Lafarge solve M25 hard shoulder retaining wall dilemma

Lafarge are currently installing hard-shoulder slipform retaining walls at locations within the motorway widening programme being undertaken by the Skanska Balfour Beatty joint venture, on behalf of the Highways Agency, at Junctions 27 to 30 on the M25 motorway in Essex.

The M25 DBFO contract is a rapid road-widening scheme requiring completion of three projects: widening between junctions 16–23; widening from junctions 27–30 to be completed by July 2012; and refurbishment of the Hatfield Tunnel, which was completed by mid-2011. The contract also includes 30 years’ operation and maintenance of the M25 motorway, which Connect Plus Services took over from September 2009.

The widening programme is designed to provide an increase in the number of traffic lanes from three to four in the clockwise and anti-clockwise directions, without the purchase or acquisition of any additional land.

The slipform retaining walls, which are up to 2.3m high, offered main contractors Skanska Balfour Beatty the opportunity to solve two problems with one innovative solution, saving on materials and CO2 emissions. The original plans involved the use of traditional steel piling in locations where significant earth retention was required, with concrete slipform barriers to be placed in front. But looking for an alternative to steel, Skanska Balfour Beatty turned to Lafarge Readymix – suppliers of concrete to the scheme.

The idea was to provide a concrete barrier high and solid enough to act as both a retaining wall and crash barrier. Readymix spent three months trialing concrete formulations before construction began to be able to slipform the wall in one unit.

Mike Thomas, materials manager for Lafarge Readmix said: ‘Our challenge was to come up with material which was workable enough to be moulded into a slipform, but stable enough not to sag under its own weight. Usually slipform barriers are around 1m tall, but for the hard shoulder slipforms to also act as retaining walls this height needed to be doubled. Technically, this was quite tricky but we were able to refine our mix until we achieved a suitable material. As far as we know these hard shoulder slipform retaining walls are the tallest in Europe.’

Slipform sub-contractors PJ Davidson made a number of engineering changes on their paver machine to allow the higher concrete wall to be laid. Parts of the machine were strengthened to cope with the increased pressure of the concrete and the mould was re-profiled to accommodate the extra material. 

Graham Keddie, Lafarge Readymix mobile contracts manager with responsibility for the M25 batching plant, said: ‘This is an innovation we will carry forward on to other projects. I think we are all proud of the team for coming up with a solution which not only maximizes safety, but also minimizes construction time, cost and materials.’