TUNNKLLTNf! TITHOUOH VARIOTTS RTT7ATA. 



149 



heavy clays, which swell on exposure to tlio air, as tlio cost 

 of pumping is more than connterbalancod Ly the extra 

 weight of the ground and difficulty in timbering. The 

 safest and cheapest mode of tunnelling such ground is, 

 where the tunnel is too long for time to admit of working 

 from both ends only, to sink shafts, and then take out the 

 full-sized tunnel in suitable lengths, care being taken to 

 follow up as closely as possible with the brickwork or 

 masom-y lining, a sufficient length of top heading only being 

 driven to admit of the drawing of the crown bars. It may 

 be contended that, in some instances, it is n(it possible to 

 sink shafts, owing to the height of the hill (causing great 

 expjense), or from other causes. The author does not con- 

 sider, however, that such cases are often met witli, as, where 

 the height of the hill above the line of the tunnel is very 

 great, it is not often that such heavy ground as that to 

 which he refers is met with in any but short lengths, and 

 ho would then advocate the taking out, if possible, and 

 lining of the lengths of soft groiTnd, as they are met with in 

 driving the bottom heading, if materials for doing so can 

 by any means, without great expense, be bi'ought to the 

 place. Great trouble and expense would often bo saved 

 if this were done, especially througli faulty ground in the 

 coal measures, as, although these strata are not necessarily 

 bad tunnelling ground, and are often found to be very good, 

 fissures and faults are often met with of considerable 

 extent, filled with soft rubbish, which, when the heading is 

 first driven through, does not show any signs of difficulty, 

 but which, after exposure to the air for any length 

 of time, swells and makes a way for considerable bodies 

 of water, which may have lodged in some basin above, and 

 which the filling in the fissure is often able to retain when 

 undisturbed, but afterwards gives great trouble in taking 



