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.
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