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Reinforced embankments for the causeway for Pont Briwet Project, Penrhyndeaudreath, North Wales, UK

The Pont Briwet Project consisted in the replacement of an historic timber trestle bridge with a new 136m long concrete viaduct and the upgrade of the approach road to the north and south of the crossing. The site is set within the flood plain of the Afon Dwyryd in north Wales, with tidal levels up to 4.6m. A key challenge of the project was building the viaduct within the tidal estuary whilst meeting, at the same time, the requirements on flood risk and water quality within an Area of Conservation of Natural Resources. Reinforced earthwork embankments using Tenax Geogrids with specific granular material were used as a solution to construct the causeway allowing the installation to take place directly into open water. The height of the embankments ranged between 2.0m to 4.10m with slope angles of 1V:1.5H and 1V:2H. The causeway working platforms were required to support large tracked piling rigs and cranes, in addition to a 800 tonne mobile crane to lift the bridge beams into place. During the construction the causeway was completely submerged on various occasions yet needed to be back in use immediately after the tidal events. The design of reinforced working platforms used as “safe areas” on the ramp approaches, above the predicted flood levels, allowed the tracked plant to remain safe during these events. The project was finished in July 2015 and the new viaduct was open to the public in August 2015.