the pipe being
supplied by the City.

LOCATION AND DESIGN OF RETAINING WALLS.

The plans, from the earliest stages, contemplated founding the
retaining wall on the surface of the rock, where of suitable quality,
and afterward excavating the rock in front of the toe of the wall
to sub-grade. This plan was definitely adopted soon after the borings
were completed, on account of the great danger of blasting out large
quantities of rock in timbered trenches close to buildings founded on
soft material, and also to avoid the additional cost and delay that
would have been caused by carrying the walls to sub-grade. The retaining
walls in Seventh Avenue, south of the viaduct, and in Ninth Avenue,
north of the viaduct, were not governed by the same conditions as in the
streets. The dip and quality of the rock at both points required that
the walls be carried to sub-grade, and they are, in fact, face walls;
the Ninth Avenue wall, in particular, having little thrust to sustain,
is very light.

The results aimed at in the design and location of the retaining walls
in 31st and 33d Streets were:

_First._--A perfectly stable wall under all conditions that might
reasonably be expected;

_Second._--As much room as possible at the elevation of the top of
rail;

_Third._--The least necessary interference with adjoining property
during construction; and,

_Fourth._--The most economical wall that would fulfill the other
conditions.

As stated in the paper by Alfred Noble, Past-President, Am. Soc. C. E.,
the third stipulation required the relinquishing of a portion of the
space under these streets granted by the City, but it was finally
decided not to approach the south house line of 31st Street with the
back of the walls nearer than 9 ft., while on 33d Street the extreme
position of the back was fixed at the north line, as there were no
buildings, except those belonging to the Railroad Company, on the house
line at the low points in the rock.

The assumptions made in designing the wall were as follows:

_First._--Weight of concrete, 140 lb. per cu. ft.

_Second._--Weight of material from the surface of the ground to a
depth of 12 ft. (which was shown by tests made in bore-holes to be
the elevation of the ground-water surface), 100 lb. per cu. ft.;
and angle of repose, 30 degrees. The distance of 12 ft. below
the surface was the depth of the inverts of the sewers, which
undoubt