encountered about 60 ft. west of Madison Avenue. (See
General Viele's map, Plate IX.) Timbering was used for a short distance,
and then the heading and bench were narrowed to 18 ft., and steam-shovel
excavation was abandoned. As the heading advanced the rock grew steadily
softer, the difficult conditions in this locality culminating when a
slushy disintegrated feldspar was met, requiring poling and breasting.
Thereafter the rock improved markedly, but near the east side of Fifth
Avenue its thickness above the roof was found to be only 1-1/2 ft.,
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. Considerable settlement took place while shifting the
posts, and eventually showed on the street surface and in the adjacent
sidewalk vaults, but no damage was done to the structural portions of
the buildings.

While the above work had been going on westward from Fifth Avenue, the
excavation of the Twin Tunnel eastward from the end of the open-cut
section at Sixth Avenue had been proceeding rapidly, and, toward the end
of the difficult Fifth Avenue work, it was being attacked from both
directions.