Embankments on Piles
UK Highways Agency
Capita Symonds (Grontmij)
The Mossband Embankment forms part of the new M6 motorway extension, crossing the West Coast Main Line railway near Guardsmill, Cumbria. The geotechnical design involved piling, basal reinforced platforms, ground improvement and reinforced soil slopes. The M6 Extension project between Carlisle and Guardsmill completes the M6 motorway to the southern end of the M74 which runs northwards into Scotland. This section of road was formerly dual carriageway, (A74T), posing safety problems and conflicts between local and agricultural vehicles and high speed traffic, especially at entry / exit points.
The 9km long project was essentially a widening process to increase the running lanes of the A74T to three in both directions. The new embankment at Mossband was constructed offline to carry the motorway across the West Coast Main Line, to the west of an existing viaduct, which it replaced. Additionally a new bridge crossing the River Esk, to the south of Mossband, was also constructed.
The geotechnical challenges unique to the Mossband Embankment were primarily the depth and weakness of the alluvial deposits under the proposed route.
In total approximately 4300 piles were used for the approach embankments. In areas where the embankment was not piled the design specified the use of band drains on a variable triangular spacing. Approximately 18,600 drains were installed through a Class 6C drainage layer /working platform down to the sands and gravels thereby allowing the dissipation of excess pore water pressures in the alluvium to occur as the embankment fill was placed. Above the VCC piles a low strain, high strength, 1200kN/m, geosynthetic reinforcement geogrid was used to provide lateral restraint across the pile caps and transfer the vertical loading to the piles themselves.
The side slopes were again reinforced with low strain, BBA certified geogrids, with strengths ranging from 110kN/m to 35kN/m. The reinforcement was placed at 600mm vertical lifts with no wrap around to the front face. A 3D geomat was detailed to retain a veneer of topsoil over the face of the embankment to support the subsequent vegetation of the slopes.
• Reduction in embankment piled footprint by steepening side slopes with geogrid reinforcement
• Use of site won materials within embankment to enhance sustainability
• Low strain high styrength basal reinforcement used for compatibility with concrete piles
• Customised roll lengths minimising waste and increasing site efficiencies.