he 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. When the bench had been removed to a point 61 ft. west of
Fifth Avenue, settlement was detected in the street surface above. Bench
excavation was suspended and a section of the permanent lining, 35 ft.
long, was placed. The space between the lining and the beams and between
the beams and the roof was filled with rubble masonry. Grout pipes were
built into the masonry and later all voids were filled with grout. Fig.
3, Plate LIX, shows the first section of the concrete lining completed
and part of the rubble in place; and Fig. 4, Plate LIX, shows details of
the work above the tunnels. A second section of bench was next removed
and more lining was placed. Work was continued in this way until all the
roof at the old three-track headings had been secured. In this portion
of the work the posts were embedded in the concrete.

Between Fifth and Sixth Avenues there were two more sections of bad rock
where it was necessary to support the roof with steel beams. At these
latter points there were no complications with the excavation for the
Three-Track Tunnel, and the work was much simpler. To avoid leaving the
center posts in the permanent work, two rows of temporary posts were
placed, as shown by Fig. 1, Plate LX, the center wall and skewback were
built, and the posts were removed, as shown by Fig. 2, Plate LX, before
placing the remainder of the lining.

In 32d Street the normal progress of the excavation was frequently
interrupted by encountering soft and unsound rock. In the excavation
between the East River and the Intermediate Shafts it was possible to
overcome these conditions by temporarily narrowing the excavation on one
side and supporting the roof on 16 by 16-in. transverse timbers caught
in niches in the rock at the sides, leaving sufficient room for the
steam shovel to work through. In order to save time, the height of the
excavation was not increased before placing these timbers, so that,
previous to the concreting, they all required to be raised to clear the
masonry lining and were then supported on posts on the center line
between the tunnels. This permitted the remainder of the excavation to
be made, and such additional timbering as was required was placed. At
most of these sections a brick arch and water-proofing were used, o