Immersed Tunnel under the Shannon in IrelandThe Limerick southern ring road in the southwest of Ireland is comprised of 10 km of two lane dual carriageway, 11 bridges, four interchanges, six underpasses and a twin bore immersed tube tunnel running under the river Shannon between Coonagh and Bunlicky, a few kilometres before the river flows directly into the Atlantic. At a cost of approximately EUR660 million, this is a major piece of infrastructural development. Developed as a public private partnership (PPP), the project comprises of six shareholders: Allied Irish Bank, John Sisk & Son, Lagan Holdings, Kellogg Brown & Root, Meridiam Infrastructure, Roadbridge and Strabag. The project was awarded the title "European PPP Deal of the Year" in 2006. The Limerick tunnel is the second phase of the scheme connecting the N7 at Rossbrien to the N18 Ennis Road at Cratloe Castle. Work started on the tunnel in August 2006 and is due for completion in September 2010. The main contractor for the tunnel is Strabag. Van Oord has been awarded the marine element of the works as subcontractor to DirectRoute. The contract value for Van Oord is some EUR24 million. The scheme is due to commence in the summer of 2008 and is programmed to take 46 weeks to completion. Van Oord's assignment include dredging a trench across the Shannon between the extremities of the cofferdam / float-out structures; the flotation, transportation and immersion of five reinforced concrete tunnel elements; and backfilling of the tunnel trench. Click ie/18. Visit www.limericktunnel.com, www.directroute.ie and www.vanoord.comThe Limerick tunnel will take up to 27,000 vehicles per day off the streets of Limerick at start-up increasing ultimately to 40,000, thus allowing the city to breathe and to realise its true potential through future innovative city planning. The tunnel will improve access times for commuters to the city as well as access to Shannon Airport, Galway, Cork, Kerry and Dublin. The removal of this huge volume of daily traffic will also lower CO2 emissions and improve the air quality of the city.Including the north and south entrance and exit ramps, the tunnel measures a total of 915 metres, 675 metres of which under the Shannon. The tunnel will consist of two bores, one for southbound traffic and the other for northbound traffic. Each of the two bores comprises two lanes with emergency walkways on each side. The two tubes are each ten metres wide and six metres high. Formwork and scaffolding specialists 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. Visit www.peri.deFor 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. View pictures here.Immersed tunnelFor the so-called immersed tunnel, a total of five tunnel elements - each with a length of 100 m - are completely precast using a 600 m-long and 33 m-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 m reinforced concrete precast elements, each weighing 20,000 tonnes, 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 m-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.For the stopend formwork, the PERI formwork solution takes into consideration both the shear key as well as two different sealing variations. The stopend formwork accommodates the water stops for the casting joints between the individual 20 m sections. With the other version, the element joint at the ends of each 100 m tunnel element is a solid steel frame for the large-volume sealing profile, a so-called Gina profile, which is integrated into the PERI stopend formwork.Four of the five tunnel elements (TE5, TE4, TE3 and TE2) are now structurally complete and construction of the base of element TE1 is now underway. Fitting-out of the elements for immersion in the Shannon has commenced, involving installation of temporary internal ballast tanks, pumps and pipework and the fitting of steel bulkheads to both ends of each element, and will continue untilcommencement of immersion.On the south bank of the river, the south cut-and-cover cofferdam has been dewatered and construction has commenced on this in-situ section of the tunnel which will receive the first floated-out tunnel element when immersion commences. The first element will be connected to the southern portal section built in-situ adjacent to Bunlicky Lake using a cut-and-cover design. The immersed tube tunnel elements are then connected and the river bed is reinstated above the tunnel. Finally the northern portal section is built using also cut-and-cover design similar to that used on the southern portal. 120-year service lifeThe Limerick tunnel has been designed for an unusually long life span of 120 years. This places enormous demands on the building materials used as well as on quality standards for site personnel and the construction progress itself. With extensive pile foundations and the carefully implemented measures for maintaining watertightness, the DirectRoute consortium headed by Strabag has fully taken this into consideration for this integral part of the contract. Special efforts are also being undertaken in order to produce crack-free concrete. This means, independent of the surrounding temperature, the curing temperature of the concrete is controlled via complex cooling measures using an internal tubular cooling system in the bottom plate, walls and slab over a period of three to four days. The high concrete cover of 60 to 80 millimetres along with the electrical insulation of all mounting parts also ensure maximum protection against corrosion. Apart from the covering, additional ballast concrete in the area of the plate serves to prevent any possibility of the structure lifting.SafetyXella's Aestuver division will supply 12,000 square metres of fire protection for tunnel ceiling in accordance with HCM 120 and ISO 240 requirements. This fire protection is 30 mm anthracite-coloured boards set in concrete, with 10 mm joint backing. Visit www.aestuver.deIn addition, a tunnel design and safety consultation group was formed to ensure effective design, construction and operation within the context of safety. 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