Network Rail has a very thorough policy of “lifeing” for all trackside facilities which includes the thousands of bridges, of many different categories, that span across the miles of rail track in the UK.
This means that the individual history of a bridge is maintained to show when, how and of what it was onstructed. This matrix of data identifies the lifespan of the materials, especially iron and steel, and when replacement is required. This triggers the reconstruction of the bridge and usually this operation integrates design updates with modern materials to extend the lifespan going forward.
This was exactly the scenario for the Pheasants Overbridge which carries the B4239 Lighthouse Road in Newport over the South Wales Main Line. The original overbridge consisted of a single steel plate girder bridge at a skewed angle to the railway line, set between masonry abutments and wingwalls. The span comprised of a “U” shaped steel profile which was significantly corroded and life-expired. In addition, it no longer met the latest Railway standards for design in terms of loading.
A further element in the decision to replace this aged structure was to increase the clearance under the bridge to enable the improvements being carried out to the electrification on this main line route. The clearance from track to underside of the bridge was to be improved by nearly one metre so substantial increases in the embankments were required.
The approved design was completed by HBPW Consulting and Carillion were selected as the main project contractor. The replacement superstructure comprised of a single reinforced pre-cast concrete “U” shaped span sitting on new pre-cast concrete cill-beams as bearers. In turn these sit on the existing retained masonry abutments and wingwalls following the demolition of the original superstructure.
The raised height of the span required substantial modifications to the earth abutments that lead down to road level on either side. This is where the attributes of Leca® Lightweight Expanded Clay Aggregate helped achieve the required height without increasing the loading on the original substrates.
Paul Monaghan, project designer at HBPW civil and consulting engineers, based at Retford, Nottinghamshire, was impressed with the attributes of Leca® Lightweight Expanded Clay Aggregate: “The requirement to lift the bridge deck 800mm would have substantially increased the loading on the base substrates and this would not have been acceptable. By cutting down the existing embankments, putting in the retaining trough, and filling with Leca® Lightweight Expanded Clay Aggregate, we have a zero weight profile meaning that we have gained the height required without adding any weight at all to the foundations. Leca® Lightweight Expanded Clay Aggregate gave us a brilliant answer to this problem. It compacts perfectly to give a good stable surface, ideal for the geotextile and roadway capping.
A totally natural product, Leca® Lightweight Expanded Clay Aggregate is formed by heating and firing natural glacial clay in a rotary kiln at temperatures up to 1150°C. This process transforms the clay into lightweight ceramic granules with a hard shell and porous core. With a bulk density of just 0.3 tonnes per cubic metre, Leca® Lightweight Expanded Clay Aggregate has excellent insulation properties, is free draining, fire resistant, frost resistant and chemically inert with no hazardous properties. Used as a lightweight aggregate fill in many civil engineering and construction applications, Leca® Lightweight Expanded Clay Aggregate reduces the weight on weak substrates and against retaining structures by up to 75% over traditional fill and eliminates expensive settlement delays, is easily handled and quickly installed.
Seth Messenger, site manager for Carillion on the Pheasants Overbridge project, was also enthusiastic about the ease of use of Leca® Lightweight Expanded Clay Aggregate. “We could not use traditional fill to raise the road embankment height so a pre-cast concrete trough was dug into the top of the existing earth embankment and filled with the Leca® Lightweight Expanded Clay Aggregate. Cost and availability played a big part in our selection decision. When we found that stocks are held nearby in Newport, very close to the site, it made the delivery scheduling very easy. Leca® Lightweight Expanded Clay Aggregate is easy to handle with a 360 excavator machine which we also used to compact the material into place. A geotextile membrane went over the Leca® Lightweight Expanded Clay Aggregate and then the roadway sub-base and finished surface. The job is neat and tidy for the client and the new overbridge looks stylish and was quickly back into operation.”