Carmarthenshire Council faced with the challenge of building a new highway across the main London to Fishguard rail line has solved the problem with a novel solution.

A large span modular corrugated steel rail bridge has been designed and installed. The construction of the largest and most lightweight corrugated steel bridge, is only the second of it's type in the country.

The bridge has a record-breaking span of 15.5 metres, measures over 52 metres in length and weighs 129 tonnes. As steel is being used instead of traditional concrete or stone, the project has attracted the interest of highways and engineering magazines from around the world.

Now the bridge is in place, the three sections have been joined together and the backfill operation has begun. The bridge installed over the open railway is now being covered with imported material, the roadway will then span the bridge.

Main contractor Costain, appointed innovative retaining structure specialist Phi Group to install the company's load transfer platform technology and reinforced earth solutions to provide support for the road, approach embankments and portals.

Phi Group has designed and built slopes up to 55°using a combination of reinforcement goegrids, and geotextiles within the fill material and incorporated a cellular matting to contain and support the topsoil on the face of the slope.

For the approach road and embankments Phi Group is installing 2620m² of the company's load transfer platform technology. Also 1585m² of the company's Textomur system consisting of horizontal geogrids or geotextiles typically at 600mm vertical centres within compacted cohesive or granular fill material. The facing to the slope is formed using a steel mesh ‘formwork' backed by a geosynthetic facing fabric which retains a layer of topsoil immediately behind the face. The finished slope can then be hydraseeded to establish grass, or planted with appropriate ground cover shrubs.

Phi Group is also utilizing 256m² of it's concrete panel system as bridge abutments. The use of the rectangular panel system maximizes the benefits resulting from reinforced soil by providing a cost effective solution typically 50% of the cost of a reinforced concrete retaining wall. It also benefits from a construction method requiring no structural foundation or temporary propping with all work carried out from behind the panel facing.

Using this method of bridge construction has enabled the project to be completed without the cancellation of any trains. The link road across the bridge when complete, will open up land for development in South Llanelli, reduce traffic congestion and improve access to the National Wetlands Centre.