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Fionnuala - the Corrib tunnel boring machine

Fionnuala - the Corrib Tunnel Boring Machine

The Tunnel Boring Machine train

The Tunnel Boring Machine train

will bring materials, tools and personnel to the front of the tunnel

The onshore pipeline is the final phase of the Corrib gas project that remains to be completed.  The onshore pipeline section is 8.3km long and 4.9km of this will be installed in a tunnel, the majority of which will run under Sruwaddacon Bay.


Approximately 4.9km of the Corrib Gas Onshore Pipeline will be constructed within a tunnel. To watch our tunnelling DVD, please press play.

The building of the tunnel requires the use of a large tunnel boring machine (TBM).

Excavation of the tunnel will be in one direction, starting at a launch pit on a SEPIL-owned site in the townland of Aughoose and running to a reception site in Glengad near where the offshore pipeline reaches land.

The tunnel will have an external diameter of 4.2m and an internal diameter of 3.5m and will run at depths of between 5.5m and 12m under Sruwaddacon Bay. 

When constructed, the tunnel will be the longest tunnel in Ireland and the longest gas pipeline tunnel anywhere in Europe.

In July 2012 Fionnuala, the tunnel boring machine which would be used on the Corrib Natural Gas project began her journey from Schwanau, Germany to Aughoose in Co. Mayo.  View her journey to the Corrib project.

The TBM for the Corrib tunnel was designed and built in Schwanau, Germany by Herrenknecht, one of the world’s largest makers of TBMs.  The TBM took more than a year to design and build.

The single shield TBM is 140m long, weighs almost 500 tonnes and comprises 14 sections.

The 28-tonne cutter head drills under the bay using a combination of cutter discs, scrapers and buckets and requires two 400kw motors to turn it.

Following in a long tunnelling tradition of naming TBMs, the Corrib TBM has been named ‘Fionnuala’ after the female of the Children of Lir, one of the legends most closely associated with the Erris region.

The TBM comprises of 14 carriages, including support and back-up services for the main TBM cutter head.

Corrib tunnel

The cutter head at the front breaks the rock, sand and gravel that it meets while tunnelling. This material is pumped in a suspended mixture back through the tunnel to the surface at Aughoose where it is separated. A naturally-occurring, inert clay material (bentonite) is used for lubrication and cooling of the cutter head, and to assist in the transportation of the arisings to the surface.

As the TBM moves forward, a series of 1.2m wide concrete rings made up of precast interlocking concrete segments is erected.   These concrete rings, which are fabricated in Ireland, will eventually line the entire tunnel. 

As the cutter head rotates, hydraulic cylinders attached to the spine of the TBM propel it forward a few feet at a time.

Personnel, tools and materials including the concrete segments are brought from the tunnelling compound to the front of the tunnel using a small train.

The TBM is controlled by an operator, who sits in a control cabin at the front of the machine. The operator’s role is to ensure the TBM stays on course. There are two other monitors on the surface to ensure the trajectory of the tunnel is correct; manual surveys are also conducted routinely.

The tunnelling compound at Aughoose has been designed and constructed to minimise its environmental impact, particularly the impact on Sruwaddacon Bay which is a designated conservation site, a candidate Special Area of Conservation (SAC) and Special Protection Area (SPA).

Tunnel excavation - evening

Construction of the compound commenced in July 2011. The initial construction work was completed by Roadbridge Ltd, entailing an earthworks programme and civil works including the construction of foundations to support equipment installation.

The earthworks programme involved the excavation of peat, some of which was removed and transported to a Bord na Móna peat deposition site at Srahmore, near Bangor Erris. The remainder of the excavated peat is being stored on site for reinstatement of the site upon project completion.

Once all earthworks and foundations were complete, work started on putting in place the various facilities and pieces of equipment required to support the tunnelling operation.

One of the most significant areas to be constructed on the Aughoose site was the start shaft, from where the tunnel boring machine was to be launched.

Work on the launch shaft got under way in late spring 2012 and continued until the autumn. In October 2012 the TBM was brought to the Aughoose site and work started on its installation into the shaft.  Following this, the TBM was commissioned and the initial concrete rings were installed in the launch pit to form the beginning of the tunnel.

The TBM commenced excavation of the tunnel in early January. The entire tunnelling operation, which is 24 hours per day seven days per week, is expected to take approximately 15 months to complete.

Shell E&P Ireland Limited MD, Michael Crothers (left) is taken through a drawing of the tunnelling site

Shell E&P Ireland Limited MD, Michael Crothers (left) is taken through a drawing of the tunnelling site

The contract for the 4.9km Corrib tunnel was awarded to a joint venture between BAM Civil and Wayss & Freytag, subsidiaries of the Royal BAM Group. The partnership brings together the vast tunnelling experience and expertise of Wayss & Freytag with BAM Civil’s expertise in civil engineering and its experience in the Irish construction industry.

Tunnelling as part of major infrastructure projects has a long history. The first TBM was invented by Brunel in January 1818, and was used to excavate the Thames tunnel, London in 1825.

By 1870, when the Tower Subway under the Thames was being constructed, tunnelling technology had already advanced. This saw a new design for a circular tunnel, which was both simpler in construction and better able to support the weight of the surrounding soil, used for the first time.

The TBMs used in tunnel construction today are highly sophisticated machines that bring together the latest advances in safety, engineering and technology.

Glengad is the area where the offshore pipeline, which was laid in 2009, makes landfall. This is also where the TBM will emerge once tunnel construction is complete. This requires the construction of a reception pit for the tunnel, and this will be undertaken in 2013.