Background to specification
The €75 million contract for the B1 extension of Dublin’s Luas light rail tram system extension was awarded by the Railway Procurement Agency to a joint venture between Bowen Group (Ireland & UK) and Somague (Portugal).
The JV was commissioned to fulfil a design build contract to complete a 7.5km extension to the existing Green Line. The contract included eleven tram stops, substations, and the track and systems infrastructure from Sandyford to Brides Glen in South County Dublin.
The contractor was intent on selecting an option which could not only deliver on performance but also maximum value and efficiency, and chose to install quick-fitting steel stormwater attenuation tanks from Tubosider.
The contractors knew from the start that Tubosider specialise in prefabricated tanks to BD loadbearing standards that could be manufactured within weeks and installed within days. Such systems have been widely used beneath heavy traffic such as at Dublin and other airports.
An in-situ concrete structure would have taken three months to build, settle and cure. The other option – concrete pipes, delivered in shorter lengths but weighing a few tonnes per section – would use more storing space, require cranes and would still require an in-situ structure to serve as a manifold.
Value engineering in meeting local discharge limits, maintenance and operability requirements also played a crucial part in the decision, as did environmental and safety considerations. Steel tanks offer a reduced carbon footprint in manufacture and reduced main contractor attendances, all important factors for the client.
Given the confined nature of the tram route alignment and other local constraints, another requirement was to ensure the particular position and design of the tanks would provide safe access and maintenance in relation to the operation of the line.
Solution & benefits
Each tank had to be even more tailored than usual, being positioned beneath the track and impossible to access in the line of the trains. Specially designed with an extended manifold and access shafts at each end, their easy in-tank monitoring and maintenance would have been difficult for an in-situ concrete tank to deliver.
The first and largest tank, with a capacity of 430 m³ (14,830 ft³), was sized and designed and delivered in three weeks, then installed in just four days – a major asset to the Track and Civil Works in progress. Another two tanks were also delivered from the five commissioned from Tubosider for the B1-400 phase.
The steel sections were lighter, easier and safer to handle than other systems, and did not require lifting cranes or special equipment to install.
“We have seen a number of great benefits from choosing Tubosider’s system,” said the project surveyors. “It has proved a more cost effective option than concrete, but equally the turnround time and the ease with which the tanks can be installed have been a major factor.”