Exceptionally detailed site organisation, combined with tight logistics, has allowed contractor group MVM to finish an M1 motorway widening ahead of schedule and with minimised traffic disruption. Adrian Greeman reports.
On paper it seems such a simple concept. Take an existing stretch of motorway, put in an extra lane but without extending the original road and do all of this while keeping one of the UK’s busiest routes flowing. These demands could give the most experienced engineer a headache and a secret longing for three wishes from a genie or the loan of Harry Potter’s wand.
But this is what Vinci has pulled off with its “compact motorway” project on the M1, carried out in conjunction with Morgan Sindall and Sir Robert McAlpine. It refers both to slightly smaller permanent lane widths in a newly widened four-lane section of the motorway, and to the reduced lane widths used during the works to keep traffic running on the busy road, without having to impose contraflows.
“The motorway width is kept to 13.95m rather than 14.7m,” says Steve Wrenn, project director on the £340M scheme for client, the Highways Agency. The reduction is taken from the outer three lanes.
By also using narrower than usual lanes for the temporary 50mph section during construction, motorway traffic flows could be better maintained with cars and lorries passing by the site works on their own “natural” side of the road throughout the project, without changing direction.
“And that is a big advantage in traffic fluidity,” says Wrenn. “The problem with a contraflow is that it requires each driver to make a decision about which lane to stay in, and when he turns.”
The accumulation of tiny hesitations by drivers can have a surprisingly big effect on traffic streams, causing slowdown wave effects that can cause serious congestion fairly rapidly, especially when traffic is busy.
High flows and congestion are already a problem for much of the M1 and the rational for widening it in the first place. On this 23.5km long project section the road was carrying 65,000 vehicles daily between Nottingham and Sheffield - junctions 25 to 28 - on a motorway that was originally designed for 13,000 vehicles/day. Now completed as four lanes each way, it can handle 85,000.
When MVM, the joint venture between Morgan Est, Vinci and Sir Robert McAlpine, proposed using a different construction method during the “early contractor involvement” stage of its contract, it was well received.
“The problem with a contraflow is that it requires each driver to make a decision about which lane to stay in, and when he turns”
Steve Wrenn, Highways Agency
The contractor, which had a detailed design and construct brief for the project, suggested moving the three running lanes over towards the central reservation and reducing their width by a couple of hundred millimetres each. That would leave a narrow strip of the inner lane free, together with the hard shoulder and the existing verge as working space.
Once the road was widened the lanes would be moved over and the central reservation could be opened up in its turn for the road rebuild and the replacement of the wider central reservation with new step type concrete barrier, giving a little more width.
But despite the advantages, the scheme posed a number of challenges. For the contractor it meant that there would be only a very narrow strip of land to work in, around 3m from the hard shoulder plus 1m of blacktop, and another 2.5m perhaps of soft ground verge. Within that space MVM needed to find room for a haul road of nearly 4m width and working space.
There was no option for reducing the site road size either, because motorway emergency services required it for full access through the works at all times.
Fortunately, just about all the space could be used. MVM worked behind Varioguard steel barrier, which is heavy enough and strong enough to allow work to proceed in relative safety.
“It is not anchored except at 500m end points and will move slightly under impact, so you have to leave a 600mm wide free deflection zone,” says Vinci’s Pierre Villard, construction manager for the project. “But otherwise the space is free.”
Also, the works to be done, with one exception, did not include any complex structures and few foundations, mainly piling for abutments. This was the result of deliberate policy by converting the hard shoulder to a fourth lane and remaking the verge as a new hard shoulder. At the 22 overbridges the hard shoulder stops and it is now discontinuous, with warning markings on the road in front of a bridge abutment; it resumes on the other side. The measure means that the existing structures could be left intact and the widening carried out without any major rebuilding.
Reduced land take
The overall scheme is also designed to reduce land take in other ways. Rather than extend the road, embankments have been steepened or retaining walls added to keep within the existing road edges rather than add to the overall corridor width. For underbridges, the abutments were extended and then precast concrete beams were used to form additional road deck.
MVM’s early involvement with the outline design reduced the structural work further. Using more detailed ground investigation all possible locations were found where the ground could cope with a steeper batter, or soil nailing could be deployed, to minimise the need for retaining walls or other complex measures.
It was doubly important to complete these works at the beginning of the project because they created additional width for the follow-on work gangs and subcontractors, on drainage, road base, blacktop, lighting and finishing. But even with extra space the control and logistics of the site were critical.
“For traffic safety reasons the only access onto site was at the ends where the junction slip roads were located,” explains Villard. “There were some coned entrance points along the sections but these were only for emergency vehicles.”
A specialist traffic and logistics department was set up to plan and control all traffic movements, spacing them out as much as possible, and arranging for stops at designated passing places if needed. All the work crews had to arrange and clear site deliveries with the department.
“For traffic safety reasons the only access onto site was at the ends where the junction slip roads were located “
Pierre Villard, Vinci
Alongside this the joint venture instigated what it referred to as “work space booking” - a detailed micro-management system for the different work gangs. Each one was allocated an exclusive working area in the corridor, changing on a daily basis, within which it had control. Layout of stopping points, ingress and egress was set down.
“Inside, perhaps a 150m length, they could determine operations, even sending dumpers back and forth down the haul road,” adds MVM chief engineer Chris Ilsley. “But otherwise they had to stay clear.”
All this meant major possessions were kept to a minimum and these were restricted to one extra lane, done at night during low flow periods. Two major 12-hour possessions of the whole road were needed; one for the only overbridge to be replaced to be demolished, and then one some months later when its replacement was lifted in.
“It was a junction bridge with slip roads and had to be widened,” says Villard.
Once the sides had been widened, the traffic lanes were moved over and the central section could be started.
This was a wider space, which was “like a breath of fresh air” says Ilsley, although it had its owncomplications, particularly for site access.
“You are in the middle of the road, so slip road access is not possible and you must have entrance and exit into the traffic stream,” says Villard.
One haul road ran on each side of the centre, with two entry points and two exits, each with a speed-matching lane coned-off to allow trucks to accelerate or decelerate, before joining or leaving the “civilian” traffic flow.
As a result throughout the 31 months of the project there was not a single incident involving site interactions with the motorway traffic.
Both contractor and client are pleased with the outcome particularly as reorganising the site sequences and the over micro-management helped achieve a half-year time saving.