'One weekend closure. 400 tonnes of steelwork. One River Thames diversion. One stunning sunrise. One bridge widened: the M4 smart motorway upgrade has reached its halfway point with its most complex work yet.'
Highways England's M4 upgrade team has shared insights from a weekend engineering wonder in an exciting new video, describing the culmination of around four years of planning and investment.
The Thames Bray widening work - an engineering centrepiece of the M4 smart motorway scheme - was managed by a Balfour Beatty VINCI joint venture on behalf of Highways England. Bridge experts Cleveland Bridge carried out the work.
The work on the 60-year-old structure saw three bridge beams installed, with work taking place over the weekend 30 May-1 June and completed in a further 24 hours the following weekend.
Mike Grant, Highways England delivery director, said: 'Creating extra lanes on the M4 requires replacing 11 bridges over the M4 so they are wide enough to fit the upgraded motorway. The Thames Bray bridge was different. It doesn’t carry a local road or footbridge over the M4, it carries the motorway over the River Thames. Of course, we couldn’t just knock this bridge down and rebuild it, this would put the M4 out of action for months. An innovative solution was needed to overcome this challenge.
'The only option was to build the extended bridge elements on a nearby site and then assemble them over a single weekend to accommodate the extra lanes required for the four running lanes on each side of a smart motorway. This was a great example of how to keep England’s strategic road network open while making vital new capacity improvements for our road users. We’re proud to say that this is what we achieved over the weekend.'
The existing bridge was originally built in the 1960s and needed to be widened to accommodate the two extra running lanes for the smart motorway upgrade. This created the huge engineering challenge of tying three new beams into the existing structure.
The beams – east, west and central – weighed nearly 400 tonnes combined, and were placed to the north of the existing bridge and connected by structural bracing, using a technique known as ‘splicing’, which allows the bridge to be asymmetrically widened to the north.
Design work was done in line with Environment Agency and River Authority regulations and is sensitive to local ecology, river navigation and wildlife.
Peter Anusas, Balfour Beatty VINCI project director for the M4 smart motorway upgrade, said: 'Our integrated team and supply chain partners have proven their industry-leading, advanced engineering capabilities, while implementing our COVID-19 site operating procedures to ensure the continued health, safety and wellbeing of all those who work with and for us, as well as the general public. Our culture of teamwork and technical excellence saw us successfully reopening the bridge to traffic on Monday morning with minimal disruption.'
Mike Fowler, Balfour Beatty VINCI engineering director said: 'The new widened structure was designed to behave in the same way as the existing structure while replicating the arched effect from an aesthetic perspective.
'In preparation for the weekend’s beam lifts, we built two new abutments (bridge supports) on either side of the river. A dry dock working environment was needed for this, so small sections of the Thames were sealed off and water pumped out to create this. Temporary sheet piled walls were then installed next to the existing bridge to ensure everything remained stable during the construction of the new east and west abutments.'
Each beam took around 13 hours to lift into place and tension. When each bridge section had been lifted into place, they were surveyed using GPS to ensure correct positioning and all the sections fitted together.
Large tension cables on the east and west back spans secured the beams onto the abutments with large anchor points within the foundations. Each cable has a tension capacity of 295 tonnes to counterweight the weight of the bridge and the associated traffic and provide the facility to adjust the profile of the bridge during the installation.