Jack Lynch Tunnel

The Jack Lynch Tunnel (named after the former Taoiseach of the Republic of Ireland, Jack Lynch) is an immersed tube tunnel and an integral part of the N25 southern ring road of Cork in Ireland.

It takes the road under the River Lee. North of the tunnel, the ring-road joins the N8 road to Dublin (north), and the city centre (west), with the N25 continuing east to Waterford. The tunnel was completed in 1999, and carries nearly 40,000 vehicles per day as of 2005, this figure is increasing every year and may rise further with the N25 ring-road being upgraded, notably with the recent opening of the Kinsale road roundabout Flyover and planned flyovers on the Sarsfield Road and Bandon Road Roundabouts.

The tunnel has footpaths and a central bore for use in an emergency only, pedestrians and cyclists are expressly forbidden from using the tunnel. The exclusion of cyclists has been somewhat controversial as the feeder road is only a dual-carriageway and so is open to cyclists, but due to space limitations and the obvious danger of cyclists in an enclosed tunnel this decision is understandable.

File:Jack Lynch Tunnel Southbound Approach.jpg

Southbound approach to the tunnel

History

The idea of a crossing of the River Lee downstream of the city came from civil engineers employed by Cork Corporation and the central government’s Department of the Environment in the late 1970s. Cork’s suburbs were expanding and traffic was rising as car ownership increased, but the city centre’s street plan, laid out in the late middle ages, was ill equipped to cope. The engineers reasoned that the congestion in the city centre and its radial routes was quickly reaching intolerable levels. They pushed through Cork’s “LUTS” - Land Use and Transportation Study – plan, in 1978, to lay down a long-term plan that would combine construction of elements of a ring road, a downstream crossing, and computerized management of traffic on existing roads. This group of engineers later became the Technical Steering Committee for the tunnel and at that time consisted of Sean McCarthy, the City Engineer, W.A. Fitzgerald, his successor, M.W. Mullins, Cork’s County Engineer, John O’Regan, his deputy, B.J. O’Sullivan, the Cork Harbour Engineer, and Sean Walsh and Declan O’Driscoll, the two Assistant Chief Engineering Advisers at the Department of the Environment responsible for the region.

No road development in Ireland prior to that date had required such a large investment , and therefore the plan met with some opposition on the grounds of cost. In 1980, the engineers succeeded in having Cork Corporation commission DeLeuw Chadwick O’hEocha, engineering consultants, to undertake a feasibility study of options for a major highway crossing of the River Lee downstream from Cork city centre. The first stage of the report established that the crossing should be located in Dunkettle, rather than near Tivoli, close to the city centre. The second stage endorsed the immersed tube tunnel as the preferred scheme versus a bridge. The primary reasons were that a tunnel would have a lower construction cost relative to its utility, short approach gradients, lower environmental impact and no effect on shipping once built . While the construction costs for a two lane tunnel were maginally higher than for a two lane high-level bridge, the steep upward ramp for a high-level bridge would slow down cars and trucks as they climbed the bridge, thus reducing its peak capacity dramatically versus a tunnel .

A further feasibility study was recommended by the Government in 1985 to re-examine the same questions and the connection to the planned South Ring Road. This feasibility study was done at a cost of approximately IR£340,000 . A local public inquiry into the application by Cork Corporation for a bridge order relating to the provision of the two-lane tunnel was held in October 1985. DeLeuw Chadwick O’hEocha later assisted at two public inquiries, supervised a major geotechnical investigation, conducted a hydraulic study of the river and carried out an environmental impact study . Finally, the required inspector's report was submitted in January 1987 to the then Minister of the Environment, Padraig Flynn. At that point, the report sat on the minister’s desk for some time. The formal reason for this was that the minister “decided that the question of the timing of the crossing should be considered in the broader context of the preparation by the Department of proposals for the medium to long term development of national roads. In this context the operational programme for roads … includes the proposal for the commencement of the downstream crossing during the programme period 1989-1993” .

After public enquiries in 1985 and 1990, and an Environmental Impact Study in 1991, work began in 1995. DeLeuw Chadwick O’hEocha, by this time part of Mott MacDonald, an engineering consultancy based in Croydon, England, worked in association with Symonds Travers Morgan, a London-based project management firm, produced a conceptual design and tender documents for Cork Corporation, prepared on a design-and-build basis .

