cavation was
completed.

METHODS OF EXCAVATION.

The distribution of materials to be excavated, as previously outlined,
divided the excavation into three distinct classes, for which different
methods had to be developed.

These three classes were:

_First._--All-rock section.
_Second._--Rock in the bottom, earth in the top.
_Third._--All-earth section.

The extent of the second and third classes was much greater than that of
the first, and they, of course, determined the use of the shield.
Shields had not previously been used extensively in rock work, either
where the face was wholly or partly in rock, and it was necessary to
develop the methods by experience. The specifications required that
where rock was present in the bottom, a bed of concrete should be laid
in the form of a cradle on which to advance the shield.

_All Rock._--At different times, three general methods were used for
excavating in all-rock sections. They may be called: The bottom-heading
method; the full-face method; and the center-heading method.

The bottom-heading method was first tried. A heading, about 8 ft. high
and 12 ft. wide, was driven on the center line, with its bottom as
nearly as possible on the grade line of the bottom of the tunnel. It was
drilled in the ordinary manner by four drills mounted on two columns.
The face of the headings varied from 10 to 30 ft. in advance of the
cutting edge. After driving the heading for about 10 ft., the bottom was
cleared out and a concrete cradle was set. The width of the cradles
varied, but was generally from 8 to 10 ft.

The excavation was enlarged to full size as the shield advanced, the
drills being mounted in the forward compartments of the shield, as
shown by Fig. 1, Plate LXVII, which represents the conditions after the
opening had been cut in the bulkhead, but before the new methods,
mentioned later, had been developed.

[Illustration: PLATE LXVIII]

The sides and top were shot downward into the heading. The area of the
face remaining behind the heading was large, and a great number of holes
and several rounds were required to fire the face to advantage. As soon
as firing was started at the face, the heading was completely blocked,
and operations there had to be suspended until the mucking was nearly
completed. The bottom-heading method was probably as good as any that
could be devised for use with the shields as originally installed. All
the muck had to be taken from the face b