752 REPORT — 1894. 



the same means. In all these tunnels compressed air has been used in conjunction 

 with shields almost continuously, as the amount of water met with has been large, 

 and it has been necessary in most cases to avoid all subsidence of the ground above 

 as much as possible. 



The tunnel under the Thames at Blackwall which is being built for the London 

 County Council under the direction of their chief engineer, Mr. A. R. Binnie, has 

 now been under construction for more than two years, and although the greatest 

 difficulties have probably yet to come, still an account of the present state of the 

 work and the difficulties met with up to date will, it is hoped, not be without 

 interest. 



Before dealing with the Blackwall Tunnel the author made a few remarks on 

 previous tunnels constructed by one or both of the methods under consideration. 



The tunnel under the Thames between Wapping and Rotherhithe, constructed 

 by Brunei between the years 1825 and 1842, was the first tunnel constructed by 

 means of a shield, and its history is so well known that it will not be necessary to 

 refer further to it. It may be noticed, however, that in a patent of Brunei's taken 

 out in 1818 he had at that time conceived the idea of a tunnel made of cast iron 

 with an inside brick lining, and constructed by means of a shield which had a 

 tail lapping over the completed portion of the tunnel and shoved forward by means 

 of hydraulic rams. 



Cochrane took out a patent for using compressed air in the construction of 

 shafts and tunnels in 1833, but it was not used for the former until 1839, and was 

 not used in any tunnel before 1872 or 1873. 



The use of compressed air in conjunction with a shield, and the construction of 

 the tunnel itself with cast-iron rings, although the latter may not be so important 

 in some cases, may be considered the key to tunnelling in loose or soft ground filled 

 with water. 



The question of settlement, especially in towns, is a very important one. 

 When pumping has to be done the water is naturally drawn down in the adjacent 

 strata, and in addition quantities of sand often come with the water, and settle- 

 ment occurs from the first or from both of these causes. When compressed air is 

 used no pumping, of course, is necessary, and tlierefr)re there can be no settlement 

 imder that head. Probably the most fruitful cause of settlement in ordinary 

 tunnels iscaui^ed by the fact that more ground is taken out than the tunnel actually 

 fills, and although the utmost care is taken in supporting the ground and packing 

 all cavities, a certain amount of settlement invariably occurs. With a shield the 

 excavation is reduced to almost the net section of the tunnel, and therefore no 

 settlement can take place to any appreciable extent. As regards safety in working, 

 it is evident that when only the face of the excavation is open, and that, perhaps, 

 only in small areas, and the water is kept b.ack by compressed air, the maximum 

 of safety is assured. The great advantages of constructing the tunnel of cast-iron 

 segments are, that it is much quicker to build than anything else, and that it has 

 its full strength as soon as it is built ; and this latter is a very important matter in 

 soft ground, which exerts a heavj' pressure on the tunnel. 



The Tower Subway, 7 ft. 1^ in. in external diameter, constructed by Mr. Peter 

 Barlow in 1869, is interesting as being the fiist tunnel in which a shield shoved 

 forward as one structure was used, and for the construction of which cast iron was 

 adopted. It was driven through London clay, no water had to be dealt with, and 

 no ditficulties were encountered. In 1870 an experimental length of tunnel 8 ft. 

 in external diameter was driven under Broadway, New York City, by means of a 

 shield, to demonstrate the practicability of constructing tunnels by this method 

 without injuring buildings by settlement. Between 1870 and 1874 tunnels of 6 ft. 

 diameter were driven by the same means under the streets of Cincinnati for drainage 

 purposes, and for 1^ mile under Lake Erie for the supplv of water to Cleveland, 

 Ohio. 



The first large tunnel completed by means of shields and compressed air was 

 the St. Clair Tunnel, finished in 1890. It was constructed with cast-iron segments 

 of an outside diameter of 21 ft., and compressed air up to a pressure of 32 lb. was 

 used. This tunnel was principally through soft clay, and the maximum progress 

 in one month at one face was 382 ft. 



