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| Underwater tunnel Limerick in six-day cycles  As part of the new M7 motorway development in the southwest of Ireland, the four-lane route crosses the Shannon near Limerick, a few kilometres before the river flows directly into the Atlantic. Centrepiece of the ten kilometre long "Limerick Southern Ring Road "is the required tunnelling with a length of 675 m. Including the north and south entrance and exit ramps, the tunnel measures a total of 915 metres. Completion of the around ten kilometre long section and the Shannon tunnel is planned for autumn 2010. Formwork and scaffolding specialist, PERI, provided two different tailored tunnel formwork solutions for the same tunnel cross-section based on the requirements of the contractor as well as the construction sequence itself. For the immersed tunnel and the three adjacent construction phases in the north to be cast in situ, a customized special formwork carriage is the most cost-effective solution. As a result, the total of 28 concreting sections can be very efficiently constructed. At the same time, in the area of the south portal, an additional tunnel section is planned which is to be completed in six cycles. This takes place using the cut-and-cover method. Due to the few concreting sections to be completed and the short rental period involved, the PERI VARIOKIT modular construction system can really prove its worth. Then, the use of mainly rentable standard elements means less overall costs for the contractor. For both solutions, identical double-tube rectangular cross-sections are semi-monolithically constructed in each case, whereby the bottom plate is completed first and then the walls and slabs are concreted in one pour. The two tubes are each ten metres wide and six metres high. For the so-called immersed tunnel, a total of five tunnel elements - each with a length of 100 m - are completely pre-fabricated using a 600 metre long and 33 metre wide dry dock positioned at the north portal, and then sealed off by means of bulkheads at the ends. After flooding the dock, the 100 x 25 x 8.50 metre reinforced concrete pre-cast segments, each weighing 20,000 tons, are floated out and then lowered into the exact position in a specially prepared channel on the river bed. The floating and sinking procedures take place by means of a hoisting system and integrated ballast tanks. A hydraulic line and pumping system, which independently fill and empty the eight tanks inside the tunnel tubes with water, provide the required lateral trim during transportation and the lowering process. With the two PERI tunnel formwork carriages, these elements - five 20-metre long concreting cycles in each case - can be manufactured cost-effectively, accurately as well as guaranteeing the watertightness in six-day cycles. The hydraulic lifting and lowering mechanism guarantees a very efficient moving process whilst wall element operations are also carried out by means of hydraulic cylinders. Hinged attachments are used to fix these to the slab girders via a horizontally-positioned guide. Integrated transverse launching units facilitate an accurate set-up procedure for each new concreting section. write your comments about the article :: © 2008 Construction News :: home page |