TUNNELLING THROUGH VARIOUS STRATA. 



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to ensure the wall being as sound as if built up in the usual 

 way, and his own experience has led him to doubt that the 

 work is usually done in a proper manner without the closest 

 supervision, which it is very difficult to give in person, or 

 to be certain of obtaining it from others; and ho always feels 

 inclined to discourage any method of working which is diffi- 

 cult of supervision, and of the soundness of which there is 

 never any satisfactory conviction. An instance of a tunnel 

 being completed in a remarkably short space of time, is the 

 case of the Combe Down Tunnel, on the line between Bath 

 and Bournemouth, about two miles from Bath. This tunnel 

 is through sand of the Inferior Oolite, with beds of rock, 

 and underlying the Eullors Earth. The site of this tunnel 

 Was extremely well chosen, as it was nearly dry throughout 

 its entire length, and in constructing the tunnel it was 

 found that it lay between two large water basins, as in 

 places it appeared in the bottom and in others at the top of 

 the tunnel, and slight alterations were made in the gradient, 

 so as to avoid the water both at the top and bottom of the 

 tunnel. When this tunnel was first projected, it was thought 

 that it would, turn out to be unusually wot, as large streams 

 of water issue from the hill, on the Midford side of Coinljo 

 JJown especially, but at a much higher level than that of 

 the tunnel ; and it was considered likely that some of this 

 water might find its way into the tunnel. 



The length of the tunnel is about 1,900 yards, and a 

 bottom heading was driven throughout its entire length, 

 from both ends, and the tunnel completed without the 

 aid of shafts, within eighteen months from the com- 

 mencement. The sand was of a liard and compact nature, 

 and would stand in most places with little or no tim- 

 bering, being easily excavated with a pick, and could, in 

 fact, be cut out with a penknife, although powder was 



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