diameter lining, 3 ft. thick, with
the brickwork forming the air passage = to 36.5 cubic yards per yard
forward. Excavation to outside of brickwork 36 ft. diameter = to 113
cubic yards per yard forward. Three tunnels 17 ft. diameter and 18 in.
brickwork. Brickwork lining for three tunnels = 24.5 cubic yards per
yard forward. Excavation outside brickwork for the same 105 cubic yards
per yard forward. It is assumed that three 17 ft. tunnels are stronger,
more conveniently formed, and involve less risks in construction than
one of 30 ft. diameter; at the same time there is no difficulty in
making the latter. The above shows the saving in the three tunnels of
23 per cent. in brickwork, and about 7 per cent. of earthwork, compared
with one of 30 ft. With regard to ventilation, it is well known that the
power required to force air along passages is practically as the cube of
the velocity; and as the area of the air passages in the single tunnel
is 106 ft. with speed ten miles per hour, and that of one of the 17 ft.
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