Madam, - Seamus Brennan certainly deserves credit for looking at the Madrid metro as a model of cost-effective construction - we don't often look beyond our nearest neighbour for inspiration. However, as a geologist and geophysicist who has lived in Madrid for 10 years and worked on aspects of the Madrid and Valencia metros, I would like to point out a factor which appears to have been overlooked in the debate over comparative costs.
Madrid is underlain by more than a kilometre of crumbly sand deposits through which tunnelling is relatively easy. Furthermore, the water table is at a depth of about 60 metres below the surface, so the tunnelling is mainly through dry sand.
Likewise, Paris and London are underlain by thick, wet, clay deposits which are nevertheless easy to bore through. Dublin is underlain by a thin layer of boulder clay less than 10 metres thick for the most part which overlies hard limestone rock, mostly below sea level.
A much better comparison could be made with the metro costs of cities such as Stockholm, which are underlain by a hard rock, granite in this case, and which are close to sea level. As far as tunnelling through limestone goes, the real experts in Ireland are to be found, not in the ranks of the road and rail engineers, but in the mines of Navan, Galmoy and Lisheen, where hard rock tunnelling is a daily activity. - Yours, etc.,
GEORGE A. REYNOLDS, M.Sc., MBA, (Consulting Geophysicist), Annamoe, Co Wicklow .