10                                           THE   TECHNOGRAPH.

A DEEP PILE FOUNDATION.
 

By C. J. Mitchell, '91, Asst. Civil Engineer I. C. E,. R.
 

The new Illinois Central Railway Passenger Station and Office
Building at 12th street, Chicago, is 180 by 220 ft.; and consists of
the office portion 9 stories high, the tower 13 stories, and the
station 3 stories high. The train shed with connection to the
station covers 8 tracks, and is 680 ft. long.

Borings taken on the site showed from 10 to 20 feet of mis¬
cellaneous rubbish which had been dumped there immediately after
the great fire. Below this made ground were found irregular layers
of stiff blue clay and quicksand. Rock was not struck until a depth
of more than 60 feet was reached. Soil of this character could hardly
be trusted to carry the ordinary Chicago type of independent pier
foundation, more especially since, owing to the nature of the building
and the arrangement of the tracks, trains running near and
under a portion of it would have a tendency to cause a settling of the
foundations.

It was determined to use a deep pile foundation the peculiar feature
of which consists in driving long piles to a firm bearing and sawing the
tops off ''at a level below that of any conceivable deep drainage
system that many become desirable or necessary in the city."

Naturally the rather uncertain quality of the soil made it neces¬
sary to use a large number of piles, about 1 700 being driven. The
piles were arranged in groups or clusters under each column. In the
head-house portion consisting of three stories above the passenger
tracks, these groups were of three sizes; viz., 8, 13 and 16 piles each.
In the office portion (9 stories high) the groups contained 20 to 42
piles each; while under the tower (13 stories) one of the corner
clusters has 73 piles. The piles in the groups were sometimes ar¬
ranged on the square, lining both ways; but usually alternating in
rows 18 inches apart making the distance between centers 25^ inches.
This is as close as these piles could be driven, and sometimes in hard
ground the position of the tops when cut off bore no resemblance to
that in which the tips were started, while in a few clusters the piles
did not vary more than 1 inch from where they were started.

The piles varied from 11 to 16 inches in diameter at the butt end,
and were from 40 to 60 feet in length, averaging about 51 feet. It was
specified that the piles should have a uniform taper, and no crooks,bends,
knots, windshakes, ato.; but before driving care was taken to reject