was
attached to each.

The method of working in soft ground from Long Island City is
illustrated by Plate LXXII. The full lines at the face of the shield
show the position of the earth before a shove of the shield, and the
dotted lines show the same after the shove. The face was mined out to
the front of the hood and breasted down to a little below the floor of
the top pockets of the shield. In the middle pocket the earth was
allowed to take its natural slope back on the floor. Toward the rear of
the bottom pockets it was held by stop-planks. The air pressure was
always about equal to the hydrostatic head at the middle of the shield,
so that the face in the upper and middle pockets was dry. In the lower
pockets it was wet, and flowed under the pressure of shoving the shield.
By this method 4,195 lin. ft. of tunnel was excavated by the four Long
Island shields in 120 days, from November 1st, 1907, to March 1st, 1908.
This was an average of 8.74 ft. per day per shield.

The rate of progress, the nature of the materials, and the methods
adopted are shown in Table 2.

_Preparations for Junction of Shields._--As previously mentioned, the
Manhattan shields were stopped at the edge of the reef. Before making
the final shove of those shields, special polings were placed with
unusual care. The excavation was bell-shaped to receive the Long Island
shields. The arrangement of the polings is shown by Figs. 4 and 5, Plate
LXXI. After the shields were shoved into final position, as shown at the
right in Fig. 5, the rear end of the polings rested over the cutting
edge and allowed room for the removal of the hood. After the latter had
been accomplished, the temporary bulkheads of concrete and clay bags
were built as a precaution against blows when the shields were close
together. An 8-in. pipe was then driven forward through the bulkhead for
distances varying from 30 to 100 ft., in order to check the alignment
and grade between the two workings before the shields were actually
shoved together. The errors in the surveys were negligible, but here, as
elsewhere, the shields were not exactly in the desired position, and it
took careful handling to bring the cutting edges together. The Long
Island shields were driven to meet those from Manhattan.

TABLE 2.--RATE OF PROGRESS, NATURE OF MATERIALS, AND METHODS ADOPTED IN
CONSTRUCTION OF EAST RIVER TUNNELS.

LINE A, LONG ISLAND.

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