PROPOSALS FOR a second Channel Tunnel are pushing the limits of current tunnel boring technology, the scheme's backers admitted this week.
The feasibility study put forward to the French and UK governments by Eurotunnel, operator of the existing link, outlines plans for a single bore 15m excavated diameter, 38km long tunnel.
Eurotunnel project manager for the second tunnel, Patrick de Montigny, said that recent tunnelling projects in Hamburg and Tokyo, where tunnel boring machines have achieved bores of 14.2m and 14.14m diameter respectively, indicate the second Channel tunnel proposal is realistic. But its length would mean pushing new frontiers.
Consultants for the feasibility study, Maunsell and French firm Scetaroute, are basing their outline design on anticipated advances in TBM technology. It is unlikely the scheme will go ahead before 2025, 'if at all', said Eurotunnel deputy managing director Alain Bertrand.
Maunsell/Scetaroute proposes using two earth pressure balance TBMs, working in opposite directions. Spoil removal proved a challenge during construction of the first, twin bore Channel Tunnel. The increase in area of the cutting face in tunnel two promises to heighten logistical problems.
The consultants speculated that in future TBMs would be capable of forward probing and ground treatment. Although no ground investigation has been carried out, tunnellers are likely to encounter chalk marl and water ingress which was a significant challenge in the first tunnel.
Road and rail options, calculated to cost £2.6bn and £3bn respectively, are being considered for the new tunnel. Unless volumes of freight carried by rail increase dramatically in the next two decades, Eurotunnel's preferred option is the road scheme.
Both road and rail schemes will undergo major reappraisal in 2010, when the governments can intervene to appoint an alternative backer for the second tunnel.