Image: courtesy of Iain Ferguson
RPS acted as the contractor’s designer on behalf of Transport Scotland for this major £400m DBFO project comprising a new M8 dual three-lane motorway section between Bailliestown and Newhouse, with three new grade separated motorway junctions and amendments to two existing motorway junctions.
The Raith Interchange is an extremely complex interchange of strategic importance to Scotland with over 100,000 vehicle movements per day. Designers faced a unique engineering challenge at Raith in developing a new elevated free flow rotary above the existing roundabout and the construction of an underpass beneath it, all in a flood plain adjacent to an area of environmental sensitivity, massive artesian pressures, variable ground conditions and live traffic conditions.
Earthworks for the underpass includes a propped secant pile cutting 600m long and 15m deep in loose sands and gravels under artesian conditions which required complex temporary works design and dewatering. The underpass, located in the Clyde floodplain, presented significant flotation problems requiring a dense array of ground anchors secured into bedrock. The design team of permanent and temporary works designers, structural and traffic management engineers, hydro-geologists and environmental scientists worked closely to achieve the optimum solution.
As part of the production of the GIR, we undertook a detailed desktop study of the available geotechnical information. This included 12 phases of ground investigation for the project, ground investigations from the 1960s for the original motorway construction, as built records, mining records, including coal authority reports, historic maps, contamination assessment reports for adjoining developments and other geological maps and memoirs.
The footprint of the M8/M73/M74 covers an area of dense historic mine workings, with a multitude of shafts/adits and worked out seams at shallow depths that posed a significant risk to the construction works. A programme of remediation, comprising extensive cavity probing and consolidation grouting was implemented prior to construction. The project was awarded the ACEI Design Excellence Award (Overseas) in 2018.