Test holes were drilled above this heading and to the sides. The
results indicated that there was sufficient rock cover of fair quality
to enable the Twin Tunnel to be driven without great risk. The new plan
(continuing the Twin Tunnel westward at a lower grade) was adopted in
March, 1907, and work was immediately resumed at Fifth Avenue.
The relation between the cross-sections under the old and new plans at
that point is shown by Fig. 3. Before the new section was excavated it
was necessary to support the timber work in the old headings. The plan
adopted is also shown by Fig. 3. The rock was excavated under the center
heading, as shown in cross-section, for a length of about 3 ft. A girder
composed of two 18-in. I-beams was then put in position over each line
and supported on the sides by posts. The ends at the center lines
between the tunnels were supported on short posts bearing on the rock
bench. The support of the timbering in the headings was then transferred
to the girders by additional posts. Blocking was also inserted between
the tops of the beams and the rock walls between the headings. Fig. 2,
Plate LIX, gives a good idea of the timber work in the top headings
above the I-beams. When the roof had been made secure, the removal of
the bench was begun. As the work advanced it was necessary to replace
the short posts at the center of the tunnel by others of full height,
and there was considerable settlement in the I-beams during this
operation.
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