Explaining tunnel construction by joint mapping

Modern-day railway tunnels are a superb legacy from a bygone age, built by eminent Victorian tunnel engineers. The great skills and abilities of these engineers and their workmen, even in very poor working conditions and with very limited technology, have resulted in these tunnels being in constant use up to the present time and beyond. But how were they built to last so long? No plans or as-built drawings survive, and only very limited written accounts of their construction are available. Attempts at looking deeper through the linings with ground-probing radar and electrical resistance techniques have failed to reveal any of the secrets. Destructive testing such as coring has been used, but the overall picture is still missing. The only remaining signs of the construction works are the open shafts and the joints. The shafts indicate where some of the access and construction points were situated along the tunnel, and the joints mark the lengths of excavation used in building the tunnel. The joint lengths ...