Subsidiary Development

SUBSIDIARY DEVELOPMENT;--STATIONS; CROSSCUTS; LEVELS; INTERVAL
BETWEEN LEVELS; PROTECTION OF LEVELS; WINZES AND RISES. DEVELOPMENT
IN THE PROSPECTING STAGE; DRILLING.

SUBSIDIARY DEVELOPMENT.

Stations, crosscuts, levels, winzes, and rises follow after the
initial entry. They are all expensive, and the least number that
will answer is the main desideratum.

STATIONS.--As stations are the outlets of the levels to the shaft,
their size and construction is a factor of the volume and character
of the work at the levels which they are to serve. If no timber
is to be handled, and little ore, and this on cages, the stations
need be no larger than a good sized crosscut. Where timber is to
be let down, they must be ten to fifteen feet higher than the floor
of the crosscut. Where loading into skips is to be provided for,
bins must be cut underneath and sufficient room be provided to
shift the mine cars comfortably. Such bins are built of from 50 to
500 tons' capacity in order to contain some reserve for hoisting
purposes, and in many cases separate bins must be provided on opposite
sides of the shaft for ore and waste. It is a strong argument in
favor of skips, that with this means of haulage storage capacity
at the stations is possible, and the hoisting may then go on
independently of trucking and, as said before, there are no idle
men at the stations.

[Illustration: Fig. 15.--Cross-section of station arrangement for
skip-haulage in vertical shaft.]

[Illustration: Fig. 16.--Cross-section of station arrangement for
skip-haulage in vertical shaft.]

It is always desirable to concentrate the haulage to the least
number of levels, for many reasons. Among them is that, where haulage
is confined to few levels, storage-bins are not required at every
station. Figures 15, 16, 17, and 18 illustrate various arrangements
of loading bins.

CROSSCUTS.--Crosscuts are for two purposes, for roadway connection
of levels to the shaft or to other levels, and for prospecting
purposes. The number of crosscuts for roadways can sometimes be
decreased by making the connections with the shaft at every second
or even every third level, thus not only saving in the construction
cost of crosscuts and stations, but also in the expenses of scattered
tramming. The matter becomes especially worth considering where
the quantity of ore that can thus be accumulated warrants mule
or mechanical haulage. This subject will be referred to later on.

[Illustration: Fig. 17.--Arrangement of loading chutes in vertical
shaft.]

On the second purpose of crosscuts,--that of prospecting,--one
observation merits emphasis. This is, that the tendency of ore-fissures
to be formed in parallels warrants more systematic crosscutting
into the country rock than is done in many mines.

[Illustration: Fig. 18.--Cross-section of station arrangement for
skip-haulage in inclined shaft.]

LEVELS.

The word "level" is another example of miners' adaptations in
nomenclature. Its use in the sense of tunnels driven in the direction
of the strike of the deposit has better, but less used, synonyms in
the words "drifts" or "drives." The term "level" is used by miners
in two senses, in that it is sometimes applied to all openings on one
horizon, crosscuts included. Levels are for three purposes,--for a
stoping base; for prospecting the deposit; and for roadways. As a
prospecting and a stoping base it is desirable that the level should
be driven on the deposit; as a roadway, that it should constitute
the shortest distance between two points and be in the soundest
ground. On narrow, erratic deposits the levels usually must serve
all three purposes at once; but in wider and more regular deposits
levels are often driven separately for roadways from the level
which forms the stoping base and prospecting datum.

There was a time when mines were worked by driving the level on ore
and enlarging it top and bottom as far as the ground would stand,
then driving the next level 15 to 20 feet below, and repeating the
operation. This interval gradually expanded, but for some reason
100 feet was for years assumed to be the proper distance between
levels. Scattered over every mining camp on earth are thousands
of mines opened on this empirical figure, without consideration
of the reasons for it or for any other distance.

The governing factors in determining the vertical interval between
levels are the following:--

_a_. The regularity of the deposit.
_b_. The effect of the method of excavation of winzes and rises.
_c_. The dip and the method of stoping.

