1896.1 The Tunnel under the Thames at Black wall. 81 



WEEKLY EVENING MEETING, 

 Friday March 6, 1896. 



Sir Benjamin Baker, K.C.M.G. LL.D. F.R.S. M. Inst. C.E. 



Manager, in the Chair. 



Alexander R. Binnie, Esq. M. Inst. C.E. F.G.S. M.B.L 



Chief Engineer L.C.C. 



The Tunnel under the Thames at Blachvall. 



The subject of this evening's discourse, the tunnel under the Thames 

 at Blackwall, at once defines and narrows it to an account of the 

 construction of a subaqueous tunnel ; and although I shall describe 

 the whole work, yet my remarks will be more particularly directed to 

 that part of the tunnel which is situate under the Thames, A tunnel 

 may be defined as a horizontal or inclined subterranean perforation 

 or boring, generally constructed for the accommodation of a roadway, 

 a railway, or a canal. It will be noticed that I use the word perfora- 

 tion or boring, by which I mean a subterranean excavation carried 

 out in a horizontal or inclined direction underground, either from its 

 two ends or from the bottoms of shafts sunk to the proper depth upon 

 its centre line. I make this definition lio prevent confusion with 

 another very similar class of work, to which I shall have to allude, 

 which is constructed by first sinking or digging a horizontal trench 

 to the required depth, in which the roadway is formed and arched 

 over, the excavation or trench afterwards being filled in above it. 

 This mode of construction is termed cut and cover work, and is the 

 way in which the sewers in our streets are generally built, and most 

 of our underground railways were carried out as cut and cover work. 

 In tunnelling, therefore, at the outset of our description, I wish 

 you to bear in mind that the work divides itself naturally into 

 two main portions : (1) the excavation, digging or blasting of the 

 material to be removed ; and (2) into lining or arching in the exca- 

 vation, so as to prevent the sides, top and bottom from foiling in or 

 being pressed upwards by the weight of the superincumbent earth 

 or rock. It will at once be noticed, therefore, that the mode of con- 

 structing any particular tunnel will difier very much according to 

 the nature of the material to be excavated, be it" rock, clay, gravel, or 

 quicksand, and that in construction the whole work will be rendered 

 much more costly and difficult if it has to be carried through £^round 

 Vol. XV. (No. 90.) g 



