TUB SEVERN TUNNEL. _ 75 



<;hc aide walls and tlio invert, may bo Luilt as strongly as 

 any part of tho work. 



And now as to the effect of wator-prcssuro — 



The river Severn is natnrally the lowest drainage level of 

 the district. In any works going below that level, the water 

 met with must cither bo pumped out or forcibly kept out 

 under pressure. 



In a land tunnel the water runs out of itself at one end or 

 the other, and is of no further trouble after the work has 

 been completed, the brickwork has only to support the 

 weight of tho ground overhead ; but in the Severn Tunnel 

 he water-pressure is the main consideration. 



In discussing this question, it may be as well first to say 

 a word as to Land Tunnels. 



If the Tunnel should bo in rock or self-supporting strata, 

 no lining is needed for the actual support of the ground ; 

 but it has been found better to put in a thin lining of brick- 

 work in railway tunnels even in this case, for in blasting the 

 rock, some pieces are so far loosened that though they do 

 not come away at once, they do fall subsequently at an un- 

 certain date after the tunnel has been opened for traiiio, and 

 thus become a siuirtto of groat danger to tho trains. No 

 invert is needed in this case, and sometimes no side walls ; 

 but in regard to side walls, tho cost of dressing tho rock 

 on tho sides, and of forming a good springing for tlie arch, 

 make tho saving so small that it is usually better to build 

 side walls as well as an arch. 



In soft or yielding ground an invert is also wanted with 

 heavier arch a;nd side Avails ; but it is a fact well known to 

 practical engineers that tlio greatest weight on the brick- 

 work is always met with in shallow tunnels; that is, when 

 tliey are only sixty foot or so below tho surface ; the ground 

 then breaks up to the top, and the whole superincumbent 



