The Jack Lynch Tunnel was built using the immersed tube method, this involved the construction of a large casting basin on the southern (Mahon) side. The excavation of a million tonnes of material including 150,000tonnes of limestone rock and the preparation of the casting basin covering 21.6 acres or 8.75 hectares was itself an unprecedented engineering challenge in this country. Inside the 350 metre long, 250 metre wide, casting basin were built the five tunnel units – each weighing 27,000 tonnes – along with the northern approach ramp “boat” section.
Each tunnel unit is 122 metres long, 24 metres wide and 9 metres high and is built with reinforced concrete. A total of 120,000 cubic metres of concrete were poured in constructing the five tunnel units and approximately 8,000 tonnes reinforcing steel was used. The twin traffic bores, each comprising two lanes of uni-directional, are divided by a Central Service/Emergency Escape passageway. While the units were being constructed within the casting basin, a trench was dredged across the river and below the river bed.
This involved the excavation of 785,000 tonnes of silt and alluvium which was not suitable for re-use within the works and which was disposed of at sea under licence from the Department of the Marine.
Further dredging of 300,000 tonnes of the underlying sands and gravels then occurred, which material was used within site for backfilling and reclamation.
On completion of the pre-cast reinforced concrete works within the casting basin, the units were prepared for flotation with the installation of temporary steel bulkheads at each end to seal each unit. Temporary water ballast tanks were also constructed within each unit.
When all the units were prepared, the basin was allowed to flood over the Easter week-end of March 1997 by pumping water from the river into the basin.
The finished sealed sections floated out into the estuary and, one by one, placed into position in the special channel located deep below the bed of the river.
The positioning of each unit, between June and August of 1997, was assisted by the use of flotation pontoons and the immersion of the units r3equired the use of water ballast tanks. This operation required some restrictions on shipping movement for six 48 hour periods and was done in co-operation with the Port of Cork Authority.
The sections were linked together using the “Gina” and “Omega” gasket – methods well proven in international submersed tunnel construction. When the tunnel units were firmly in position, the temporary bulkhead seals were removed from inside the tunnel and the trench was later backfilled to restore the river bed level.
The casting basin was then backfilled for landscaping and the creation of a valuable local amenity. The reclamation areas adjacent to the southern portal of the tunnel at the Mahon side were also landscaped to create a riverside amenity walk.
During 1998 and the earlier part of 1999, the assembled and sealed tunnel was completed and fitted from the inside. This included installations of all the requisite lighting, ventilation, fire fighting, drainage, control, monitoring and safety equipment, along with internal finishes and road surfacing. In addition the southern approach ramp to the tunnel was constructed in situ within the eastern side of the casting basin