RAILWAY.
191
Fig- 7-
Section of Tunnel, shewing Drainage.
The following prices shew how much they vary in expense, viz.
Name of Tunnel. Price per yard.
Birkenhead Tunnel (single line) . . . . £ 32 in red sandstone
Box Tunnel, on Great Western, . . .
. 100 „
oolitic rock
Bletchingley, South Eastern, ....
72
Cheltenham.
34
Clay Cross, North Midland.
. 100
Grovely Hill, Bristol and Birmingham , .
. 32 „
marl
Kilsby, London and North Western, . .
. 125
Leeds, Leeds and Selby,.
• 25,,
shale & coal measures
• 80 „
red sandstone
Royston, North Midland,.
. 50 „
red sandstone
Saltwood, South Eastern,.
Summit, Manchester and Leeds, . . .
. 97
Whitehall, Exeter,.
53
The expense of the Kilsby and Saltwood tunnels was increased by meeting withlarge quantities of water.
It may be mentioned here, that in tunnelling through chalk, 1 pot-holes’ are fre-quently met with, which are cylindrical shafts, worn apparently by the rotatoryaction of water and stones, and filled generally with gravel or some other foreignsubstance; and the removal of the chalk from below the bottom of these, leaving onlya thin shell to support them, frequently causes their contents to break through, andoccasionally to do serious damage to the tunnel.
Viaducts .—In the construction of viaducts, either instead of embankments or tocarry a railway over roads, rivers, &c., Engineers appear to concur in considering thatbrick and stone are the most desirable materials to use; but questions of economy,and the necessity of crossing large spans with a level soffit, often compel them toadopt wood and iron: and where the foundations are treacherous, as in the miningdistricts, or where the substrata are compressible, a flat soffit affords capabilities forrepair which are highly advantageous.