76 



THE SEVEEN TUNNEL. 



load comca upon tlio arch. But in (loop tunnels, the ground 

 does not break up very far above the arch, and therefore it 

 mainly supports itself. 



The chief precaution to bo adopted in order to insure 

 permanent stability in a tunnel arch and side walls is to 

 have the space at the hack, between the excavated ground 

 and the brickwork, quite built up with bricks and mortar. 

 There must always be a space loft over the arch where the 

 timbers are withdrawn, even if the ground has been got out 

 with the greatest truth; but, in addition to this, miners 

 have usually a tendency to take out too much ground at the 

 haunches of an arch, for some hidden reason which. ii.ppca,rs 

 to be common to mining judgment, and if this space is not 

 completely filled up, the effect can readily be imagined. An 

 arch, it must be remembered, is a Jialanced structure and of 

 great strength when properly loaded ; but in the case now 

 supposed, it will be found that as the ground by-and-by 

 naturidly comes down heavily where it is l(«ist supported — 

 that is, over the crown of the arch— if the space above the 

 haunches has not been (jnito filled up, the crown of the arch 

 sinks under the load and the haunches rise until they do get 

 a bearing against the ground above, and the arch is badly 

 crippled, more especially if the brickwork has been built in 

 rings, for the inner rings then gape at the crown, and some 

 of the bricks often fall out. When land tunnels fail, it is 

 almost always be(!a,uso this bat^k filling has been neglected; 

 and this can ordy be seen to during the progress of the work, 

 and not afterwards. 



In the Severn Tunnel, on the other hajid, the cliief load 

 on the brickwork is from the water pressure, which differs 

 entirely from the land pressure, as described above ; in that 

 the pressure is from all directions, and precisely according 

 to the head of water at each point. Referring again to the 



