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Fri May 07 2021

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Military precision on M3-M25 interchange

28 Apr Meticulous planning and the careful choice of materials have vastly improved the speed and efficiency of maintenance work on the motorway network. David Taylor reports

One of the expansion joints is inspected prior to its install
One of the expansion joints is inspected prior to its install

Closing one of the busiest motorway junctions on the network for routine maintenance isn’t something you undertake lightly. Wherever possible, the work is done in phases in order to keep at least one lane clear so that traffic continues to flow – albeit at a frustratingly slow rate.

But temporary lane closures are popular with nobody. Every driver knows the aggravation caused when coned-off bottlenecks slow traffic flow for weeks on end; and every road maintenance worker will appreciate the potential danger of working in close proximity to live traffic.

This article was first published in the March 2021 issue of The Construction Index magazine.  Sign up online.

Nevertheless, this looked likely to be the situation at the M3-M25 interchange when it became necessary a couple of years ago to begin replacing several expansion joints. Expansion joints span the entire carriageway so there’s little scope for reducing traffic disruption; nevertheless Jackson Civil Engineer, the framework contractor responsible for the works, had a cunning plan.

Rather than carry out the joint replacements in phases, Jackson decided that if it closed the carriageway completely and worked round the clock, it could replace each joint in a single weekend. Of course, this would mean diverting traffic, but only for a relatively short period of time when the junction was at its least busy.

Jackson calculated that doing it this way meant not only that lane closures could be flagged up for road users well in advance, but they could also be reduced by more than 90%. Safety would be improved because workers would spend less time on the road and avoid proximity to live traffic.

The main challenge, though, was the meticulous planning necessary to ensure snag-free execution of the works. Jackson planned what became known as the “weekend wonder-shifts” to run from 10pm on Friday evening to 5am the following Monday. 

The work, which has recently been completed, saw six teams working in relays around the clock for 55 hours straight. Doing it in one hit meant also that there were none of the delays associated with conventional working patterns because there was no need to mobilise and demobilise the sites between shifts.

“Working in shifts isn’t new of course,” says Jackson’s framework manager Ryan Smith, “but it’s never been explored in this way on the M25.” Generally speaking, work cannot be done outside of permitted hours and Highways England’s payment mechanism (‘paymech’ for short) imposes hefty charges should this happen. Cramming such an ambitious operation into a narrow 55-hour window has never been attempted before, says Smith.

“Previously we’d have installed a complex temporary works system using metal plates acting like individual drawbridges so that we could work under them in sequence,” he explains. That method was an innovation in itself, says Smith, but it still drags the joint replacement operation out over an extended period. “This is much quicker and much safer,” he says.

Smith explains that the key to making the ‘weekend wonder-shifts’ work within the 55-hour timescale was the concrete mix employed. Jackson’s concrete supplier, Axtel, had previously worked with the contractor to perfect a rapid-strength mix for pavement-quality concrete (PQC) bay replacements on sections of road that are paved with concrete rather than blacktop.

Smith says that Jackson realised that it could use that same concrete mix – slightly ‘tweaked’ for the application – on joint replacements. Whereas a standard concrete mix for this type of work would take 21 days to cure properly, Smith says Axtel’s rapid-cure mix can reach the required strength “in six to eight hours”.

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While the rapid-cure concrete made the job possible, the challenge of planning and executing the ‘weekend wonder-shifts’ remained. Jackson organised the work into six consecutive shifts working back-to-back continuously over the weekend. Not only did the contractor have to ensure that each change-over happened smoothly and without undue delay, but also it had to ensure that its suppliers could deliver on time and without any difficulties.

The work was planned literally minute-by-minute, says Smith, with numerous contingencies and escalations allowed for and strategic hold-points built into the programme. “It was like a military operation,” he says; “we had to be able to give Highways England an assurance that everything was going to complete on time”.

The first two joint replacements were completed towards the end of 2019 and Jackson replaced two more identical joints at the same junction in November and December 2020.

“Productivity during the ‘weekend wonder-shifts’ is exceptionally high, as they give us the opportunity to get the bulk of the removal and replacement of the joint done in one go,” says Smith. “It’s been a real win for our client,” he adds. 

Adopting the ‘weekend wonder-shifts’ allowed the contractor to deliver the schemes for just 30% of their original predicted cost and with only 10% of the man-hours originally envisaged. Jackson estimates that the new method cut around three months from the project programme.

Road users also benefited hugely. “Essentially the traditional method would require approximately 160 road closures whereas the new method took around 12 closures give or take a few,” says Smith. “The closures were over a shorter period and so were less impactful than over a four-month period per joint.”

So impressed was Connect Plus with the success of this new method that it has changed its joint replacement strategy for the remainder of the contract. Besides the cost reductions and efficiency gains, the use of ‘weekend wonder-shifts’ improved safety, with workers spending less time on the road. 

In fact, says Jackson, no incursions or safety observations relating to traffic management were reported on this job.

Other benefits have included environmental gains, with a reduction in the use of materials including asphalt, concrete and steel bolts and an estimated 60 tonnes of CO2 being saved across the project. 

This article was first published in the March 2021 issue of The Construction Index magazine.  Sign up online.

In recognition of the new method’s achievements, Jackson was awarded the London and South-east regional Construction Excellence awards for the project last year. Last month the team was declared the winner of the national awards.

Meanwhile Jackson Civil Engineering has shared its knowledge with other supply chain partners and areas beyond Highways England’s Area 5.

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