16fl 



TtlNNELTJNO TIinOUOH VARIOUS STRATA. 



has remonstrated with the contractor or his mon on this 

 point, had the reply, " What does it matter, so long as the 

 heading is not outside the tunnel." It does matter ; as, if 

 you had a heading all on one side of the tunnel, when you 

 proceed to enlarge to the full size, it makes a very great 

 difference whether your heading be in the centre or not, 

 as you have all the hard driving on one side, and none on 

 the other, therefore, your heading is not the benefit it 

 should be, besides having to remove the excavated material 

 all over to one side to fill it into the wagons. The author 

 is indebted for much sound practical advice on these matters 

 to his father, whose experience in tunnelling has been very 

 great; and although they may appear trifling to some, yet 

 in these days of competition and cutting down of cost, they 

 are not to be lost sight of, and, taken together, make all the 

 difference in the economical construction of a tunnel. 



In rock tunnels, it is often unnecessary to have side walls, 

 as the ground will generally stand well enough without, 

 although an arch is very seldom dispensed with nowadays, 

 although it was foi'merly done to a large extent. Where 

 arches only are adopted, they should have as small a rise 

 as is consistent with safety, so as to spring directly off of 

 the sides of the tunnel. It is very essential in these eases 

 to have arches of large radius, as they then do not bear any 

 downward weight on the sides of the tunnel, and are not 

 endangered by any after displacement if the sides should 

 at any time require facing up with brickwork or masonry, 

 which often happens in tunnels of this kind. It being 

 borne in mind that in rock the sides require, if any, mere 

 face walls, and which will not have to bear any direct 

 thrust, but only to keep any stray lumps which may become 

 loose from the vibration of passing trains. It is very 

 essential in all tunnels, but i)erhap3 more ]iarticularly so 



