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