Financing

The IR£70 million design and construction contract was awarded by the Cork Corporation on behalf of the National Roads Authority. The NRA received financial assistance from the Cohesion Fund of the European Union. As is typical on large-scale project of the type, the chosen contractors was a joint venture, Tarmac Walls JV, formed by Tarmac, now a division of Carillion plc, and Walls. Subcontractors then handled many of the key tasks of construction.

Construction

Plans for a downstream crossing of the River Lee have been active since the late-1970s, construction work began in 1994. Construction involved the excavation of a large casting basin where the tunnel elements or pieces were constructed. After construction of elements was complete, the casting basin was filled with water and joined to the adjacent River Lee, each element was floated out and sunk into position into a carefully dredged river bed. The road surface was laid and 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 610m long reinforced concrete immersed tube tunnel is made up of five elements, each around 120m long, 24.5m wide and 8.5m high. These were constructed in a casting basin located partially on the line of the tunnel south of the river at Mahon. The northern approach was formed by a 120m 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 contract is stated by them at € 12,390,000. 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. Dredging work was completed between May 1996 and March 1999.

785,000 tonnes of silt and alluvium and 300,000 tonnes of sand and gravel were excavated during the project .

The trench was dredged primarily by a series of specialized vessels in several stages and with different equipment. Much of the main trench for the Lee Tunnel was dredging by the backhoe dredger “Zenne”. Two barges were used to transport the dredged material 12 miles downriver and from there to the 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 bund. A rock layer, encountered over part of the trench line, was dealt with by the jack-up platform “Zeebouwer”.

The “Big Boss” 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

The construction involved the pre-casting of 3 tunnel segments. Each was 122m long and weighed approx 27,000 tonnes . Contractors Tarmac Walls JV engaged RMD Kwikform, a global construction formwork and shoring solutions company, for the formwork on the tunnel and open section elements of the river crossing .

Each box section measures approximately 24m wide x 8.5m high x 122m long and comprised two 9.8mm wide dual lane traffic tubes and a 1.35m wide twin walled central tubes for services and emergency access. The 1200mm thick base slab was cast first, followed by the 500mm thick central walls. The outer walls and roof were cast together in a single operation, in six nominal 21m lengths, using special travelling formwork. This was struck inside the formed section, moved along to the next length and then jacked up into position, each within a 72 hour cycle. Each 1000 cubic metre pour required approximately 1500 square metres of formwork. The inside shutters for the outer walls were vertical steel section panels and Alform Beam walers, which were attached to the top slab of the special steel traveller, while the outer shutters were crane handled vertical steel section panels with Alform Beam or steel channel walers.

The open top boat units, measuring 42m wide x 122m long and up to 10m high were cast in two operations using standard formwork. All the elements and boat sections were cast from grade 40 ggbfs concrete, with reinforcement provided by 16 x 40mm diameter rebar .

Tunnel Operation

The road surface was laid and the tunnel opened for traffic in 1999, roughly 20 years after the first formal studies had been prepared. The tunnel has two separate dual carriageways (each carriageway 3.75m wide). There is a service walkway about 1 metre across which doubles as an emergency escape. During periods of maintenance, one bore may be closed and the other used for bi-directional traffic.

The environment within the tunnel is controlled by an array of monitors and a closed circuit TV and traffic control system. External photocells linked to the computer management system provide a level of tunnel lighting most compatable with ambient light levels outside. To assist drivers in adjusting their eyes, lighting gradually brightens as they approach an exit.

Jet fans located within niches in the roof of the immersed tube sections provide longitudinal ventilation, and the tunnel is equipped with fire and life safety equipment.

Despite the cost of its construction and operation the tunnel is not tolled. The Feasibility Study for the tunnel envisaged that the crossing would have a toll of 30 pence (in 1981 IR£), but the concept of a toll was dropped after political opposition. Tolls could be introduced in the future on the Northbound bi-lane, which is suitable, having the required space and roundabout free tailback capacity.

See also

• Roads in Ireland