shield was driven above grade and it was desired
to descend, the passage of the shield over the summit produced a like
effect. In all these movements, with the space between the tail of the
shield and the iron packed tight with pugging, the upward thrust of the
shield tended to flatten the iron in the bottom and occasional broken
plates were the result. The free use of the taper rings, placed so as to
relieve the binding of the lining on the tail of the shield, forces the
tunnel to follow the variations in the grade of the shield, but reduces
greatly the injuries to the rings from this action.

In Tunnel _D_, where very high vertical leads were required through the
soft sand, combined with a marked tendency of the shield to settle, the
shield was badly cramped on the iron and dragged along it at the top.
The bearing of the iron on its soft foundation tended to thrust up the
bottom in this case also, as shown by the opening of the bottom
cross-joints when the bolts were slackened to relieve the strain during
a shove. The anticipated cracks in the crown plates, which have been
more frequently observed in other tunnels, did not occur here, and were
not found elsewhere except in one place in Tunnel _B_ where they were
traced to a similar action of the shield. The cracks resulting from the
movements of the shield, as briefly described above, in this third case
were not confined to any particular type, but occurred more frequently
at the extreme end of the circumferential flange than at any other
point.

The number of broken plates occurring in the river tunnels was 319, or
0.42% of the total number erected. Of these, 52 were found and removed,
either before or immediately after a shove, by far the greater number
being broken in handling before or during erection. The remaining 267
are considered below.

_Repair of Broken Plates._--On the completion of a shove, the tail of
the shield lacked about 5 in. of covering the full width of the last
ring, and the removal of a plate broken during the shove, therefore,
would have exposed the ground at the tail of the shield. With a firm
material in the bottom, this introduced no particular difficulties, and,
under such conditions, a broken plate was usually removed at once. In
the sand, however, and especially on the Manhattan side where it was
quick and flowing, the removal of a plate was attended with some danger,
and such plates were usually left to be removed on the completion of