Methods of Ore-Breaking

METHODS OF ORE-BREAKING; UNDERHAND STOPES; OVERHAND STOPES; COMBINED
STOPE. VALUING ORE IN COURSE OF BREAKING.

There is a great deal of confusion in the application of the word
"stoping." It is used not only specifically to mean the actual
ore-breaking, but also in a general sense to indicate all the operations
of ore-breaking, support of excavations, and transportation between
levels. It is used further as a noun to designate the hole left
when the ore is taken out. Worse still, it is impossible to adhere
to miners' terms without employing it in every sense, trusting
to luck and the context to make the meaning clear.

The conditions which govern the method of stoping are in the main:--

_a_. The dip.
_b_. The width of the deposit.
_c_. The character of the walls.
_d_. The cost of materials.
_e_. The character of the ore.

Every mine, and sometimes every stope in a mine, is a problem special
to itself. Any general consideration must therefore be simply an
inquiry into the broad principles which govern the adaptability of
special methods. A logical arrangement of discussion is difficult,
if not wholly impossible, because the factors are partially
interdependent and of varying importance.

For discussion the subject may be divided into:

1. Methods of ore-breaking.
2. Methods of supporting excavation.
3. Methods of transport in stopes.

METHODS OF ORE-BREAKING.

The manner of actual ore-breaking is to drill and blast off slices
from the block of ground under attack. As rock obviously breaks
easiest when two sides are free, that is, when corners can be broken
off, the detail of management for blasts is therefore to set the holes
so as to preserve a corner for the next cut; and as a consequence
the face of the stope shapes into a series of benches (Fig.
22),--inverted benches in the case of overhand stopes (Figs. 20,
21). The size of these benches will in a large measure depend on
the depth of the holes. In wide stopes with machine-drills they
vary from 7 to 10 feet; in narrow stopes with hand-holes, from
two to three feet.

[Illustration: Fig. 20.]

The position of the men in relation to the working face gives rise
to the usual primary classification of the methods of stoping.
They are:--

1. Underhand stopes,
2. Overhand stopes,
3. Combined stopes.

These terms originated from the direction of the drill-holes, but
this is no longer a logical basis of distinction, for underhand
holes in overhand stopes,--as in rill-stoping,--are used entirely
in some mines (Fig. 21).

[Illustration: Fig. 21.]

UNDERHAND STOPES.--Underhand stopes are those in which the ore
is broken downward from the levels. Inasmuch as this method has
the advantage of allowing the miner to strike his blows downward
and to stand upon the ore when at work, it was almost universal
before the invention of powder; and was applied more generally
before the invention of machine-drills than since. It is never
rightly introduced unless the stope is worked back from winzes
through which the ore broken can be let down to the level below,
as shown in Figures 22 and 23.

[Illustration: Fig. 22.]

This system can be advantageously applied only in the rare cases
in which the walls require little or no support, and where very
little or no waste requiring separation is broken with the ore
in the stopes. To support the walls in bad ground in underhand
stopes would be far more costly than with overhand stopes, for
square-set timbering would be most difficult to introduce, and
to support the walls with waste and stulls would be even more
troublesome. Any waste broken must needs be thrown up to the level
above or be stored upon specially built stages--again a costly
proceeding.

A further drawback lies in the fact that the broken ore follows
down the face of the stope, and must be shoveled off each bench.
It thus all arrives at a single point,--the winze,--and must be
drawn from a single ore-pass into the level. This usually results
not only in more shoveling but in a congestion at the passes not
present in overhand stoping, for with that method several chutes
are available for discharging ore into the levels. Where the walls
require no support and no selection is desired in the stopes, the
advantage of the men standing on the solid ore to work, and of
having all down holes and therefore drilled wet, gives this method
a distinct place. In using this system, in order to protect the
men, a pillar is often left under the level by driving a sublevel,
the pillar being easily recoverable later. The method of sublevels
is of advantage largely in avoiding the timbering of levels.

[Illustration: Fig. 23.--Longitudinal section of an underhand stope.]

OVERHAND STOPES.--By far the greatest bulk of ore is broken overhand,
that is broken upward from one level to the next above. There are
two general forms which such stopes are given,--"horizontal" and
"rill."

[Illustration: Fig. 24.--Horizontal-cut overhand stope--longitudinal
section.]

