The Indianapolis journal., 1902-01-19
NEW BRIDGE CONSTRUCTION
DIFFICULTIES OVERCOME IN DIG
GING CHICAGO DRAINAGE CANAL.
Problems Involved I'm TunneIinE IN
der Rivers and Through Quick
sands, and How Solved.

most people, the cOmmOneSt, the modern
railroad bridge, and it is hard TO realize
that nothing like it. even for highway pur-
poses, existed fifty years ago. Now almost
every one of those which were earliest
built has been found inadequate by the
enormous increase Of traffic and the weight
OF locomotives, and has been replaced by
steel-s Each span is made according to
drawings. with hundreds Of beams, grid
ers, bars. rods and columns, riveted and
forged and bored in distant shops, but
never fitted together until they arrived by
dozens Of carloads at the site, and are
quickly assembled to make a slender, self
supporting framework with the rigid pieces
which weigh thousands of pounds fitted
accurately and swiftly in place like the
mechanism of a watch. This is hard enough
to do across a storm-swept bay. over an
almost bOttomless river. where there is
a fierce current of high tides. or where,
like the Niagara gorge, the height is so
great that no support can be built up from
the bottom, and the span has to support
itself during construction; but all these
difficulties are small to the bridge builder,
compared 10 the requirement, often met,
that an old bridge of perhaps many spans
must be replaced by a new one in exactly
the same place, Of the same dimensions
and supported on the very same piers, with
out interrupting appreciably the almost
constant traffic which demands passage
nearly every hour in the twenty-four for
long freight trains or rushing expresses.
Such was the case with the Florence
bridge across the Tennessee river in Ala
bama. There were twelve spans of about
12G feet each, which carried a single track
trailroad on top and a highway between
the trusses. The old wooden spans could
not even support the new steel ones dur
ing erection, although the masonry piers
had ample strength. Wooden towers
were built the TOP edges of the old
bridge, outside the track, above two adj
= Jacent piers. Above these towers nearly
twenty-five feet over the track a complete
new temporary span of wood and iron as
long as the longest span of the old bridge
was built new permanent span was as
sembled together, suspended from the un-
der side Of it, clear Of the trains which
passed uninteruptedly through it and un-
der the temporary span.
NEW BRIDGE OVER OLD.
When it was finished and track laid on
top Of it the under side was lashed to
the upper side of the old span, and it was
additionally suspended from the overhead
temporary span by four vertical three-and-
one-half-inch steel screw rods, about thirty
feet long. one at each corner. which had
nuts bearing on top of the temporary span.
A suitable interval between trains was Sc
lected. the ends OF the old span chopped
away, the lashings between the new and
temporary spans removed, and then the
nuts on the great screws were Stacked off
by gears operated by a steam engine at
F,-fB 4
smoothly and safely together, about twen-
ty-two feet. in less than an hour; the old
one dropping down almost TO the water s
edge and the new one following 10 occupy
its former place and rest on masonry seats
which were prepared meantime \ few
minutes later trains were running across
the new span. the Old one was removed at
leisure. and the overhead span was rolled
along until its towers rested above the
next piers. another span was replaced, and
50 on. until the whole superstructure had
been replaced with about 100000 pounds of
steel. It took thirty-four men only eighty
days TO rebuild the bridge
The difficulty and expense Of building
solid foundations for bridge where the
water is very deep or swift or the bottom
unstable is very great. In soft soils the ut
most effort is made TO prevent any danger
OF settlement or Of undermining by floods
and swift current. and where piles are not
used it is generally necessary to carry the
masonry down 10 solid rock or 10 a safe
distance In earth and sand. This Is always
very difficult on account Of the tremendous
pressure of the earth and water al a great
depth. and the danger of working there
even when il IS possible to exclude the
water. In this country cribs or caissons
are often sunk by mechanical processes or
BY men working in pneumatic pressure and
then are filed with concrete.
ADAPTATION To NATURE.
In India. where the soft alluvial soil ex-
tends 10 great depth and is saturated
with water and Will flow like mud, they
make 1t support heavy foundations partly
by its stickyness. For many years they
have built hollow cycindrical brick piers
from twenty 10 one hundred feet deep
which hold heavy loads chiefly BY the fric-
tion on their sides. Originally these hollow
piers. a few feet in diameter. were built up
on wooden rings laid on the ground in the
bottom OF pits dug down TO water level A
native diver went inside and dug up the
bottom with an adze-like tool. laboriously
undermining the walls, so that they grad
Ually settled a few inches a week as the
top. were built on high above the surface
of the ground. Afterwards the excavation
was done by dippers attached to long poles
and sometimes by buckets with valves at
the bottom. Recently large pier founda-
tions for railroad bridges have been made
with two or three brick cylinders twelve or
fifteen feet IN diameter. which were shod
with circular iron girders having sharp
cutting edges, and horizontal shelf TO re-
ceive the brickwork Steam dredging ma
chimes were set on top OF the piers. and
undermining the shoes. allowed the piers TO
settle forty or fifty feet below the river bed.
The friction on the sides OF the pier be-
came at last so great that the weight Of
the briekword built high above the surface
OF the ground would rot sink il. and the
last OF the descent was accomplished by
loading the top os the finished pier with