I can confirm for the Deputy that the Corrib developers have notified my Department about depressions in Sruwaddacon Bay, which are caused by air escaping during tunnel boring machine "intervention" maintenance, and I wish to confirm that these depressions are not sinkholes. To clarify, the tunnel boring machine, TBM, being used by the developers to tunnel under Sruwaddacon Bay for the purposes of laying the onshore segment of the Corrib gas pipeline requires periodic inspection and maintenance of the mechanical components of the TBM cutter head. Inspections and maintenance on the TBM cutter head are carried out during interventions at above atmospheric pressure and are carried out in accordance with standard industry practice for tunnelling and Health and Safety Authority guidelines.
The air pressure is adjusted and stabilised to safeguard the health and safety of the tunnel operatives and maintain tunnel face stability within the excavation chamber, and is tailored to the anticipated ground conditions at each location. The duration of interventions will vary depending on the nature and extent of the works within the tunnel boring machine. There is always a potential for small scale air loss from the intervention activities, the air loss being a function of the permeability of the ground conditions, and where more permeable material or conduits are present this air loss may migrate to the surface. Local depressions in the seabed can arise as a result of the air loss. These are depressions in the sand and they can be about 50 cm deep when the tide is out. As set out in the works method statement provided by the developers, should such local depressions occur, mitigation by local raking will be undertaken when tidal and site conditions are suitable to accelerate the natural sediment reinstatement process.
If there is anybody left following "Oireachtas Report" at this stage, a Cheann Comhairle, they will certainly be fast asleep. I am nodding off myself.
Additional information not given on the floor of the House
The areas in which depressions have been observed are in intertidal areas of Sruwaddacon Bay, that is, in areas which dry out during low tide. As the TBM has progressed through the sands, there has also been some minor localised consolidation of less than 10 cm depth of sands along the tunnel trajectory. This consolidation was predicted in the environmental impact statement supporting the application to construct the pipeline and is expected to reinstate itself over a short duration as the TBM progresses.
My Department’s consultant tunnelling expert from Environ conducted his most recent site visit last week and a draft report on this visit, including his comments with respect to these depressions, will be published on the Department of Communications, Energy and Natural Resources website as soon as possible. It is important in the meantime to note that the depressions appear to have occurred as a result of air releases during TBM maintenance interventions with no evidence of the tunnel sealant bentonite release to the surface; the depressions have been small relative to the overall context of the bay and there is no evidence of contamination or of any significant environmental harm having occurred as a result of the these events; and in the current circumstances there is no foreseeable way that any of the air release could affect the integrity of the completed tunnel.