Evening public ledger. [volume] (Philadelphia [Pa.]) 1914-1942, 1915-08-21
SUBWAY ENGINEERS
FIND WAvE-LIKE ROCK
UNDER THE CITY HALL
Nature of Soil Determined by
Engineers of Director
Taylor's Depart
ment in Tests
TO WORK IN CAIssoNs

Approximately 100000 cubic yards OF
earth will have TO be excavated for the
construction of the Broad street subway
beneath City Hall This estimate was
given today by s. NJ. swaab, the engineer.
who will have charge OF the work for the
Keystone State Construction Company.
The material TO be removed varies from
soft loan, directly below the street sur.
face. to solid rock at the bottom Of the
tube.
The nature and quality of the soil has
already been determined by the engineers
Of the Department of City Transit, who
have made 10 borings and dug test pits
under City Hall. The deepest boring was
carried down to a depth of so feet below
the level of the street and the deepest pit
was dug Is feet below the basement floor
of City Hall, and rs feet below the street
level.
The most remarkable discovery made
by the engineers in the borings was the
great variance of depths at which hard
rock is encountered. At one point d.
rectly below the arch, at the West Mar
ket street entrance to the City Hall, hard
rock was found to be 50 feet below the
street level, while 200 feet north of this
point near the northwest corner of the
building, the hard rock depth was found
to be only 20 feet below the surface. In
no two borings was the rock level found
the same.
Directly above the bed rock was found
layer OF mica. schist (disintegrated
rocb) varying in thickness from llk to
11Ik feet. Above the mica schist was
found a gravel bed of such formation
that the engineers believe II was depos-
tied there during the glacial period. Be-
tween this and the street level at
varying depths was found sand, clay,
common gravel and loan.
At one point the engineers encountered
! a pocket of sand which had every indi-
| cation of having been quicksand formerly
! For some reason the water had all
| drained from this place, leaving the
| quick-sand now simply common sand, al
| though ground very fine. No other evil
! dence of quick-sand formation was found
beneath the City Hall.
The four-foot reinforced concrete walls
| of the subway will rest on solid bedrock,
no matter at what depth this may be
| found to lie. The base of the tube itself
will he about 46 feet below the street
level. In some places where the bedrock
| is higher than this, blasting will be
necessary but the danger of the blast
| ing will be reduced to a minimum Il
will be done in caissons.
AS the high-water level of the city
| is about 3S feet below the surface at
| City Hall, all eXcavating below this
depth will have to be carried on i'm
caissons. The workmen will experience
no great inconvenience in working in the
caissons, however, as the depth will not
require heavy air pressure.
In digging the test pits it was found
! that the foundations of City Hall at no
point, not even beneath the great tower
rest on the solid rock. The foundations
| of the tower are laid 80 feet below the
| surface, which Is believed to be ten or
more feet above the level of bedrock At
other points the City Hall foundations
vary from depth of IA to 26 feet