M7/M8 Motorway - Portlaoise, County Laois, Ireland

Leca® LWA lightweight aggregate provided a simple solution for the Portlaoise Joint Venture between BAM Contractors and Dragados when faced with the problem of replacing the new Dublin to Limerick motorway over a small rail tunnel.  The expansion of Ireland’s motorway network included the junction of the M7 and M8 at Portlaoise and part of this section had to cross the Clonkeen railway underbridge, a small arch tunnel over a Bord Na Móna rail line. The design called for the new road level to be raised 3m above the top of the arch. The archway is 6.1m wide at the base with a crown height of 4.3m. Using traditional fill material for the raised road level would have imposed excessive load on the structure. Core samples were taken to determine the strength and rebar pattern of the precast matiere concrete construction. These tests proved that the archway would not be able to support the greatly increased load. The options that had to be considered by joint venture consultants AECOM and Roughan O’Donovan included demolishing the arch and replacing it with a more substantial structure, building a cover slab on new supporting piles above and independent of the existing arch, or find a lightweight material for the embankment that would be substantially lighter than general class 1 fill. These options were analysed for cost, design capacity and the time required to keep the programme on schedule. Leca® LWA proved the most effective solution in all three parameters and the joint venture contractor welcomed the fast build solution it provided. Leca® LWA is a lightweight expanded clay aggregate formed by heating and firing natural glacial clay in a rotary kiln at up to 1150°C. This process transforms the clay into lightweight ceramic granules that have a hard ceramic surface and porous core. The material is extremely light with a bulk density of a fraction of traditional fill material. 1300m3 of Leca® LWA was delivered to site in bulk and placed and compacted by a tracked mechanical shovel.