,
and the advance was stopped, pending a decision as to a change of plan.
[Illustration: PLATE LX, FIG. 1.--DOUBLE ROW OF POSTS UNDER I-BEAMS,
SUPPORTING ROOF IN BAD ROCK SECTION.]
[Illustration: PLATE LX, FIG. 2.--CENTER WALL AND SKEWBACK UNDER I-BEAMS,
AFTER REMOVAL OF DOUBLE ROW OF POSTS.]
[Illustration: PLATE LX, FIG. 3.--TIMBERING IN FULL-WIDTH HEADING OF
THREE-TRACK TUNNEL.]
[Illustration: PLATE LX, FIG. 4.--UNDERPINNING WALLS IN OPEN-CUT
SECTION.]
After some delay, an exploration drift, similar to the one already
described, was driven through to Sixth Avenue, and a change in plan was
made, substantially the same as for the 33d Street tunnels. Enlargement
to full size was at once started, but, for 400 ft. the rock was very
soft and poor, and required extremely careful handling. The exploration
drift was widened out to the full Twin-Tunnel width, and I-beams were
placed and supported, in much the same manner as in 33d Street. The rock
was so soft that it was frequently necessary to drive poling boards
ahead as the face was mined out with picks and shovels. The load was
very heavy, and the work the most difficult encountered in the tunnels.
After this stage of the enlargement was reached, the excavation of the
bench and the placing of the lining proceeded alternately, with the
I-beams temporarily supported on long posts while the concrete core-wall
was being built.
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