Evening public ledger. [volume] (Philadelphia [Pa.]) 1914-1942, 1917-01-12
BEDROCK REACHED
UNDER CITY HALL
First of Concrete-Filled
Steel Piers to Support
Subway Walls
GREAT ENGINEERING FEAT
Mighty Foundation Which Will
Resist Pressure of
80 Tons

The first of the series Of concrete-elled
steel piles upon which the walls OF lhe
Broad street subway under CITY Han VIII
rest was driven down TO bedrock today IN
one Of the basement excavations near me
southwest corner Of The building. \ 35oV-
pound compressed-air hammer delivering
$50 blows per minute. drove the steel smell
down through twenty feet N hard graNcl
and sand until bedrock was found az q
point about sxty-two feet below the level
Of Broad street
The niles which are fifteen inches In
diameter. are driven down as hollow shells
The gravel inside is first loosed by a COM
pressed-air hose and is then removed In
small buckets When the pile has reached
rock and tho earth Is all removed concrete
IS poured into the shell. making solid col
umn which will support weight OF several
tons to the square inch.
About 2OOO of these piles will he driven
into place before the work Is completed
under city Hail Each pile is to be tested
to show IL II can bear the required weight
of ninety tons The two outside walls Of
the subway under the hall viii rcst upon
two double rows Of piles. placed between one
and q half and four feet apart from center
10 center The three inside walls scparai-
ing the four tracks Of the subway VIII be
thicker than the outside walls, and accord -
lngly will rest upon three parallel rows OF
piles.
The sinking of the sixty-foot "sump' or
drainage pit which is the first step on con-
tract 1O2 awarded December to the Key
stone state Construction Company, has been
almost completed. Contract lO2 ts, I'm real
lity. n subdivision of contract l0l, the di-
vision being made necessary to cover the
cost Of foundation, all rebuilding not con-
sidered necessary when the original con
tract was let.
The drainage stump will be dug down al
most to bedrock The water from the sur
rounding soil will drain naturally to the
low level of the pit and two submerged
centrifugal pumps. each Of coo gallons per
minute capacity, will then lift out the water.
Three similar sumps have already been
constructed under city Hall. These pits
will remain permanently in the subway eX-
cavatlon to drain the tube after II has been
placed in operation. since the tracks on the
subway will be several feet below the ctty's
water level.
The work was abandoned temporarily
under the northwest corner Of city Hall
last summer; the nvmps i'm the one sump
in this section were flowed to stop. Soon
more than twenty feet Of water was stand
ing I'm the pit. Tho pumps were started
again several days ago, but it will be two
or three weeks before the water Is lowered
to the level necessary before work can pro
coed
Half dozen or the huge reinforced con
crete girders which will rest diaGonally
across the walls of the subway and support
the weight Of the entire western end of city
Hall have just been completed. These
girders range from twenty-eight inches
seven feet i'm thickness and between ten
and eleven feet in vertical depth. They are
each twenty-seven feet long,