The contractors planned to erect the iron lining with erectors of the
same pattern as that used on the shield under the river, mounted on a
traveling stage. These will be described in detail in Mr. Japp's paper.
Two of these stages and erectors worked in each tunnel at different
points. The tunnel was attacked from so many points that these erectors
could not be moved from working to working. The result was that about
58% of the lining was built by hand. At first thought, this seems to be
a crude and extravagant method, as the plates weighed about 1 ton each
and about 20,000 were erected by hand. As it turned out, the cost was
not greater than for those erected by machinery, taking into account the
cost of erectors and power. This, however, was largely because the hand
erection reduced the amount of work to be done by the machines so much
that the machines had an undue plant charge.
The hand erection was very simple. A portable hand-winch, with a 3/8-in.
wire rope, was set in any convenient place. The wire rope was carried to
a snatch-block fastened to the top of the iron previously built; or,
where the roof was in soft ground, the timbering furnished points of
attachment. The end of the wire rope was then hooked to a bolt hole in a
new plate, two men at the winch lifted the plate, and three or four
others swung it into approximate place, and, with the aid of bars and
drift-pins, coaxed it into position and bolted it. Where there was no
timbering above the iron, sometimes the key and adjoining plates were
set on blocking on a timber staging and then jacked up to place.