Contractors for Ireland’s Dublin Port Tunnel last month launched the TBM that will drive the US$410M twin-bore road tunnel.
Over the next 30 months, the 11.8m-diameter Herrenknecht hard rock TBM, christened ‘Grainne”, will drive two 2.3km tunnels between its Whitehall launch shaft and a reception chamber at Fairview Park.
On breakthrough at Fairview, the 1600t TBM will be disassembled and turned round, a process expected to take two months, to enable it to tunnel back to Whitehall.
The 56.5m diameter, 30m deep Whitehall shaft is the focus of construction activity and marks a change in geology at the tunnel face from limestone to stiff boulder clay.
Once Grainne has disappeared into the limestone, an 11.7m diameter shielded triple backhoe named Meghan will head off from the shaft in the opposite direction constructing a further 300m of the twin-bore tunnel through the boulder clay. Diaphragm wall supported cut and cover sections at either end will provide the tunnel’s approach ramps.
JV contractor on the design and build project is Nishimatsu/Mowlem/Irishenco (NMI) with Nishimatsu carrying out all tunnelling works
At tender stage in January 2000, NATM was widely tipped as the likely construction method. But NMI and its designers Charles Haswell firmly rejected this in favour of a bored tunnel with a deeper alignment, between 18-24m of cover, to allay resident’s fears over subsidence, noise and vibration.
Sean Wynne project engineer with client Dublin City Council, says it will monitor local properties throughout the operation to safeguard householders from nuisance or disturbance. According to the council, vibrations will be kept below levels at which "even cosmetic damage could occur."
When open in early 2005, the National Roads Authority funded tunnel will provide two lanes of traffic in both directions. It will run from the existing M1 motorway at Santry to link with Dublin Port and the local road network. It is expected to divert 9,000 trucks and heavy goods vehicles from the city each day.
