i il 

 p 



I 148 



TUNNELLTNO 



thuouoh 



VARTOUS 



STKATA. 







medium 



ground, 



heing neither 



hard nor 



soft ; and 



hard 



[ ground or rock. 















' I What 



is called 



soft 



ground is- 



by far the most 



difficult, 



and requires tlie most care on the part of tlie constructors. 

 Under this head may be classed running sand, heavy clays 

 and faulty ground in the coal measures. 



In tunnelling through very heavy ground, it is no doubt 

 undesirable to drive a bottom heading, owing to the difF.- 

 culty in supporting the ground and the very heavy timber- 

 ing required, it being necessary, in some strata, to close 

 pole the heading all round; and whore a heading has to 

 stand exposed to the air for any length of time, there 

 is a tendency in the ground to swell, and perhaps disturl) 

 the ground above, making it a very difficult and costly 

 matter to afterwards take out the lengths of full-sized 

 tunnel. 



The author remembers a tunnel, with which ho was con- 

 nected afl.or the occurrence, which ran in to the surface, 

 letting in part of a farmyard and buildings, owing to the 

 constructors having lost their reckoning ; and in endeavour- 

 ing, by driving side headings, to find their wny to a shaft 

 which had been sunk on the centre line, and which they 

 failed to meet. This multiplicity of headings was the cause 

 of the mishap, owing to weakening the ground through too 

 many headings, and perhaps bad. timbering in addition. It 

 was extremely difficult to tunnel through this portion 

 afterwards ; the whole of the ground seemed full of timbers 

 of all sines, and we came upon headings in most extraor- 

 dinary places. 



In cases where much water may be expected, a bottom 

 heading is no doubt very desirable, as it so completely 

 drains all the workings of the tunnel ; but even in such 

 cases the author does not advocate it in such strata as 