REGULARITY OF THE DEPOSIT.--From a prospecting point of view the
more levels the better, and the interval therefore must be determined
somewhat by the character of the deposit. In erratic deposits there
is less risk of missing ore with frequent levels, but it does not
follow that every level need be a through roadway to the shaft or
even a stoping base. In such deposits, intermediate levels for
prospecting alone are better than complete levels, each a roadway.
Nor is it essential, even where frequent levels are required for
a stoping base, that each should be a main haulage outlet to the
shaft. In some mines every third level is used as a main roadway,
the ore being poured from the intermediate ones down to the haulage
line. Thus tramming and shaft work, as stated before, can be
concentrated.

EFFECT OF METHOD OF EXCAVATING WINZES AND RISES.--With hand drilling
and hoisting, winzes beyond a limited depth become very costly to
pull spoil out of, and rises too high become difficult to ventilate,
so that there is in such cases a limit to the interval desirable
between levels, but these difficulties largely disappear where
air-winches and air-drills are used.

THE DIP AND METHOD OF STOPING.--The method of stoping is largely
dependent upon the dip, and indirectly thus affects level intervals.
In dips under that at which material will "flow" in the stopes--about
45° to 50°--the interval is greatly dependent on the method of
stope-transport. Where ore is to be shoveled from stopes to the
roadway, the levels must be comparatively close together. Where
deposits are very flat, under 20°, and walls fairly sound, it is
often possible to use a sort of long wall system of stoping and to
lay tracks in the stopes with self-acting inclines to the levels.
In such instances, the interval can be expanded to 250 or even 400
feet. In dips between 20° and 45°, tracks are not often possible,
and either shoveling or "bumping troughs"[*] are the only help
to transport. With shoveling, intervals of 100 feet[**] are most
common, and with troughs the distance can be expanded up to 150
or 175 feet.

[Footnote *: Page 136.]

[Footnote **: Intervals given are measured on the dip.]

In dips of over 40° to 50°, depending on the smoothness of the foot
wall, the distance can again be increased, as stope-transport is
greatly simplified, since the stope materials fall out by gravity.
In timbered stopes, in dips over about 45°, intervals of 150 to
200 feet are possible. In filled stopes intervals of over 150 feet
present difficulties in the maintenance of ore-passes, for the wear
and tear of longer use often breaks the timbers. In shrinkage-stopes,
where no passes are to be maintained and few winzes put through, the
interval is sometimes raised to 250 feet. The subject is further
discussed under "Stoping."

Another factor bearing on level intervals is the needed insurance
of sufficient points of stoping attack to keep up a certain output.
This must particularly influence the manager whose mine has but
little ore in reserve.

[Illustration: Fig. 19.]

PROTECTION OF LEVELS.--Until recent years, timbering and occasional
walling was the only method for the support of the roof, and for
forming a platform for a stoping base. Where the rock requires no
support sublevels can be used as a stoping base, and timbering
for such purpose avoided altogether (Figs. 38, 39, 42). In such
cases the main roadway can then be driven on straight lines, either
in the walls or in the ore, and used entirely for haulage. The
subheading for a stoping base is driven far enough above or below
the roadway (depending on whether overhand or underhand stoping
is to be used) to leave a supporting pillar which is penetrated
by short passes for ore. In overhand stopes, the ore is broken
directly on the floor of an upper sublevel; and in underhand stopes,
broken directly from the bottom of the sublevel. The method entails
leaving a pillar of ore which can be recovered only with difficulty
in mines where stope-support is necessary. The question of its
adoption is then largely one of the comparative cost of timbering,
the extra cost of the sublevel, and the net value of the ore left.
In bad swelling veins, or badly crushing walls, where constant
repair to timbers would be necessary, the use of a sublevel is a
most useful alternative. It is especially useful with stopes to
be left open or worked by shrinkage-stoping methods.

If the haulage level, however, is to be the stoping base, some
protection to the roadway must be provided. There are three systems
in use,--by wood stulls or sets (Figs. 19, 30, 43), by dry-walling
with timber caps (Fig. 35), and in some localities by steel sets.
Stulls are put up in various ways, and, as their use entails the
least difficulty in taking the ore out from beneath the level,
they are much favored, but are applicable only in comparatively
narrow deposits.

