A465 Heads of Valleys Carno Arch
The Heads of Valleys Dualling project is one of the Welsh Government’s key drivers for promoting social and economic regeneration in the area. The new road crosses over the Carno valley on an embankment of earth, to maintain the flow of the existing spillway down the valley from the Carno reservoir, and to provide access to the Welsh Water treatment works an arch underpass was designed and supplied by Reinforced Earth Co. Ltd. (RECo). The precast arch structure RECo provided has a span of 18m and a rise of 9.6m. The arch has more than 22m of backfill above the crown. Reinforced Earth arches are twin precast concrete segmental arch units which when erected connect at the crown. The arch strength is developed by soil/structure interaction. The geometry of the arch and the large backfill made this a complex structure to design. The arch has a 1:20 longitudinal fall. This created a problem for erection stability, RECo designed and supplied special unit geometries to enable the units to be erected quickly and safely. The arch portal units have traditionally been designed with a reinforced concrete collar beam, this can make the installation complex and involves substantial temporary works and working at height. To eliminate these hazards RECo designed the portal units as free-standing cantilever structures with a small in situ joint.
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