uired
to be stable against the full pressure before the superstructure was
erected, and in case it should ever be removed; or, to state the matter
more clearly, the reaction from the superstructure was so great in
comparison to the weight of the wall, that, if it fell in front of the
resultant of all the other forces, the width of base required would be
greatly increased to make the wall stable after the superstructure was
erected; whereas, if the reaction from the superstructure fell back of
the resultant of all the other forces, the width of base could not be
correspondingly decreased without danger of the wall being overturned
before the superstructure was erected. The least batter that would
answer those conditions was found to be 2 in. per ft.

For convenience in designing, and economy in constructing, the
steelwork, the faces of the bridge seat and of the backwall were laid
parallel to the center line of the Terminal, and in elevation on line
parallel to the top of the curb and as near to it as the economical
depth of steel would permit, without bringing the finished construction
above the plane fixed in the ordinance. As there is a variation of 13
ft. in the elevation of the top of the curb of 31st Street above the top
of rail and a variation of 18 ft. in 33d Street, a uniform batter, with
the top parallel to the center line, would produce a toe varying in
distance from it and from the other constructions. It was decided,
therefore, for the sake of appearance, to make the face of the wall
(or wall produced) at the top of rail parallel to the center line,
and to vary the batter accordingly, using the 2-in. batter previously
mentioned as the minimum. This gave a maximum batter of 3 in. per ft. The
variation is so gradual that it is unnoticeable, and is not sufficient
to introduce any complications in construction.

The wall was designed with a stepped back, primarily to allow the
water-proofing and brick protection to be held in position more readily.
The first step was put at 13 ft. below the surface of the ground. This
gave a vertical back above that point for a 3-in. battered face, and a
slightly battered back for sections having a less batter in front. Below
that point a step was added for each 5 ft. of depth to the elevation
of the top of rail, or to the foundation of the wall if above that
elevation. As the horizontal distance of the heel of the wall, at its
greatest width, from its face at the top of rail w