Shape of Mining Shafts

SHAPE AND SIZE OF SHAFTS; SPEED OF SINKING; TUNNELS.

SHAPE OF SHAFTS.--Shafts may be round or rectangular.[*] Round
vertical shafts are largely applied to coal-mines, and some engineers
have advocated their usefulness to the mining of the metals under
discussion. Their great advantages lie in their structural strength,
in the large amount of free space for ventilation, and in the fact
that if walled with stone, brick, concrete, or steel, they can be
made water-tight so as to prevent inflow from water-bearing strata,
even when under great pressure. The round walled shafts have a longer
life than timbered shafts. All these advantages pertain much more to
mining coal or iron than metals, for unsound, wet ground is often
the accompaniment of coal-measures, and seldom troubles metal-mines.
Ventilation requirements are also much greater in coal-mines. From
a metal-miner's standpoint, round shafts are comparatively much
more expensive than the rectangular timbered type.[**] For a larger
area must be excavated for the same useful space, and if support
is needed, satisfactory walling, which of necessity must be brick,
stone, concrete, or steel, cannot be cheaply accomplished under
the conditions prevailing in most metal regions. Although such
shafts would have a longer life, the duration of timbered shafts
is sufficient for most metal mines. It follows that, as timber
is the cheapest and all things considered the most advantageous
means of shaft support for the comparatively temporary character
of precious metal mines, to get the strains applied to the timbers in the
best manner, and to use the minimum amount of it consistent with
security, and to lose the least working space, the shaft must be
constructed on rectangular lines.

[Footnote *: Octagonal shafts were sunk in Mexico in former times.
At each face of the octagon was a whim run by mules, and hauling
leather buckets.]

[Footnote **: The economic situation is rapidly arising in a number
of localities that steel beams can be usefully used instead of
timber. The same arguments apply to this type of support that apply
to timber.]

The variations in timbered shaft design arise from the possible
arrangement of compartments. Many combinations can be imagined,
of which Figures 9, 10, 11, 12, 13, and 14 are examples.

[Illustration: FIG. 9. FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG.
14.]

The arrangement of compartments shown in Figures 9, 10, 11, and
13 gives the greatest strength. It permits timbering to the best
advantage, and avoids the danger underground involved in crossing
one compartment to reach another. It is therefore generally adopted.
Any other arrangement would obviously be impossible in inclined
or combined shafts.

SIZE OF SHAFTS.--In considering the size of shafts to be installed,
many factors are involved. They are in the main:--

_a_. Amount of ore to be handled.
_b_. Winding plant.
_c_. Vehicle of transport.
_d_. Depth.
_e_. Number of men to be worked underground.
_f_. Amount of water.
_g_. Ventilation.
_h_. Character of the ground.
_i_. Capital outlay.
_j_. Operating expense.

It is not to be assumed that these factors have been stated in
the order of relative importance. More or less emphasis will be
attached to particular factors by different engineers, and under
different circumstances. It is not possible to suggest any arbitrary
standard for calculating their relative weight, and they are so
interdependent as to preclude separate discussion. The usual result
is a compromise between the demands of all.

Certain factors, however, dictate a minimum position, which may
be considered as a datum from which to start consideration.

_First_, a winding engine, in order to work with any economy, must
be balanced, that is, a descending empty skip or cage must assist
in pulling up a loaded one. Therefore, except in mines of very
small output, at least two compartments must be made for hoisting
purposes. Water has to be pumped from most mines, escape-ways are
necessary, together with room for wires and air-pipes, so that at
least one more compartment must be provided for these objects.
We have thus three compartments as a sound minimum for any shaft
where more than trivial output is required.

_Second_, there is a certain minimum size of shaft excavation below
which there is very little economy in actual rock-breaking.[*]
In too confined a space, holes cannot be placed to advantage for
the blast, men cannot get round expeditiously, and spoil cannot be
handled readily. The writer's own experience leads him to believe
that, in so far as rock-breaking is concerned, to sink a shaft
fourteen to sixteen feet long by six to seven feet wide outside
the timbers, is as cheap as to drive any smaller size within the
realm of consideration, and is more rapid. This size of excavation
permits of three compartments, each about four to five feet inside
the timbers.

