas,” says site supervisor Craig Lennon from Connect Plus Services, the division responsible for the design and supervision of the surfacing works. “This meant the tunnel had to open an hour earlier on Saturday mornings in December. Altogether 14 surfacing shifts were planned for, with a nominal target of 100m of surfacing being carried out each shift – the tunnel being just over 1,400m long.”

The reason for the resurfacing project was the deterioration of the western tunnel’s existing 11-year-old surfacing, and in particular this tunnel’s nearside lane. It takes the vast majority of HGVs travelling north.

Hard working asphalt

The asphalt under the lorries is pounded 24 hours a day, seven days a week, so it is not surprising this lane in particular became heavily wheel tracked.
The surfacing also started to pothole badly. Inevitably, the problem had as its root cause the presence of water, brought in on vehicle tyres or having leaked in through the tunnel lining, and then pumped into the asphalt by traffic-induced hydraulic pressure.

When Connect Plus looked into the failure mechanism, it found that ‘weep holes’ through the road deck at subsurface level were inadequate to cope. This led to saturation and failure of a 20mm thick sand carpet situated between the asphalt and the deck’s waterproofing. This in turn caused potholing at the asphalt surface and damage to the deck waterproofing.

“The concrete deck below the waterproofing is still in good condition. It isn’t that old – having been replaced in 1999/2000,” says Waugh. “Also road salt is not used at Dartford which helps the situation.”

It was clear that the existing surfacing had to be replaced, new waterproofing was required and the subsurface drainage had to be enhanced. The challenges were how to do all this in the limited time available; and with what materials to ensure a durable, long-lasting solution.

An NEC (Option C) Early C0ntract Involvement contract was drawn up by Connect Plus, with Jackson Civil Engineering the main contractor and Bardon Contracting responsible for surfacing.

As far as the materials were concerned, two fundamentals had to be sorted out: how to waterproof the deck and what constituents should go into the asphalt mix.

“We believed this membrane would provide sufficient water proofing to the deck, and trials bore this out”

Amir Khalilzadeh

“The problem with a conventional HAPAS approved spray applied waterproofing system was that it took time to install. It couldn’t give us the necessary short turn around we needed during a surfacing shift,” says Connect Plus Services project manager Amir Khalilzadeh.

One product that could meet the tight time constraint was Eurovia’s Flexiplast, marketed in Britain as a crack inhibitor system. Flexiplast is a heavily polymer modified bitumen membrane with good elastic properties and low thermal sensitivity.

“We believed this membrane would provide sufficient water proofing to the deck, and trials bore this out,” Khalilzadeh says. The Flexiplast, laid 2mm thick, is protected by a 3mm layer of Gripfibre, an emulsion slurry. Both could be laid rapidly by tanker and spray bar, covering the full width of a lane in one pass. Installation took minutes. Even more to the point, it could be covered by asphalt an hour or so after, to have little inhibiting effect on general resurfacing progress.

The Highways Agency was persuaded of the product’s suitability and agreed to its use.

A great deal of thought also went into the asphalt mix design. The deck to Dartford’s western tunnel is made of precast concrete panels, each one 3.5m long, which rest on elastomeric bearings.

HGVs “like fingers across piano keys”

HGVs running over the panels have the same effect as fingers travelling across piano keys, according to Jackson’s site supervisor Jonathan Cass.
Whatever asphalt was laid, it had to be one with good flexural characteristics, resistant to fatigue. It also had to display high resistance to deformation, to combat the effect of the HGVs.

These differing (not to say conflicting) requirements called for an exceptional asphalt binder and the debate – which included the technical staff and laboratories of Barden parent company Aggregate Industries plus asphalt guru Ian Walsh – eventually settled in favour of Endura Z2, an extremely competent polymer modified product from bitumen specialist Nynas.

“One would typically have expected such a strong durable binder to result in a stiff asphalt at the time of laying but a perhaps surprising characteristic of Endura Z2 is its ability to remain workable at relatively cool temperatures,” says Aggregate Industries’ area technical manager Chris Marchesi. “This was particularly important because the lack of headroom within the tunnel meant our tipper trucks could not deliver asphalt straight to the paving machine.”

“The work area is very small, there are lots of people, lots of plant, lots of stress. You lose one operation, you lose the night.”

Steve James

Instead the material had to be discharged at the tunnel portals, then loaded into dumpers, then delivered to the work face in small quantities. A major trial of how the resurfacing was to be done was carried out in July last year, advance pre-surfacing works – such as removing ironwork and expansion joints – started in September and the 14 surfacing shifts, during which time the tunnel was closed to traffic, began in October.

Three planned shifts had to be rescheduled, twice because of bitterly cold weather and once the shutting of Dartford crossing’s bridge. This required the western tunnel to remain open to carry northbound traffic, southbound traffic being diverted through the east tunnel.

Each shift is run as a military operation. At 8pm – as long as northbound traffic had fallen below 3,500 vehicles per hour, one of the conditions for closing the tunnel – personnel, machines and materials pour through the portals. For each 100m of carriageway to be surfaced, five planers, 14 eight-wheeler wagons, two pavers, two rollers, nine 6t dumpers, two low loaders, three sweepers and umpteen trailers queue up for their turn.

“The work area is very small, there are lots of people, lots of plant, lots of stress,” says Connect Plus project manager Steve James. “You lose one operation, you lose the night.” Contingency plans and back-up equipment are in place to ensure nothing goes disastrously wrong.

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