78 Proceedings of the Royal Irish Academy. 



X. — Ow A Mode of SuB-AaxjEous Tunnelling. By Sir Samuel 

 Ferghtson, Q. C, LL.D., President. 



[Read, December 10, 1883.] 



TuNNBLS under water-ways have always hitherto been constructed by 

 driving a masonry-built culvert through the subjacent strata. 



The conditions most favourable for such works are where the strata 

 are impervious to water, as rock, chalk, or tenacious clay. Those least 

 favourable are, where the river bottom consists of mud, sand, or gravel ; 

 and it may be doubted if engineering skill at present would be equal to 

 the construction of a drift tunnel through any considerable flow of 

 free water. 



But the more unfavourable such conditions are for the drift tunnel, 

 the more they seem to invite to another method which would appear 

 sufficiently practicable to make it worth the attention of engineers, and 

 which may be thought to involve enough of scientific speculation to 

 commend it to the attention of a learned society. 



It is simply the deposit, by sinking, of an iron-built tubular sub- 

 way, which, after being settled in its bed in the river bottom, should 

 be pumped out and connected with the underground approaches. 



A tidal river or estuary, in which the work should be kept out of 

 the way of navigation, will best illustrate the proceedings which should 

 be taken in carrying out such an operation. Let it be required to sink 

 such a subway under the navigable bed of a tidal river having a bottom 

 of mud, sand, gravel, or other material easily dredged ; a depth say of 

 16 feet in mid-channel at low water, and a width of 400 feet between 

 walled wharves, up to which the approaches are assumed to have been 

 executed, having their roadways 22 feet below the river bottom, and 

 terminating in cross- walls of wood or puddle at the back of the quay 

 wall at either side ; and let the tunnel be 1 6 feet in height under 

 6 feet of solid material between the crown of its arch and the deepest 

 part of the river channel. 



The operations would consist — {a) in the excavation of a transverse 

 trench in the bed of the river in the line and down to the level of the 

 roadways of the approaches ; {h) in the preparation, at either end, of 

 recesses on the face of the wharf wall, down which the ends of the 

 tubular tunnel should be guided in its descent into the trench so pre- 

 pared ; {c) in the construction of the iron tubular tunnel, having a 

 flat floor and arched section, floating it into position, sinking it into its 

 bed, and making good the river bottom alongside and over it; {d) in 

 luting the tube at either end in the recesses, and pumping it and them 

 out ; and (e) in cutting away the cross walls and completing the con- 

 nexions. 



Each operation would have its own practical difficulties. A level 

 bottom under the tube (in this case supposed to be a straight bar) 

 would be essential, as well for the avoidance of strains as for the 



