the tunnel opened for traffic in 1999. Construction techniques The chosen method of construction was the immersed tube technique. In this method, a trench is dredged in the bed of the water channel. Tunnel sections are constructed in the dry, for example in a casting basin, a fabrication yard, on a ship-lift platform or in a factory unit. The ends of the section are then temporarily sealed with bulkheads. Each tunnel section is transported to the tunnel site – usually floating, occasionally on a barge, or assisted by cranes. In the Jack Lynch Tunnel, the 610-metre-long reinforced concrete immersed tube tunnel is made up of five elements, each around 122 metres long, 24.5 metres wide and 8.5 metres high. The northern approach was formed by a 120-metre-long floated open 'boat' section – the first of its kind. Excavation and dredging Dredging International was the chosen dredging contractor, and the value of the dredging contract is stated by them at €12.4 million. The scope of works consisted of dredging the main tunnel trench and subsequently backfilling the completed construction with sand and gravel. The contract provided also for restoration of the riverbed profile and the addition of a rock protection layer. Between May 1996 and March 1999, the company excavated 785,000 tonnes of silt and alluvium and 300,000 tonnes of sand and gravel. The trench was dredged primarily by a series of specialised vessels, in several stages and with different equipment. Much of the main trench for the Lee Tunnel was excavated by the backhoe dredger "Zenne". Two barges were used to transport the dredged material 19 km (12 mi) downriver and from there to a disposal site four miles offshore. The cutter dredger "Vlaanderen XIX" removed the underlying fluvioglacial material. A second cutter dredger, "Vlaanderen XV", was deployed to breach the casting basin perimeter, or "bund". A rock layer was encountered on part of the trench line, which was dealt with by the jack-up platform "Zeebouwer". The "Big Boss" vessel was employed to remove the rock. This backhoe dredger was equipped with a "Backhoover" (in effect, a "mini" precision dredger). This system proved extremely effective in removing very thin layers of recently deposited material immediately prior to immersion of the tunnel elements. Constructing the immersed tube Building the tunnel structure required the pre-casting of 5 tunnel segments and an open boat unit. These were constructed in a casting basin located