150 



TUNNELLING THEOUOH VARtOUS STItATA. 



out the full-sized tunnel. Tlie dryness or wetness of a 

 tunnel is often the result of the ground above being undis- 

 turbed or not, and lengths of tunnel which, when taken out, 

 have been perfectly dry, have afterwards been very wet 

 through water having found its way from above, through 

 cracks caused by the swelling of the ground, owing to care- 

 lessness in tunnelling. 



A very important point, in taking out lengths of tunnel 

 in heavy ground, is to place the crown bars high enough 

 above the crown of the arch, as the bars invariably come 

 down somewhat, owing to the great weight compressing the 

 props and sending the footblocks, on which they stand, into 

 the soft ground below. When sufficient allowance is not 

 made for tliis, it generally necessitates what is termed 

 poling back, that is, removing the poling boards one or two 

 at a time, excavating the ground to a sufficient height, and 

 then replacing the boards. This often leads to a disturbance 

 of the ground, thus throwing additional weight on the tim- 

 bering; and where the ground has already swelled before 

 the poling back, it renders it a dangerous operation, and one 

 which has caused the collapse of many a length, especially 

 as it must be remembered that the whole weight of the 

 ground has to be supported by propping off the centres ; and 

 where those are not strong enough to bear it, thoy become 

 distorted, and distort the portion of the arch already turned. 

 It shoiild, therefore, be the first care in soft ground to have 

 the crown bars placed well up, not being afraid of their 

 being a little high, which is a good fault rather than other- 

 wise. The centres used should also be of ample strength, 

 and then the miners may consider themselves prepared for 

 any emergency. 



In the coal measures, dangerous ground is often found 

 where thin beds of shale, coal and fireclay alternate. The 



