178 RAILWAY CONSTRUCTION. 



curved wing walls, as in Fig. 220, or with straight wing walls, 

 as in Figs. 221, 222, and 223. Where the approach cutting is in 

 rock, the latter form is generally adopted. 



It would be misleading to put down any average price for 

 tunnel- work. So much depends upon the locality, the description 

 of material to be excavated, the cost of masonry or brickwork, 

 and the cost of labour. Added to these come the unforeseen 

 troubles of slips and water-laden strata, creating difficulties 

 which baffle the miners for a time, and add enormously to the 

 expenditure. Some tunnels for double line have been con- 

 structed in good ground, and under favourable circumstances as 

 to building materials and labour, for as low as 32 per lineal 

 yard ; while others, carried out under adverse conditions, have 

 cost as much as 150 per lineal yard. A medium somewhere 

 between the two should represent the cost of tunnel-work 

 through ground which does not present any special difficulty. 

 At the same time it must be borne in mind that simple tunnel- 

 ling which can be done in one locality for 50 or 60 per lineal 

 yard, would be increased 20, 30, or 40 per cent, in another, where 

 building material for the lining is scarce and expensive. 



Tunnel-work abroad will generally cost more than the same 

 work at home. The native labourers may perhaps be procured 

 at low rates, but the skilled workmen must be brought from a 

 distance, and will obtain high wages. 



Another form of tunnel-work, generally termed the covered- 

 way system, is frequently adopted in towns and places where 

 land and space are very valuable. This method consists in the 

 excavating and removing of earthwork to admit of the building 

 of the side walls and arching of a suitable tunnel- way, and then 

 filling in over the top to a depth of three or four feet, or up to 

 the level of the original surface of the ground. This work may 

 be carried out by either removing the entire width of the earth- 

 work before the commencement of any building operations, or 

 by first forming two deep, well-shored trenches, in which to 

 build the side walls up to about arch-springing. In bad ground 

 the latter arrangement has the advantage, as the shoring and 

 strutting to hold up the sliding material is limited to the widths 

 of the two naiTOW trenches, and the centre block of earthwork 

 is left untouched as a support to the strutting. When the side 

 walls have been built sufficiently high the upper portion of the 

 centre block of earthwork can be removed to allow of the 



