How do you widen a dual three-lane highway to dual four lanes and add paved shoulders, all within existing highway limits? That was the dilemma faced by contractor Balfour Beatty Civil Engineering Ltd. and designer Gifford Graham and Partners when they entered a design-build contract to widen one of the busiest sections of the M25 motorway in Surrey, England—the 11.8-mile stretch between Junctions 8 and 10. In addition to the tight site constraints, three lanes of traffic had to be maintained in both directions during road widening. The two-year project was completed in the summer of 1997.

Because the road had to be widened within existing boundaries, space was at a premium, particularly at existing structures and at cut/fill transition areas. To help overcome space limitations, two solutions were adopted:

  • As much widening as possible took place within the median, which minimized the amount of outer right-of-way widening required and the impact on the existing earthworks and landscaping. This was achieved by installing two permanent concrete barrier walls in the median, one alongside each new lane.
  • The installation of a linear slot-drain system within each paved shoulder, which made the most effective use of the land available and allowed reuse of as much of the existing cross-drain system as possible.

To facilitate construction, Balfour decided to place both the concrete barriers and slot-drain system by slipform paver. Despite the confined work area, the contractor slipformed nearly 1,500 linear feet of slot drain per shift, more than could have been achieved with a conventional piped drainage system.