The
work of excavation was arduous, and proceeded very slowly, on account of
the care with which it was executed. Only a small amount of sand entered
the tunnel, but the lining was placed as soon as the excavation was
completed. Rubble masonry packing and grout ejected through pipes built
into the arch were used to fill the voids above the roof. As a further
precaution against the settlement of the subway, 2-in. pipes were washed
down from the street above the point where soft ground was exposed in
the roof of the tunnel, and through them grout was forced into the
ground at various depths. Careful levels show that no settlement of the
subway has taken place.
West of the Intermediate Shaft the tunnel was excavated for full width
until bad rock was 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.
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