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Cracking resurfacing

COVER STORY - M11: Crack and seat and strict enforcement of a 40mph speed limit was the secret of a motorway resurfacing contract which finished 30 days early. Ty Byrd reports from the M11.

Junction 6 northbound on the M11 is where M25 traffic seeking Stansted Airport and other points north can join the London to Cambridge motorway. It is marked by sliproads of tight radius and in recent weeks, thousands of cones. Cones mean roadworks and roadworks mean frustrated motorists.

The Highways Agency is acutely aware of the impact of its roadworks, not least when there is another set 15 or so kilometres further on. Two junctions up the Stansted turnoff is being rebuilt (see page 27).

In fact, any angst was kept to an absolute minimum by astute planning, intelligent traffic management and competent contracting. The result was that the Junction 6 job - let as a 66 day project - was completed last weekend, 30 days early, a triumph for all those involved.

'We wanted the M11 work carried out safely for both passing traffic and work people. We wanted it done quickly and in a way which gave us best value, ' says the Highways Agency's Arvind Ranpuria. Ranpuria is project manager for M25 Sphere, the maintenance manager partnership including the HA and Mouchel which is responsible for the orbital and its radial connections.

A 6km stretch of the northbound carriageway of the M11 was to be rebuilt, from just above Junction 6 towards Junction 7. The original concrete surface displayed all the noise, joint thumping, rough riding and mud pumping which characterise highway slabs close to their sellby date.

'By last year, it had reached a critical stage, ' Ranpuria says. 'If we left it much longer, the carriageway would have required full depth reconstruction. But if we moved quickly, crack and seat topped by an asphalt overlay was an option.'

Crack and seat is a technique where the existing concrete slab is cracked transversely at 1m centres to create mini slabs.

These are then relatively easy to seat into the existing pavement granular base by loading with a 20t pneumatic tyred roller. This creates a good sound foundation for a flexible overlay and prevents reflective cracking of the new surface.

The process is much less expensive than full depth reconstruction, much quicker and, because it does not require removal and disposal of the existing concrete, much more environmentally friendly. Topped by asphalt, it is also highly effective in improving driver comfort and reducing traffic noise. Two sections of the M20 near Maidstone that were 'crack and seated' prompted letters of praise to the local press from grateful motorists and nearby residents.

So, a happy choice when the circumstances are right. Engineers from M25 Sphere specified crack and seat in the M11 bid documents, the concrete to be topped by a minimum of 150mm of dense bitumen macadam (BDM) with 30mm of proprietary thin wearing course above that.

Some carriageway reconstruction was required, at overbridges to maintain clearance, and at underbridges to avoid overloading the structure. In addition, existing drains had to be refurbished and new ones built between the three lane carriageway's inside lane and hard shoulder to relieve hydraulic pressure.

'Draining the subformation was a problem we had to tackle as well, ' says Mouchel's Patrick Jones, serving as M25 Sphere's deputy project manager on the job. 'This was the source of the groundwater that was being pumped by the slabs.'

M25 Sphere decided to progress the M11 works as a lane rental scheme, with a good bonus for each clear day saved and a hefty penalty for days lost.

The two envelope system was employed and out of the slightly complex 40% quality, 60% price calculation came a clear winner - RMC Surfacing Southern.

'We weren't the lowest on price, but we did achieve the highest quality mark, ' says RMC regional civils manager Richard Butterfield. 'As a contractor, we are good at pulling the right people together with the right skills and as a consequence, are very capable of meeting high pressure demands.'

What came out of the teamwork early on was a highly effective traffic management system.

RMC subcontractor Chevron (London) was responsible for traffic management, but all the principal players contributed to the system's development, joined by Essex police, the transport associations and BAA, the latter concerned about impact of the roadworks on motorists with flights to catch.

There were three main aspects to the traffic management: reducing speed to improve safety, providing access for RMC to the right bit of carriageway at the right time while maintaining traffic flows, and general signing to warn of the roadworks.

Thanks to 20 Tascar cameras and the determination of Essex police to prosecute motorists who drove through the roadworks at speeds greater than 40mph, the clear limit, speed reduction was very successful.

'There were few shunts among the travelling public - none of them serious. A major benefit of the 40mph speed limit was that our people on site felt a great deal more secure, ' says Butterfield.

Keeping vehicles flowing in the right parts of the site was helped by two factors - extensive use of CCTV, in particular to gauge traffic build-up, and give early warning of events likely to impact on traffic flows, such as an accident on the nearby M25;

and by a tidal flow system in which the direction of flow showing the biggest hold ups got given an extra lane.

'We may have only been resurfacing the northbound carriageway but, of course, this affected southbound traffic with our contraflows, ' Butterfield says. 'We always had to keep five lanes open, two in either direction, with the fifth available depending on weight of traffic.'

While crack and seating operations were relatively simple - a guillotine breaker plus roller achieving 3km/lane/shift - RMC's reconstruction and overlay operations were complex, involving up to five layers of bituminous pavement construction.

Up to three 90 to 110mm layers of DBM 50 base had to be put down, followed by 50mm of DBM 50 binder course and then a 30mm surface course of RMC Aggregates' HAPAS approved Viatex thin surfacing.

Each layer was laid to a predetermined width, to provide three running lanes plus a hard shoulder. The different widths were to ensure discontinuity of construction joints through the full thickness of the overlay; to provide sufficient lateral traffic clearance in reconstruction areas and to distance construction joints from wheel tracking zones.

'We also faced the prospect of replacing failed concrete pavement bays, ' Butterfield says.

Highways consultant TRL examined the treated concrete sections prior to overlaying, by coring to ensure cracks had passed right through the old carriageway, and with a falling weight deflectograph to determine stiffness.

Slabs which failed to achieve the required dynamic stiffness were then hurriedly removed and replaced in structural concrete with preinstalled crack inducers to mirror the crack pattern of the adjacent bays, prior to continuing with overlay works.

'We soon got into the swing of things, however, and at peak were laying 5000t of bituminous material a day, ' Butterfield reports. 'We were extremely lucky with the weather and other factors that could have held up our activity.'

RMC's contract was initially worth £6.3M with the final figure set to be boosted substantially due to the company's rapid progress. 'It's not all profit though, ' says Butterfield. 'To accelerate the programme cost us money, spent on the additional resources we employed.'

Ranpuria has the last word on the matter. 'Cost benefit analysis says the additional cost of speeding up the work is greatly exceeded by the benefits to the taxpayer of the maintenance being completed and the roadworks cleared early.'

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