82 Mr. Alexander E. Binnie [March 6, 



highly charged with water ; and when, as in the case of the Blackwall 

 Tunnel, it has to be executed through gravel under a wide river like 

 the Thames, the cost, difficulty and dangers of the work approach 

 the limit of engineering skill to carry it successfully to completion. 



It will be at once obvious that if the tunnel is of any considerable 

 size, and the soil to be excavated is of a soft nature such as clay, sand, 

 gravel, or the like, considerable difficulty will be experienced in 

 supporting the face, sides and top of the excavation, before the 

 lining is built into its place. If the work be of small dimensions it 

 is often called a heading, and its top is supported on cross timbers 

 resting on side props. Should it, however, be of larger size, the 

 timbering becomes much more complicated and costly, and requires 

 great skill in its design and management. When, between the 

 years 1818 and 1825, Sir Mark Isambard Brunei was thinking out 

 the mode of constructing the old Thames Tunnel between Rother- 

 hithe and Wapping, he designed several pieces of apparatus, which 

 he termed shields, to obviate the use of all the mass of timber 

 usually required. Some contrivance of this description became 

 necessary, for it was imperative that as far as possible, if settle- 

 ments or subsidences in the bed of the river were to be avoided, 

 no more material should be excavated than was just required to 

 receive the brickwork of the tunnel. Besides which, it is very pro- 

 blematical if the mode of timbering usually adopted would withstand 

 the varying strains to which it would be subjected under the varying 

 pressures due to the different depths of water at high and low tide. 

 The shield he ultimately adopted was a structure of iron, which could 

 be pushed forward in front of the work as it progressed, a model of 

 which stands on the table. It was so designed as to afford platforms 

 on which the men could work at different levels ; it afforded a means 

 of supporting the face and roof during excavation, and a place of 

 safety in the rear of the shield in which the brickwork of the tunnel 

 could be built up, and it could be pushed forward gradually in sections 

 by means of screw-jacks. I have now, I hope, made clear the general 

 subject, and must proceed to the particular work before us to-night. 



Position of the tunnel. — During the past ninety years many pro- 

 posals have been made for crossing the Thames below London Bridge, 

 where the port of London, with its river, ships and docks, forms a 

 barrier to vehicular or pedestrian traffic between its opposite banks. 

 The first work of the kind attempted, but not completed, was Yazes' 

 tunnel at Limehouse, in 1805. We then have Brunei's tunnel, 1825 

 to 1841. Then the Tower Subway for foot passengers, 7 feet in 

 diameter, carried through the London clay, in 1869-70, by Messrs. 

 Barlow and Greathead. And finally, the late Metropolitan Board of 

 Works obtained an Act in 1887 to construct a tunnel under the 

 Thames at Blackwall, six miles from London Bridge. This tunnel 

 crosses the river IJ miles below Greenwich and 3 miles above Wool- 

 wich, and will bring these growing and populous places into direct 

 communication with Poplar and the East and West India Docks on 



