Speed restrictions are hardly necessary approaching the intersection of the M25 and A2 in Kent. If traffic is heavy, vehicle speed is reduced to a crawl.
Outside peak time, the sight of mammoth cranes, excavators and quarry trucks hard at work acts as a natural brake as drivers indulge in a spot of rubbernecking.
For the past nine months contractor Costain has been at work on a £120M project to upgrade the connection between the two roads at junction 2 of the M25 (NCE 22 March).
Some 135,000 vehicles a day hit the M25, with 9,000 of them using junction 2, says Highways Agency project manager John Martin. Much of the traffic at the junction is travelling between London and Dover, putting huge pressure on the link between the M25 north and the eastbound A2. The two roads are connected by slip roads and a roundabout.
'We're aiming to take about 40% of the traffic out of the junction by building link roads that'll carry vehicles directly from one road to the other, ' Martin says.
The upgrade is designed to cope with projected 2022 traffic volumes. 'It's a key part of the Thames Gateway regeneration, ' he adds.
Two viaducts connect the roads. The one linking the M25 southbound with the A2 eastbound is a relatively modest structure, clearing the slip road that ties into the junction.
But the flyover linking the A2 westbound to the M25 northbound must leap over the A2, the M25 and junction slip roads. Its 470m length curves on a tight radius and making the link within geometric tolerances suited to motorway traffic loadings and speeds was a tight fit, says Costain project manager Rob Phillips.
Threading the structure's piers between the lanes of existing roads and lifting in its steel deck beams without bringing traffic to a grinding halt was even more challenging, he adds.
Costain won the contract in July 2005 under an early contractor involvement agreement. 'We had good relationships with Foster Yeoman, Bachy and Fairfield Mabey ([materials supplier, piling subcontractor and steelwork fabricator/erectors respectively) from earlier projects such as the Porth relief road in south Wales, ' says Phillips.
'We scrutinised the scheme design and looked for ways of value engineering it.' 'We were after speed of delivery, safety and minimal congestion, ' explains Martin. 'And we wanted a design that we were totally comfortable with and really wanted to build, ' Phillips adds.
The project involves extensive road widening and resurfacing (see box), but the construction of the flyovers has been the show stopper. Structurally they are straightforward - pairs of cast insitu reinforced concrete piers founded on bored piles, supporting a composite deck of Corten weathering steel beams topped by a reinforced concrete deck cast insitu in permanent GRP shuttering. Spans are by no means large.
But coming up with something that could be built on one of the UK's busiest highway junctions without bringing everything to a standstill has been no mean feat of design and logistical planning.
'In the schematic design, the deck was supported by sets of four columns which we reduced to pairs of columns. Originally the deck had six longitudinal beams. Now there are four.
There were cross beams at the columns, but we decided to sit the longitudinal beams directly on the column heads, ' says Phillips.
'We and Faireld Mabey looked at the position of piers and the lengths of deck you could lift in one go.' The deck was split in half lengthwise, enabling all but the last section of bridge to be lifted using a 1,000t mobile crane, for fabrication and erection. All the lifts were carried out during night-time lane possessions.
Phillips says that construction was all about sequencing. 'We built the piers either side of the A2 rst, with trafc down to two lanes rather than three to give us a decent working area, and then constructed the intermediate pier, which is in the central reservation. We gained access by pushing trafc away from the centre onto the verges.' While piling and column construction was taking place the yover's reinforced earth abutments were built. North of the A2, piers were built outside the line of the existing roads, enabling relatively swift construction - each group of piles and columns took eight weeks to complete.
The first section of deck installed spanned the A2 eastbound, straddling the piers in the central reservation and on the northern side. With the first span in place, support of each subsequent span was simple, .
Ends of adjacent spans were joined with bolted connections, while the further end was supported by the next pier.
Spans lengths differ lengths, but have a constant beam depth of 2.5m to cope with variations in bending moment.Flange thicknesses range from 30mm to 80mm.
The last span was placed late last month with the aid of the UK's largest mobile crane, a gargantuan 1,200 tonner. 'The element wasn't that heavy at 140t, but the lift was at a 50m radius over the M25. We needed that much lifting capacity, ' says Phillips.
Costain is now well advanced with pouring the yover's concrete deck. Martin says that work is progressing well. Though the project as a whole is not due for completion until spring next year he is hopeful that the yover will be operational by the end of the year.
The A2 is being widened from dual three to four lane by reducing the width of the central reservation.
The central barrier has been beefed up and lighting columns mounted on top of it. There has been no need to reduce lane width, says Martin.
East of the junction the A2 runs through a cutting anked by sites of special scientic interest.
It is impossible to claw extra space from the central reservation here, so instead the embankments are being steepened with the aid of extensive soil nailing and the installation of cellular geogrids.
'We weren't allowed a millimetre of extra land take because this is such a protected environment - normally the widening would have required 40m of land take, ' says Martin.
Costain will start on a second major motorway widening contract later this month.
It is contractor for the £50M M25 junctions 1b-3 widening and resurfacing.
AFR: 0.17 Innovations:
All staff have been through a 'Safety leadership in Costain' workshop, which is about behavioural safety and getting ideas into the supply chain.
There is a feedback system for the workforce to report unsafe working practices. A back to basics campaign focuses on method statements and PPE.