The horizontal "flat-back" or "long-wall" stope, as it is variously
called, shown in Figure 24, is operated by breaking the ore in slices
parallel with the levels. In rill-stoping the ore is cut back from
the winzes in such a way that a pyramid-shaped room is created,
with its apex in the winze and its base at the level (Figs. 25 and
26). Horizontal or flat-backed stopes can be applied to almost any
dip, while "rill-stoping" finds its most advantageous application
where the dip is such that the ore will "run," or where it can be
made to "run" with a little help. The particular application of
the two systems is dependent not only on the dip but on the method
of supporting the excavation and the ore. With rill-stoping, it is
possible to cut the breaking benches back horizontally from the
winzes (Fig. 25), or to stagger the cuts in such a manner as to
take the slices in a descending angle (Figs. 21 and 26).

[Illustration: Fig. 25.--Rill-cut overhand stope--longitudinal section.]

In the "rill" method of incline cuts, all the drill-holes are "down"
holes (Fig. 21), and can be drilled wet, while in horizontal cuts
or flat-backed stopes, at least part of the holes must be "uppers"
(Fig. 20). Aside from the easier and cheaper drilling and setting
up of machines with this kind of "cut," there is no drill dust,--a
great desideratum in these days of miners' phthisis. A further
advantage in the "rill" cut arises in cases where horizontal jointing
planes run through the ore of a sort from which unduly large masses
break away in "flat-back" stopes. By the descending cut of the
"rill" method these calamities can be in a measure avoided. In
cases of dips over 40º the greatest advantage in "rill" stoping
arises from the possibility of pouring filling or timber into the
stope from above with less handling, because the ore and material
will run down the sides of the pyramid (Figs. 32 and 34). Thus
not only is there less shoveling required, but fewer ore-passes
and a less number of preliminary winzes are necessary, and a wider
level interval is possible. This matter will be gone into more
fully later.

[Illustration: Fig. 26.--Rill-cut overhand stope-longitudinal section.]

COMBINED STOPES.--A combined stope is made by the coincident working
of the underhand and "rill" method (Fig. 27). This order of stope
has the same limitations in general as the underhand kind. For
flat veins with strong walls, it has a great superiority in that
the stope is carried back more or less parallel with the winzes,
and thus broken ore after blasting lies in a line on the gradient
of the stope. It is, therefore, conveniently placed for mechanical
stope haulage. A further advantage is gained in that winzes may
be placed long distances apart, and that men are not required,
either when at work or passing to and from it, to be ever far from
the face, and they are thus in the safest ground, so that timber
and filling protection which may be otherwise necessary is not
required. This method is largely used in South Africa.

[Illustration: Fig. 27.--Longitudinal section of a combined stope.]

MINIMUM WIDTH OF STOPES.--The minimum stoping width which can be
consistently broken with hand-holes is about 30 inches, and this
only where there is considerable dip to the ore. This space is
so narrow that it is of doubtful advantage in any case, and 40
inches is more common in narrow mines, especially where worked
with white men. Where machine-drills are used about 4 feet is the
minimum width feasible.

RESUING.--In very narrow veins where a certain amount of wall-rock
must be broken to give working space, it pays under some circumstances
to advance the stope into the wall-rock ahead of the ore, thus
stripping the ore and enabling it to be broken separately. This
permits of cleaner selection of the ore; but it is a problem to
be worked out in each case, as to whether rough sorting of some
waste in the stopes, or further sorting at surface with inevitable
treatment of some waste rock, is more economical than separate
stoping cuts and inevitably wider stopes.

VALUING ORE IN COURSE OF BREAKING.--There are many ores whose payability
can be determined by inspection, but there are many of which it cannot.
Continuous assaying is in the latter cases absolutely necessary
to avoid the treatment of valueless material. In such instances,
sampling after each stoping-cut is essential, the unprofitable ore
being broken down and used as waste. Where values fade into the
walls, as in impregnation deposits, the width of stopes depends
upon the limit of payability. In these cases, drill-holes are put
into the walls and the drillings assayed. If the ore is found
profitable, the holes are blasted out. The gauge of what is profitable
in such situations is not dependent simply upon the average total
working costs of the mine, for ore in that position can be said to
cost nothing for development work and administration; moreover,
it is usually more cheaply broken than the average breaking cost,
men and machines being already on the spot.