Geology of the seabed and shallow subsurface: the Irish Sea - NERC Open Research Archive

...Seabed sediments are subdivided into regions of soft mud- (clay and silt) rich sediment in the eastern and western Irish Sea and a central gravel belt comprising coarse sand and gravel. Small areas of bedrock outcrop at seabed are also recognised.

...Very stiff diamicts (glacial `boulder clays' or tills) are present across most of the report area of variable thickness.

...The predominant bedrock lithologies in the report area are Triassic and Carboniferous sandstone and mudstone. Geotechnical properties of Triassic rocks are comparable and potentially predictable. Carboniferous rock show high lateral and vertical variability. There are a number of igneous intrusions in the report area and rock properties near to the location of these igneous bodies may differ due to alteration of the host rock during intrusion.

What does this mean for a tunnel? For example, are any of these water-impermeable?

_{*Lunatic*, n.
One whose delusions are out of fashion.}