beds of quicksand being passed through, varying in thickness from 1 to
18 ft., the latter, in 31st Street between Seventh and Eighth Avenues,
in the deepest excavation made. After encountering the fine sand in that
trench, no headway was made until a tight wooden cylinder was sunk
through the sand by excavating the material inside of it and heavily
weighting the shell with pig iron. When this cylinder had reached the
gravel, which lay below the sand, it was used as a sump, and the water
level was kept below the bottom of the excavation, which permitted good
progress. Sand continued to flow under the sheeting to such an extent,
however, that the front walls of four adjoining buildings were badly
cracked and had to be taken down and rebuilt. All the stoops along this
trench settled, and had to be repaired.

The bench method of excavating for the retaining wall was very simple,
and was used only where the rock lay near the surface and the adjoining
buildings were founded on it, the overlying material being in such case
dry, and consequently firm, little or no shoring was required. The
method was to extend the pit excavation to a width of 2 or 3 ft. beyond
the proposed back of the retaining wall, and to carry that width down to
the depth required for its base, below which the excavation was narrowed
to 1 ft. inside of the face of the wall and continued either before it
was built or subsequently.

_Retaining-Wall Construction._--The concrete walls were built in
sections 50 ft. in length, except where that spacing would bring an
expansion joint under a girder pocket or just on line with a tier of
struts, in which cases the section was shortened as required. Trenches
were never allowed to remain open at the full depth, the concreting
being started as soon after the necessary length of rock had been
uncovered as the forms and preliminary work for a section could be
prepared. Each section was a monolith, except in a few cases where
very heavy rains made it impossible to hold the laborers.

The various operations in building the concrete wall are shown on
Fig. 7. Guide-planks, "_a a_," Section "_A-A_," were securely spiked
to alternate tiers of struts for the length of the section, the face
of each guide-plank being set on line with the intended face of the
concrete wall, and 2-in. tongued-and-grooved spruce plank were laid
along the guide-plank to the height of the bottom strut and securely
braced from the front sheeting. A