The Indianapolis times. [volume] (Indianapolis [Ind.]) 1922-1965, 1926-02-15
SCHOOL
HELPS Q
Copyright. COmptOn's Pictu.

m ONE of the most daring feats
II of engineering have been
the construction of tunnels
beneath rivers, through mountains,
and under great cities. The streets
of some of our cities are underlaid
by veritable networks of such
passagewsys. There are tunnels for
sewers, gass mains, and electric
wires tunnels for acqueducts in
great water supply systems; tunnels
for under-river traffic, as in Chi
cago; and tunnels or sub-ways for
rapid subsurface transportation. as
in London. Paris and New York,
where millions of people daily whiz
in roaring trains and traI9way cars
back and forth through serpentine
tubes"
The first step In tunnel con-
struction Is to prepare a geological
map showing the nature of the
earth to be penetrated, the prob
ability of encountering water, ETC.,
for upon these difficulties the cost
of construction depends largely.
Then a guide line for excavating,
called the tunnel center line" is
established by surveyors. Next the
difference in elevation between the
two ends is ascertained;s the shape
and inside dimensions are decided
upon, and then the excavating be-
gins. Open-cut tunnels are often
constructed by first removing the
everything material (by open ex-
cavating if on land or by dredging II
under water), completing the tunnel
structure, and then refilling over it.
Open-cut tunnels are often used in
constructing subways.
Large tunnels are seldom driven
as one big hole; that is, the whole
area of a cross-section is seldom re-
moved at ONCE. Instead, the cross
section is divided into several
segments and these art started one
at a time. Thus severs gangs can
work at once, the first gang to start
always being farthest along.
Top Supported
The sides and top of a tunnel are
usually supported by a lining. even
in hard rock. Temporary lining is
always set up, when working in soft
material, to prevent "cave-ins," and
is replaced by the permanent lining
as the work progresses. In such
cases, the first structure is of tim
bers and cross-timbers. The perma-
nent masonry--brick, stone or con-
thick. the greater thickness being
used in such soft. heavy material as
wet clay and sand. Cast iron lining
is sometimes used, particularly in
tunnels at considerable depths below
the surface Of water.
The iron is made in cylindrical sec
tons from two to six feet long and
enc or two inches thick, and these
sre boIted together when put in
place. Sometimes they are further
lined with masonry.
Small dump or tram cars carry
out the excavated material. Often
vertical holes or shafts, braced with
timber, are sunk from the surface so
as to attack the excavation from ad-
ditional points, the work being ex-
tended both ways from where the
shaft is sunk. Men, materials and
tools are raised and lowered through
shafts in power elevators. Some
times the shafts are filed in again
after the work is completed; some
times they are left permanently for
ventilating the tunnel. Occasionally
the digging of tunnel starts at each
end, the worlxmen meeting in the
center. In the Rogers Pass tunnel,
British Columbia, a small pioneer
tunnel was driven first. "Sidecuts"
were then made and gangs worked
from their ends to build the main
tunnel.
Easy to Cut Rock
Hard rock is the easiest material
to tunnel through, as it is easily
broken up by blasting, and the
masonry lining may usually be COM
pleted without much timbering. Sub
marine tunneling Is more difficult.
III the water Whlch SeepS uoWn INTO
the excavation cannot be kept out
by pumping. use must be made of
compressed air, or a shield, or both.
The shield. one of the most valuable
of engineering inventions, is a steel
plate cylinder or shell, from s to 30
feet in length and 40 to 80 tons in
weight, inside of which the excava-
tto,, o,,5 t1,a oonefrt,ctiop of tho ft,n
mel lining is a carried on. Thus the
thrush of water and soft material
during the excavation is prevented.
Inside the shield is a vertical bulk
head partition or diaphragm with
openings whose doors can be quick
ly closed If necessary. Through
these, laborers excavate the material
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