rred more frequently, the disadvantage
of placing the erector on the shield was more apparent. Under such
conditions the plane of the erector's motion was acutely inclined to the
plane of the ring, and, after placing the lower portion of the ring, it
was usually necessary to shove the shield a few inches farther in order
to place the upper plates. The practical effect of this action is
referred to later.

[Illustration: FIG. 1.]

At first the erection of the iron in the river tunnels interfered
somewhat with the mucking operations, but the length of time required to
complete the latter was ample for the completion of the former; and the
starting of a shove was seldom postponed by reason of the non-completion
of a ring. After the removal of the bottom of the diaphragms, permitting
the muck cars to be run into the shield and beyond, the two operations
were carried on simultaneously without serious interference. The
installation of the belt conveyor for handling the soft ground spoil in
Tunnel _A_ was of special benefit in this respect.

Preparatory to the final bolt tightening of each ring as erected, a
15-ton draw-jack, consisting of a small pulling-jack inserted in a light
eye-bar chain, was placed on the horizontal diameter, and frequently the
erectors were also used to boost the crown of the iron, the object being
to erect the ring truly circular. Before shoving, a 1-1/4-in.
turn-buckle was also placed on the horizontal diameter in order to
prevent the spreading of the iron, previous to filling the void outside
with grout. The approach of the supports for the upper floor of the
trailing platform necessitated the removal of these turnbuckles from all
but the three leading rings, but if the iron showed a tendency to
continue distortion, they were re-inserted after the passage of the
trailing platform and remained until the arch of the concrete lining was
placed.

The cost of handling and erecting the iron varied greatly at different
times, averaging, for the river tunnels, $3.32 per ton for the directly
chargeable labor of handling and erecting, to which must be added $7.54
for "top charges." The cost of repairing broken plates is included in
this figure.

_Broken Plates._--During the construction of the river section of the
tunnels, a number of segments were found to have been broken while
shoving the shield. The breaks, which with few exceptions were confined
to the three or four bottom plates, almost invariably