place of the bulkhead across the tunnels for confining the air pressure.

The first work in air pressure was to remove the shield plug closing the
opening in the side of the shaft. This being done, the shield was shoved
through the opening, and excavation begun.

At the start the shields were fitted with movable platforms, but no
hoods of any kind were placed until after the rock excavation 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.