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Still a trailblazer

Dublin Port Tunnel is a flagship project in Ireland's five year National Development Plan, providing a high quality access route for the 20,000 vehicles that travel daily between Dublin's M50 ring road and Dublin Port.

In total the tunnel will be 4.5km long and construction is split into three sections. Two cut and cover sections to the north and south totalling 1.9km give way to a 2.6km central twin bored section running beneath the suburb of Marino. The ground is primarily stiff boulder clay, with carboniferous limestone beneath the residential area.

A surface road improvement connecting to the M1 makes up the scheme's 5.6km total length.

The tunnel is a massive investment, costing E 160 (£100) per head of population. But it will be free to HGVs to ensure they stay clear of the city centre. Cars will pay a toll.

Formal approval for instruction to proceed was first granted in November 1995. But there was immediate controversy, with the initial plan that saw the tunnel skirting past built-up areas rejected on cost grounds.

A new, straighter alignment and adoption of the New Austrian Tunnelling Method (NATM) brought costs down to £78M at 1994 prices. But the Heathrow Express tunnel collapse had a major knockon effect in Dublin. Pre-tender design had been carried out by Austrian firm Geoconsult - later found guilty of safety breaches at Heathrow - in joint venture with Arup.

Work was due to start in 1998 but residents' concerns aired at public inquiries escalated into a vociferous campaign. To allay these fears the tunnel was made deeper, from a mooted 10m below ground to between 20m and 25m.

NATM was abandoned in favour of a bored solution.

Joint venture contractor Nishimatsu Mowlem Irishenco (NMI) won the design and build contract in December 2000 for E448M (£303M). Haswell was appointed tunnelling designer with Carl Bro given responsibility for highway design, structures and M&E. Brown & Root is construction supervisor on behalf of DCC.

Work began on site on 5 June 2001. Nishimatsu took responsibility for the bored tunnel section and the complex railway crossing (see main feature).

Mowlem Irishenco (Mowlem's Irish subsidiary) took responsibility for the remainder of the work and long term maintenance.

The bored section is itself split into two sections, with access for the two 11.77m diameter TBMs a 56m diameter, 30m deep shaft 400m from the north of the bored section.

A 156m long, 1,600t shielded hard rock machine - Grainne - will complete over 90% of the tunnel.

It was officially launched on 7 May 2002, reaching the southern end in July 2003. It is now two thirds of the way back and is scheduled to return to the launch pit this July, having excavated 500,000m 3ofcarboniferous limestone.

Meanwhile a 60m long, 1,100t second TBM - Megan - has bored its way twice through the 400m of boulder clay that stands between the launch pit and Whitehall at the northern end. Megan, which uses just 400kW of power (compared to the 3,200kW cutting head on Grainne), arrived back at the launch pit in November last year.

Both sections are lined with a precast ring of six 700mm segments and a key unit, each 1.7m long with injection ports for grouting. A 275mm thick fire lining made of polypropylene fibre reinforced concrete is cast in 12m long segments and crosspassages then excavated - in all up to 140 of various shapes and sizes.

Challenges on the northern cut and cover section are provided by very tight land take constraints - it is being built alongside the existing road with just 2m separating the tunnel outer wall from the traffic.

With no access on either side, work progresses in linear fashion.

Diaphragm walls 1.2m to 1.5m thick are installed 26m to 27m deep into the boulder clay. At the far northern end cut and cover gives way to open excavation with extensive soil nailing.

Excavation is now complete and over 60% of the tunnel lining is in place. Work should be finished by August.

In the southern section, work is easier as the tunnel cuts a swathe through Fairview Park. This cannot be completed until the bored section is finished, as all limestone spoil comes out at this end where it is stockpiled for use as backfill or sub-base material. At present, some of the material is being used to surcharge existing ground before road construction.

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