M7/M8 Motorway | Portlaoise, County Laois, Ireland



Utilising Leca solves a heavy weight problem with building 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® Lightweight Expanded Clay Aggregate proved the most effective solution in all three parameters and the joint venture contractor welcomed the fast build solution it provided.

1300m3 of Leca® Lightweight Expanded Clay Aggregate was delivered to site in bulk and placed and compacted by a tracked mechanical shovel.

Leca® Lightweight Expanded Clay Aggregate solved the weight problem and 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.