152 TUNNELLTNa THROUGH VA-RTOUS STRATA. 



One of the chief causes, — in fact,, almost, the only cause, — of 

 so many tunnel arches being flat at the crown, is from the 

 preceding reason. The tunnel has been taken out too small 

 at the crown, therefore a ring or two of brickwork is left 

 out, to pass the crown bars without going to the trouble and. 

 expense of poling back, thiis weakening the crown where 

 most of the weight occurs ; the spaces behind the brickwork 

 in the haunches are loft unfilled, and in a very short time 

 after the centres are struck, the arch takes the distorted 

 and dangerous form which is so often seen. Indeed, the 

 wonder is that so many of tlio tunnels, constructed when 

 the supervision was lax in former days, have stood like 

 they have. Experience is a grand thing, but it is not every- 

 thing, and often seems to confirm some men only in the old 

 rulo-of-thumb ways learnt in their yoiitli, and which it is 

 almost impossible to eradicate. What is wanted is more 

 intelligent supervision, and not merely rule of tJiumh. 



A source of groat danger exists in tunnelling tlirnngh the 

 coal measures near collieries where the ground has been 

 undermined by the workings. Very often coal is prospected 

 for and headings driven which are afterwards abandoned, 

 and no record left of their position, or any ]dans of the 

 workings. So in tunnelling through the neigliboui'liood of 

 many abandoned coal mines, it is often impossible to tell 

 where to expect these old workings. And tliny may bo 

 met with at most unexpected times; and, being often 

 full of water, are often a source of great danger and 

 expense, in addition to the ground in their vicinity 

 having been disturbed by the rotting of the timber with 

 which they wore supported, or in many cases by the timber 

 having been withdrawn for other purposes, and the work- 

 ings loft to their fate. A short tunnel on tlio Penanr 

 Branch of the Great Western Railway, which was con- 



