The mirror., 1919-04-24

9I4IlHSl on the Mesaba Range
H90kgl soon convinced the iron
a&Ay and steel industry of the
vast possibilities of obtaining it
quickly and cheaply by means of
open-pit mining, as the deposits in
many instances we're near the sur
face, and the nature of the mater
ial comprising the overburden was
such that it could be economically
removed with the steam shovel,
thus materially reducing the cost
and substantially increasing the
output as compared to the under
ground method of mining.
The ore bodies are first thor
oughly explored by churn and dia-
mond drilling, surface and ore
contours are plated on maps, on
which complete plans for stripping
and mining are laid out before ex-
cavating begins. While the gen.
eral features of steam-shovel min
ing may appear simple, good man.
agement. close attention to detail
and systematic work are essential
to success. Intelligent planning
of approaches and track layouts
are important in order to minimize
delays and handle the maximum
amount of dirt and ore. Another
factor that enters largely into the
cost of stripping is the handling
of the material after it is loaded
into the dump.cars. Sometimes it
is deposited on an adjoining forty
(which involves heavy grades and
switchbacks if the yardage be very
great), and in some instances it is
hauled for a distance of two miles,
varying according to availability
to secure proper dumping grounds.
When quicksand and muskeg are
encountered to any great extent,
it is necessary to spill it over as
large surface as possible, in order
to maintain sufficiently solid
foundation for tracks to hold up
the heavy type of locomotives and
large dump-cars now in use. With
all the precautions used, slides fire.
quently occur which derail and oc-
casionally overturn entire trains.
The usual cut of the average
steam-shovel is twenty-five feet
deep and between forty and fifty
feet wide. Horizontal holes, COM
monty referred to in Minnesota as
Gopher holes,' are driven into
the face of the bank near the bot
tom of the cut. with an approx
mate pitch of ten per cent for a
distance of about twenty feet; they
are spaced about twenty-five feet
apart. The diameter of these holes
is from eight to twelve inches, and
they are made with jump drills
and long-handled shovels. 1t is
necessary to keep continually
springing the hole until it gets
back to the required depth. A
blasting machine and electric blast
ing caps are used for this work to
save time.
These holes are then chambered
by the use of from four to six
pounds of forty per cent Red Cross
Extra dynamite for the charge Of
blasting powder, which usually
contains from four to eight kegs,
according to the hardness of the
ground. These charges are fred
electrically, from four to six holes
at once. In loading these charges
a V-shaped trough with a box on
the end, which holds about two
kegs of powder, is used. The con-
tinual shaking of this apparatus
causes the powder to run down
where the charge is desired. An
average of about two hundred GU
bic yards of dirt are removed for
every keg of powder used, when a
one-hundred-ton steam-shovel is
operated. Where the large equip
ment of a three-h -hundred-ton shovel
is used, a great deal of the materi-