The four river tunnels, between the Manhattan and Long Island City
shafts, a distance of about 3,900 ft., were constructed by the shield
method. Eight shields were erected, one on each line in each shaft, the
four from Manhattan working eastward to a junction near the middle of
the river with the four working westward from Long Island City. Toward
the end of the work it was evident that the shields in Tunnels _B_, _C_,
and _D_ would meet in the soft material a short distance east of the
Blackwell's Island Reef if work were continued in all headings. In order
that the junction might be made in firm material, work from Manhattan in
those three tunnels was suspended when the shields reached the edge
of the ledge. The shields in Tunnel _A_ met at a corresponding point
without the suspension of work in either. An average of 1,760 ft. of
tunnel was driven from Manhattan and 2,142 ft. from Long Island City.
[Illustration: PLATE LXV, FIG. 1.--SHIELD FITTED WITH SECTIONAL SLIDING
HOODS AND SLIDING EXTENSIONS TO THE FLOORS.]
[Illustration: PLATE LXV, FIG. 2.--SHIELD FITTED WITH FIXED HOODS AND
FIXED EXTENSIONS TO THE FLOORS.]