WINZES AND RISES.

These two kinds of openings for connecting two horizons in a mine
differ only in their manner of construction. A winze is sunk underhand,
while a rise is put up overhand. When the connection between levels
is completed, a miner standing at the bottom usually refers to
the opening as a rise, and when he goes to the top he calls it
a winze. This confusion in terms makes it advisable to refer to
all such completed openings as winzes, regardless of how they are
constructed.

In actual work, even disregarding water, it costs on the average
about 30% less to raise than to sink such openings, for obviously
the spoil runs out or is assisted by gravity in one case, and in
the other has to be shoveled and hauled up. Moreover, it is easier
to follow the ore in a rise than in a winze. It usually happens,
however, that in order to gain time both things are done, and for
prospecting purposes sinking is necessary.

The number of winzes required depends upon the method of stoping
adopted, and is mentioned under "Stoping." After stoping, the number
necessary to be maintained open depends upon the necessities of
ventilation, of escape, and of passageways for material to be used
below. Where stopes are to be filled with waste, more winzes must
be kept open than when other methods are used, and these winzes
must be in sufficient alignment to permit the continuous flow of
material down past the various levels. In order that the winzes
should deliver timber and filling to the most advantageous points,
they should, in dipping ore-bodies, be as far as possible on the
hanging wall side.

DEVELOPMENT IN THE EARLY PROSPECTING STAGE.

The prime objects in the prospecting stage are to expose the ore
and to learn regarding the ore-bodies something of their size, their
value, metallurgical character, location, dip, strike, etc.,--so much
at least as may be necessary to determine the works most suitable
for their extraction or values warranting purchase. In outcrop mines
there is one rule, and that is "follow the ore." Small temporary
inclines following the deposit, even though they are eventually
useless; are nine times out of ten justified.

In prospecting deep-level projects, it is usually necessary to
layout work which can be subsequently used in operating the mine,
because the depth involves works of such considerable scale, even
for prospecting, that the initial outlay does not warrant any
anticipation of revision. Such works have to be located and designed
after a study of the general geology as disclosed in adjoining mines.
Practically the only method of supplementing such information is
by the use of churn- and diamond-drills.

DRILLING.--Churn-drills are applicable only to comparatively shallow
deposits of large volume. They have an advantage over the diamond
drill in exposing a larger section and in their application to
loose material; but inability to determine the exact horizon of
the spoil does not lend them to narrow deposits, and in any event
results are likely to be misleading from the finely ground state of
the spoil. They are, however, of very great value for preliminary
prospecting to shallow horizons.

Two facts in diamond-drilling have to be borne in mind: the indication
of values is liable to be misleading, and the deflection of the drill
is likely to carry it far away from its anticipated destination.
A diamond-drill secures a small section which is sufficiently large
to reveal the geology, but the values disclosed in metal mines must
be accepted with reservations. The core amounts to but a little
sample out of possibly large amounts of ore, which is always of
variable character, and the core is most unlikely to represent
the average of the deposit. Two diamond-drill holes on the Oroya
Brownhill mine both passed through the ore-body. One apparently
disclosed unpayable values, the other seemingly showed ore forty
feet in width assaying $80 per ton. Neither was right. On the other
hand, the predetermination of the location of the ore-body justified
expenditure. A recent experiment at Johannesburg of placing a copper
wedge in the hole at a point above the ore-body and deflecting
the drill on reintroducing it, was successful in giving a second
section of the ore at small expense.

The deflection of diamond-drill holes from the starting angle is
almost universal. It often amounts to a considerable wandering
from the intended course. The amount of such deflection varies
with no seeming rule, but it is probable that it is especially
affected by the angle at which stratification or lamination planes
are inclined to the direction of the hole. A hole has been known
to wander in a depth of 1,500 feet more than 500 feet from the
point intended. Various instruments have been devised for surveying
deep holes, and they should be brought into use before works are
laid out on the basis of diamond-drill results, although none of
the inventions are entirely satisfactory.