The general plan of enclosing the area in retaining walls having been
adopted, wash-borings were taken, for the purpose of determining the
best location for the walls, the depth of rock, and the nature of the
material overlying it. These borings were made along both curb lines of
Seventh Avenue, the east curb line of Ninth Avenue, the north curb line
of 33d Street, and the south curb line of 31st Street. The borings, as a
rule, were taken at intervals of approximately 100 ft., some deviation
in these intervals being made in order to prevent injury to water, gas,
and sewer connections, and, if the elevation of the surface of the rock,
as determined by one of these borings, corresponded fairly well with
the borings on either side of it, no intermediate borings were taken.
When a discrepancy appeared, a boring was taken midway between the
two non-corresponding ones, and if the information obtained from the
intermediate boring failed to account for the discrepancy, others were
taken at the quarter points of the original 100-ft. interval.
The dotted lines on Fig. 1 show the profiles of the surface of the rock
underlying 31st and 33d Streets, on the line of the borings, constructed
from the elevations obtained by them; the solid lines show the profiles
of the actual surface of the rock as found when uncovered. It will be
noted that, except in three cases, Borings 313, 328, and 333, the two
profiles correspond very closely at the points where the borings were
made, but they differ widely between those points, a variation of 5 ft.
being common; there is a variation of 14 ft. between Borings 324 and
327, and between Nos. 337 and 340; and of 12 ft. between Nos. 333 and
335, and between Nos. 312 and 313, while an extreme variation of 17 ft.
is shown between Nos. 303 and 305. At each of the points where the
variation is great the interval between borings is the full 100 ft., and
it is quite apparent that, if a definite idea is to be obtained of the
elevation of the surface of the rock in Manhattan, borings must be taken
at shorter intervals.
The necessary width of trench for the construction of the retaining
walls was determined by the elevation of the rock, as shown by the
borings, and only in the case of the dip between Borings 303 and 305
did the variation lead to any difficulty. The trench at that point had
to be widened after rock was reached. This depression corresponded very
closely in location to that of one arm of the creek shown on General
Viele's map of 1865,[2] the bed of that stream, or one in approximately
the same location, being clearly marked across the excavation by
smoothly-worn rock and well-rounded boulders. The original stream,
however, seemed to have turned in a westerly direction under 31st Street
to Eighth Avenue instead of crossing, as shown on General Viele's map.
[Illustration:
Fig. 1.
PROFILE OF ROCK SURFACES IN THIRTY-FIRST AND THIRTY-THIRD STREETS,
BETWEEN SEVENTH AND NINTH AVENUES]