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Tunnelling update - July 2013
Interventions a normal feature of tunnelling progress
The Corrib pipeline tunnel is being constructed up to ten metres below the seabed of Sruwaddacon Bay, which is a tidal estuary. During normal tunnelling operations, a bentonite and water mixture is used at the front of the tunnel boring machine (TBM) and no air is present. Bentonite is a fine inert clay (natural product) used with water to lubricate and cool the cutting head and to stabilise the cut. It is also the medium in which the arisings from the tunnel (rock, gravel, clay and peat) are transported for segregation in the Aughoose tunnelling compound.
Interventions are carried out regularly on all major tunnelling works to allow for tunnel operatives to access the front of the TBM for maintenance and inspection. This includes changing the cutter heads, which wear out as they cut through the rock, soils and sands.
Shallow depression due to air loss following intervention on July 3rd 2013
During an intervention, air is introduced at the front of the TBM to maintain the soil stability around the chamber at the cutting face and to allow personnel to safely access and work within the chamber. The compressed air helps to prevent the ground water, sands and soils from falling in.
Before filling the chamber with compressed air, a fresh thickened mix of bentonite is used to seal the walls around the cutting face to prevent air loss. However, due to the permeability of the surrounding soils, there is a potential for air to escape, and, in certain circumstances, this can result in air migrating to the surface and local shallow depressions appearing on the seabed in Sruwaddacon Bay. These depressions have no lasting impact on the bay.
We have been informing the relevant regulatory authorities* and Mayo County Council of these occurrences as required.
These small depressions are not ‘sink holes’; they are shallow temporary depressions of approximately one to two feet in depth and they are naturally in-filled with the surrounding sands by the ebb and flow of the tide.
The interventions are carried out in accordance with an agreed method statement, which is part of our Environmental Management Plan for the tunnel and is approved by the regulatory authorities. This sets out measures to minimise the potential for air escaping to the surface, as well as mitigation to be used should air loss lead to any disturbance of the seabed. These include local raking of sands, when practicable, to aid the natural infilling of sand through the tidal processes. Should there be concerns about public safety being compromised the area will be cordoned off until the seabed is safe.
Further interventions will be taking place throughout the Corrib tunnelling programme and, as for all our activities, they will be closely monitored.
*The relevant regulatory authorities are:
- The Foreshore Unit of the Department of the Environment, Community and Local Government
- The Department of Communications, Energy and Natural Resources