M4 junction 11: What a relief

Anyone who has driven along the M4 near Reading in recent months may well have noticed the overnight appearance of two new bridges over the motorway at junction 11. What they may not realise is that these bridges are part of a massive scheme that should make travelling through that junction far less fraught and considerably quicker in future.

Junction 11 is where the A33 − a major route in and out of Reading − intersects the M4. It carries 70,000 vehicles a day, and is a fairly notorious bottleneck, with traffic getting snarled up on the gyratory above the motorway at peak times, and even queuing back down the slip roads and onto the motorway itself.

Plans to improve the junction have been on the drawing board since 1999, and are now being put into action in the form of a major improvement scheme. In simple terms, this consists of building an entirely new, larger gyratory around the outside of the existing junction roundabout.

Bigger capacity

This will have a much bigger capacity than the existing system, with dual four lanes replacing the existing dual three. The off slip roads will also be widened to take four lanes of traffic.

Extra capacity will be created by taking buses off the gyratory and creating a dedicated bus only route through the junction using part of the original roundabout. Cyclists and pedestrians will also benefit from the improvement with the construction of new paths for all the desire lines, including four overbridges to ensure that they do not have to come into contact with the main traffic.

Incorporated within the scheme are improvements to the A33 south of the junction, including a new signalised junction and a connector road. These are designed to stop local traffic getting caught up at the motorway junction, and also to enable traffic managers to move vehicles through the gyratory in blocks controlled by signals.

“At the illustrative design stage, it was clear that we had to keep the junction live throughout construction.”

Malcolm Fillingham, Peter Brett Associates

Two local authorities − Wokingham Borough Council and Reading Borough Council − are involved in the project, as well as the Highways Agency, but it is Reading that is taking the lead. The A33 is the only road into Reading that comes under the Borough’s jurisdiction, and is seen as a gateway to the town.

Funding for the scheme was secured in 2006, and the project went out to tender as a design and build scheme in August 2007. Eight months later Laing O’Rourke was awarded a £40M contract to deliver the junction improvement, with just over two years to complete it.

Outline design for the scheme had been done by Peter Brett Associates (PBA), and the consultant took over as site representative on behalf of the client once the D&B contract was awarded. PBA project manager Malcolm Fillingham says: “At the illustrative design stage, it was clear that we had to keep the junction live throughout construction. The perfect solution would have been to demolish it and start again − but that’s not possible.”

Maintaining movement

Instead, there are 10 different stages of traffic management designed to keep traffic moving. “The whole concept is to do as much work off line as possible, then do the major traffic management switching,” explains Laing O’Rourke project leader Sally Cox. “The design has been phased so we could get the earthworks out and start filling the off line areas before the winter [2008].”

The new alignment requires the construction of substantial embankments, both for the new slip roads and the work on the A33 south of the junction. “We have used a few different types of material,” explains Cox. “We chose PFA [pulverised fuel ash] for the south east embankment because we wanted to get on and do it quickly, and it’s not weather susceptible.”

The south east embankment is the largest, measuring up to 8m in height. At the base is a drainage layer of recycled crushed glass, with the bulk of the fill − 83,000t in total − being PFA from the nearby Didcot power station. The PFA is sealed by a layer of clay on the top and face of the embankment. Topsoil is placed on the face slope.

A 2m high embankment on the A33 was built entirely from clay, as the contractor knew it would be completed before the onset of winter. For the two embankments scheduled for construction during the worst of the weather − the 6.7m high south west and 8m high north east embankments − Laing O’Rourke opted for granular material. “This is a flood plain,” explains Tim Boam, site representative for the contractor’s designer Aecom. “Once it gets wet it stays wet.” A total of 190,000t of granular fill was used in the embankments.

“The whole concept is to do as much work off line as possible, then do the major traffic management switching. The design has been phased so we could get the earthworks out and start filling the off line areas before the winter.”

Sally Cox, Laing O’Rourke

All the fill material has been sourced locally, and much of it is recycled, including the 7,000t of crushed glass being used in the south east embankment and for narrow filter drains across the site. “The clay has come from a local source, because there’s not enough good clay off the scheme,” explains Cox. “The granular fill is crushed concrete material − some of which we’ve recycled on site ourselves.”

Where the existing carriageway surface has been planed out, the planings are being re-used as capping material, with a total of 19,000t being sourced from site from external supply. A further 38,000t of recycled/crushed concrete has been used for sub base under the road construction.

The embankments presented one of the main design challenges on the project for Aecom, according to Boam. “All the embankments had to fit into the site boundaries, and they had to be designed to suit the different materials. The finished slopes vary depending on the types of material used.”

For many motorists on the M4 the most conspicuous element of construction is the two new bridges that have suddenly appeared above the carriageway.

Integral structures

These 40m single span structures will carry the new four-lane dual carriageway gyratory over the motorway and have been designed as integral structures, with the deck and abutments working as a portal frame.

Each reinforced concrete abutment sits on seven bored piles, all 24m long and 1.2m in diameter. There are six weathering steel bridge beams on each deck, which sit directly on top of the abutments and are cast into them, then topped with precast concrete planks and an insitu deck. As there are no bearings, and the weathering steel requires no painting, it should be a lowmaintenance solution.

The beams were lifted into position during two overnight closures of the M4 − one in July and one in August − with a crane sitting on the carriageway and the beams braced together in pairs.

“Construction of the new gyratory bridges is on the critical path, but there are plenty of other things that need to be done concurrently.”

Tim Boam, Aecom

Abutment construction was far from simple, as the walls sit within the embankments that support the existing slip roads, and sheet piles had to be installed so the material in front could be excavated. The next challenge is to build the wing walls, which will also require temporary sheet piles.

“Construction of the new gyratory bridges is on the critical path, but there are plenty of other things that need to be done concurrently,” explains Boam. “On the south east embankment we can’t complete the backs of the wing walls, because they hit the line of the existing gyratory. So we have to build the new slip road in order to release the rest of that work.”

On the north side traffic will also be taken onto the new slip road, but this area of the job is very complicated, as the existing road is a lot higher than the new section will be. This is one of the few areas where Laing O’Rourke is building a section of temporary road to carry traffic, to release the space where the new “superhighway” will be built − a section of the junction that will carry four lanes of normal traffic in each direction and the dedicated two-lane bus route.

“Once that’s joined all the way through, we can send traffic into Reading onto it, and release the north side of the job,” explains Cox. “What’s critical is to get traffic onto the new route [on the north east side] to release the far corner of the junction, so we’re concentrating on this.”

At the same time, the contractor is also excavating the areas where four new wetland areas will be constructed to take run-off from the new road. All new additional surface water has to drain into two existing systems and, ultimately, to an existing stream on the north west side of the junction. Maximum volumes in this stream are not allowed to exceed existing levels, so flows have to be attenuated via flow control chambers.

Other activities include continuous flight auger piling for gantry foundations, and construction of a large mound on the north side of the gyratory that is part landscaping feature and part structural support for a new cable stayed bridge on the cycle/pedestrian route. Construction of the entire junction project is due to be completed in summer 2010.

Have your say

You must sign in to make a comment.