Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Super-props brace Irish tunnel project

Six giant super-props are being used to brace a casting basin where tunnel sections will be built for the new 570M Euro (£390M) Limerick Tunnel in the Republic of Ireland (GE March ’07).
To support the sides of this excavation, Roadbridge, one of four contractors forming the DirectRoute (Limerick) Construction joint venture, has driven steel sheet piles into the ground and is using Groundforce Shorco 250t capacity struts to support lateral loads during temporary works.

The 675m long tunnel is a Public Private Partnership (PPP) scheme for the country’s National Roads Authority. The twin tunnel is designed to carry four lanes of traffic under the River Shannon and is the centrepiece of a major bypass project for the city.

Five 100m long tunnel units will be built in the casting basin on the north bank of the river. When complete, the basin will be flooded and the tunnel units floated out and sunk into a pre-dredged channel.

The tunnel is completed at each end by cut and cover sections built in situ with approach ramps. Measuring 550m long, 33m wide and 9m deep, the basin has to accommodate all five tunnel elements at once. To complicate matters, the ground conditions on the river banks are poor, with soft alluvial soils to 20m.

The construction sequence begins with rig operators driving piles down to firm strata and installing a row of raking ground anchors at between 2.5m and 3m before excavating the cofferdam to about 6m. This allows a fabricated steel waling beam to be welded inside the pile walls.

The 34m long MP250 super struts are then craned into position and hydraulically pressurised between the waling beams.

Excavation continues to formation level, typically down to 9m, where alluvium gives way to a more competent gravel material. As the excavation reaches full depth, permanent steel and concrete composite U-beams are dropped into the basin to carry the lateral loads. The struts can then be lifted out and moved on to the next section to be excavated.

Due to its exceptional length, the casting basin is being excavated in phases. "The struts are leap-frogged along the basin as the excavation proceeds towards the river," says Groundforce Ireland general manager on site Liam Brew.

The struts cross over a 45m span using 1.2m diameter tubular extensions and special tapered adaptor sections that Groundforce claims are the largest proprietary hydraulic struts available in Europe.

"The extension reduced installation time for 83 precast concrete composite struts from an estimated 20 weeks to just over 12 weeks," says Roadbridge project manager Eamonn Curran. "The hydraulic element also allowed loads to be monitored and checked against the design assumption ensuring safe construction."

Temporary works design was carried out by the Limerick office of White Young Green Ireland.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.