screwed to the back. The arches were put in, either in 15, 30 or 45-ft.
lengths, depending on what was ready for concrete and what could be done
in one continuous working. The rule was that when an arch was begun, the
work must not stop until it was finished. An arch length always ended in
the middle of a ring. The lagging was placed to a height of about 6 ft.
above the bench before any concreting was done. When the concrete had
been brought up to that point, lagging was added, one piece at a time,
just ahead of the concrete, up to the crown, where a space of about 18
in. was left. When the lagging had reached the upper hanger-bolts, they
were removed, which left only the two bottom bolts fixed in the
concrete. Most of these were unscrewed from the eye and saved, as tin
sleeves were placed around them before concreting. Two cast-iron eyes
were lost for every 5 ft. of tunnel. To place the key concrete, a stage
was set up in the middle of the floor, and, beginning at one end, about
2 ft. of block lagging was placed. Over this, concrete was packed,
filling the key as completely as possible. This was done partly by
shoveling and using a short rammer, and partly by packing with the hands
by the workmen, who wore rubber gloves for the purpose. Another 2 ft. of
lagging was then placed, and the operation was repeated, and thus
working backward, foot by foot, the key was completed. This is the usual
way of keying a concrete arch, but in this case the difficulty was
increased by the flanges of the iron lining. It was practically
impossible to fill all parts of the pockets formed by these flanges. To
meet this difficulty, provision was made for grouting any unfilled
space. As the concrete was being put in, tin pipes were placed with
their tops nearly touching the iron lining, and their bottoms resting on
the lagging. Each pocket was intended to have two of these pipes, one to
grout through and the other to act as a vent for the escape of air. Each
center key ring had six pipes, and each side key had eight. The bottoms
of the pipes were held by a single nail driven half way into the