NT OF SHIELD.]

[Illustration: PLATE LXX, FIG. 4.--HYDRAULIC ERECTOR PLACING SEGMENT.]

_All-Earth Section._--As described by Messrs. Hay and Fitzmaurice, in a
paper on the Blackwall Tunnel,[C] the contractor had used, with marked
success, shutters in the face of the shield for excavating in loose open
material. He naturally adopted the method for the East River work. When
the shields in Tunnels _B_ and _D_, at Manhattan, the first to be driven
through soft ground, reached a point under the actual bulkhead line,
work was partly suspended and shutters were put in place in the face of
the top and center compartments. The shutters in the center compartments
in Shield _D_ are shown in Fig. 3, Plate LXX, while the method of work
with the shutters is shown by Figs. 4, 5, 6, and 7, Plate LXVIII. Fig. 4
on that plate shows the shield ready for a shove. As the pressure was
applied to the shield jacks, men loosened the nuts on the screws holding
the ends of the shutters, and allowed the latter to slide back into the
working compartments. At the end of the shove, the shutters were in the
position shown in Fig. 5, Plate LXVIII. In preparing for a new shove,
the slides in the shutters were opened, and the material in front was
raked into the shield. At the same time, the shutters were gradually
worked forward. The two upper shutters in a compartment were generally
advanced from 12 to 15 in., after which the muck could be shoveled out
over the bottom shutters, as shown on Fig. 6, Plate LXVIII, and Fig. 3,
Plate LXX. No shutters were placed in the bottom compartments, and as
the air pressure was not generally high enough to keep the face dry at
the bottom, these compartments were pretty well filled with the soft,
wet quicksand. Just before shoving, this material was excavated to a
point where it ran in faster than it could be taken out. Much of the
excavation in the bottom compartment was done by the blow-pipe. During
the shove the material from the bottom compartment often ran back
through the open door in the transverse bulkhead, as shown by Fig. 5,
Plate LXVIII.

In the Blackwall Tunnel the material was reported to have been loose
enough to keep in close contact with the shutters at all times. In the
East River Tunnels this was not the case. The sand at the top was dry
and would often stand with a vertical face for some hours. In advancing
the shutters, it was difficult to bring them into close contact with the
face at the end of