_--From the end of the single-track tunnel westward
to Fifth Avenue on 33d Street, and to Madison Avenue on 32d Street, with
some exceptions, each pair of tunnels was excavated for the entire width
at one operation. Three different methods of work were extensively used.
They were the double-heading method, the center-heading method, and the
full-sized-heading method, and these differed only in the manner of
drilling and blasting. The bench was usually within 10 or 15 ft. of the
face of the heading, and was drilled and fired in the same way as in the
single tunnels. After the installation of the permanent plant, most of
the muck was handled by steam shovels.
In the double-heading method, shown on Plate LVII, the top headings for
each tunnel of the pair were driven separately, leaving a short rock
core-wall between them. The headings were drilled from columns in the
manner described for the single tunnels. The temporary rock dividing
wall between the headings was drilled by a tripod drill on the bench of
one of the headings, and was fired with the bench.
In the center-heading method, also shown on Plate LVII, only one heading
was driven. It was rectangular in shape, about 8 ft. high and 14 ft.
wide. It was located on the center line between the tunnels. In general,
the face was from 6 to 12 ft.
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