LEITCH ON IRON TUNNELS. 377 (students' Paper No. 376.) ' l Iron Tunnels." By WILLIAN ORR LEITCH, Jun., Stud. Inst. C.E. THE extraordinary increase of traffic in cities, and the demand for direct communication by the shortest route, have led to the construction of railways under the busiest streets, running close to the most valuable property, and passing on their way the maze of sewers, gas-and water-pipes, existing underground railways, under rivers, and through difficult and varying strata, in order to gain for the community, and possibly to snatch from a rivaI route, all the advantages of the most rapid transit. These conditions have led to the introduction of iron tunnels, driven in cities far below all obstruction, and from an operating base so as to involve the least interference with the street surface. For crossing under rivers and penetrating loose and water-bearing strata the design is specially suitable, as, after the material has been excavated, the tunnel is quickly built, and a t once attains its maximum strength. The use of the shield and artificial air-pressure has largely aided the success of such works. I n 1816 Sir Isambard Brunel, when ' This communication was read aud discussed at a Supplemental Meeting of Students on the 17th January, 1896, and has been awarded a MilIer Prize, Session 1895-96.. " A Memoir of the Thames Tunnel," by Henry Law, Weale's Quarterly
