SILLS.
forated there with three or four holes of
s inch diameter, was sunk under com-
pressed air into the sand until it reached
a depth varying from 16 to 19 feet. In
the air pressure supply pipe provision
was made, by means of suitable branches
and stopcocks, for connecting therewitli
an apparatus which, with the aid of an
injector device, enabled any desired quan-
tity of cement powder to be fed into the
air current. While this was being done,
the pipe was slowly withdrawn in an
upward direction, so that the cement
was thoroughly diffused throughout the
bed, which was full of natural moisture.
The cement was supplied dry and warm
air was used. Consecutive areas, from
8 to 12 incites square, were treated in
this way, and the concrete allowed suffi-
cient time to set before being built
upon.
Principal Constructive Features.—
Apart from the question of the floor
and its foundations, the following (illus-
trated in fig. 176) are the most im-
portant features in the construction of
entrances and passages generally:—
(1) the sills, (2) the platforms, (3) the
side recesses and chambers, (4) the walls,
and (5) the levelling culverts. The sub-
ject of the means adopted for closing the
entrance is reserved for an independent
chapter.
1. Sills.—If for caissons, these will
constitute straight lines in plan, normal
to the axis of the waterway ; if for gates,
each will consist of two straight or
curved lines intersecting at the centre.
The level of the sill will generally be
somewhat higher than the floor of the
chamber in order to avoid sinking the
gate or caisson platform below the floor
level. This, however, is often done,
more especially in the case of caisson
platforms, which are not so extensive
GUTER WATERWAY.
Fig. 176. —Elevation of a Dock Entrance,
25'