to
center, the tie-rods were placed midway between them, and 6 ft., from
center to center, vertically and horizontally. Fig. 8 shows the arrangement
of these rods and drains. Around the Express Building site, just west of
Ninth Avenue, on the south side of the work, the bridge seat was omitted,
and the face wall was designed 2 ft. thick from top to bottom. The batter
on the 31st Street wall was made variable, the top and bottom being
constant distances from the center line and on different grades.

The retaining walls were water-proofed with three layers of felt and
coal-tar pitch, which was protected by 4 in. of brick masonry. A 6-in.
vitrified drain pipe was laid along the back of the wall, with the joints
open on the lower half, and this was covered with 1 ft. of broken stone and
sand before any back-fill was placed on it.

The arrangement of the drains was as follows: The 6-in. drain back of the
retaining wall was connected with one of the box drains in the rear of the
face wall by a cast-iron pipe or wooden box every 24 ft., and this ran
through the base of the retaining wall. Midway between these pipes, a
connection was made at the bridge seat between the drain in the rear of the
face wall and the gutter formed at the rear of the bridge seat to carry off
rain-water coming down the face of the wall above. All the box drains,
except those connected with the drains back of the retaining wall, were
sealed at the elevation of the base of the retaining wall, as noted
previously.

The specifications required vitrified pipe to be laid through the retaining
wall, but, owing to the difficulty of holding the short lengths of pipe in
place during the laying of wet concrete, they were dispensed with, and
either iron pipes or wooden boxes were used.

_Tie-Rods._--When the excavation on the sides had been completed, movable
drilling platforms were erected, as shown by Fig. 4, Plate L. The holes
were drilled on a pitch of 2 in. per ft. with the horizontal. The depths of
the holes were decided by the engineer, and were on the basis of a minimum
depth of 5 ft. in perfect rock; the character of the rock, therefore, and
the presence of seams, determined the depths of the holes. Each hole was
partly filled with grout, and the rod, with the steel wedge in the split
end, was inserted and driven with a sledge so that the wedge, striking the
bottom of the hole first, would cause the split end of the rod to open.
Each hole was then enti