The diamond drill. (Crystal Falls, Iron County, Mich.) 1887-1996, 1911-08-26
Block Gazing and Sub Slope
System at the Tobin Mine.
BY Fred G Roberts

Of several methods. each Of which
proved more or less unsuitable TO the
peculiar nature OF the ground to be
mined
underhsnd stopping was tried but
soon discontinued on account Of the
continual falling of ground from back |
of the stope. Bookkeeping was next
tried, but the danger from falling
ground was still too great. Sub
stopping was the third method used.
and In some parts of the mine proved
satisfactory. aud is still used TO some
extent where the ore body Is too mar
row or too firm a nature to be mined
by the block caving system Block
caving was the next method used. and
proved so well adapted to the con-
ditions and physical characteristics OF
the ore that il has been adopted as the
principal means of mining.
The levels at the Tobin are 1875 feet
from floor TO floor. and the main
haulage DRIFT follows very closely the
hanging wall.
In the block to be mined by caving
parallel cross-cuts. "a feet from center
to center, are driven from the main
drift. as nearly at right angles as may
be, to the foot.wall, and are connect
ed at the foot-wsll side by a small
drift for ventilating purposes.
Throughout the length OF these oross-
cuts. chute-raises are put up alternate
ly on the right and left sides at inter
walls of 10 feet. to the sub-level 90
teet above.
The sub-level is opened % teet above
the back of the main level. A drift
is driven. parallel with and about IS
feet from the hanging wall, the entire
length of the block TO be mined. Cross
cuts are driven to the foot wall from
this sub-level drift directly above the
cross -cuts on the main level Opposite
each cross-cut on the sub-level raise
inclined about i5dsgrees is put up from
the sub-dritt to the hanging wall. the
object being TO leave an additional
thickness of the back above the main
haulage drift. The cross-cuts on the
sub -level are connected every 10 test
by drifts over the line of chutes, leav-
ing small pillars of about 10 IT. by 10
It. II is sometimes necessary to cross
cut these small pillarsagaiu, dependin
he ~ hn
drift is also driven along the foot-wall
connecting all the cross-cuts.
After the pillars have been reduced
to suitable size. they are drilled with
a.sufficient number of holes so that
they may be all blasted at once. The
pillars furthest from the manway be-
ing drilled first. Holes are also drilled
around the tops OF the chute raises
and blasted. making the raises funnel
shaped at the top. At the ends of the
block 10 be caved, IL Is necessary to
weaken the ground so that IL will cave
square with the pillar Raises are
put up from the end cross-cuts at var
ying intervals, depending on the
nature of the ground. These raises
are connected by two cross cnts 25 and
80 feet respectively above the sub-level.
After all the necessary raising, drift
ing, and cross cutting have been com
pleted, the holes il the pillars Of the
suh-level are all blasted at once. un-
dercutting the entire block, which set
ties down on the back OF the level be-
low. The ends OF the block not being
entirely cut of from the pillar, the ore
does not drop down in one solid block,
but breaks up in settling and comes
down in such shape that II can DO
handled through the funnel shaped
chute-raises with only occasional blast
ing of masses that lodge in the cbutes.
The cayed ore is drawn uniformly
throughout the level, so that IL will
settle down evenly, and this keeps the
caved rock from the ola level above
from getting mixed into the ore.
Sub.stoping is used where caving is
not practical; at the narrow ENDS OF
the ore body, ana IN pockets ana small
or deposits that are sometimes found
separated from the main body Con-
ditions vary so in these cases, that
they have to be met in different ways
The nature of the ground and dimen-
sions Of the ore body have to be taken
into consideration In each case, and
the method Of mining adapted to 1t.
The usual method OF working these
places is to drive drift on the main
level the entire length of the ore body
in question. and determine the width
OF the same by cross-cuts. Chute
raises are put up at intervals of about
10 feet. From raise. at the end OF
the ore nearest the shaft, q sub-levei
is opened 10 feet above the-main level.