[Footnote *: Notes on the cost of shafts in various regions which
have been personally collected show a remarkable decrease in the
cost per cubic foot of material excavated with increased size of
shaft. Variations in skill, in economic conditions, and in method
of accounting make data regarding different shafts of doubtful
value, but the following are of interest:--

In Australia, eight shafts between 10 and 11 feet long by 4 to
5 feet wide cost an average of $1.20 per cubic foot of material
excavated. Six shafts 13 to 14 feet long by 4 to 5 feet wide cost
an average of $0.95 per cubic foot; seven shafts 14 to 16 feet
long and 5 to 7 feet wide cost an average of $0.82 per cubic foot.
In South Africa, eleven shafts 18 to 19 feet long by 7 to 8 feet
wide cost an average of $0.82 per cubic foot; five shafts 21 to
25 feet long by 8 feet wide, cost $0.74; and seven shafts 28 feet
by 8 feet cost $0.60 per cubic foot.]

The cost of timber, it is true, is a factor of the size of shaft,
but the labor of timbering does not increase in the same ratio.
In any event, the cost of timber is only about 15% of the actual
shaft cost, even in localities of extremely high prices.

_Third_, three reasons are rapidly making the self-dumping skip
the almost universal shaft-vehicle, instead of the old cage for
cars. First, there is a great economy in labor for loading into
and discharging from a shaft; second, there is more rapid despatch
and discharge and therefore a larger number of possible trips;
third, shaft-haulage is then independent of delays in arrival of
cars at stations, while tramming can be done at any time and
shaft-haulage can be concentrated into certain hours. Cages to
carry mine cars and handle the same load as a skip must either
be big enough to take two cars, which compels a much larger shaft
than is necessary with skips, or they must be double-decked, which
renders loading arrangements underground costly to install and
expensive to work. For all these reasons, cages can be justified only
on gold mines of such small tonnage that time is no consideration
and where the saving of men is not to be effected. In compartments
of the minimum size mentioned above (four to five feet either way)
a skip with a capacity of from two to five tons can be installed,
although from two to three tons is the present rule. Lighter loads
than this involve more trips, and thus less hourly capacity, and,
on the other hand, heavier loads require more costly engines. This
matter is further discussed under "Haulage Appliances."

We have therefore as the economic minimum a shaft of three compartments
(Fig. 9), each four to five feet square. When the maximum tonnage
is wanted from such a shaft at the least operating cost, it should
be equipped with loading bins and skips.

The output capacity of shafts of this size and equipment will depend
in a major degree upon the engine employed, and in a less degree
upon the hauling depth. The reason why depth is a subsidiary factor
is that the rapidity with which a load can be drawn is not wholly a
factor of depth. The time consumed in hoisting is partially expended
in loading, in acceleration and retardation of the engine, and in
discharge of the load. These factors are constant for any depth,
and extra distance is therefore accomplished at full speed of the
engine.

Vertical shafts will, other things being equal, have greater capacity
than inclines, as winding will be much faster and length of haul less
for same depth. Since engines have, however, a great tractive ability
on inclines, by an increase in the size of skip it is usually possible
partially to equalize matters. Therefore the size of inclines for
the same output need not differ materially from vertical shafts.

The maximum capacity of a shaft whose equipment is of the character
and size given above, will, as stated, decrease somewhat with extension
in depth of the haulage horizon. At 500 feet, such a shaft if vertical
could produce 70 to 80 tons per hour comfortably with an engine
whose winding speed was 700 feet per minute. As men and material
other than ore have to be handled in and out of the mine, and
shaft-sinking has to be attended to, the winding engine cannot
be employed all the time on ore. Twelve hours of actual daily
ore-winding are all that can be expected without auxiliary help.
This represents a capacity from such a depth of 800 to 1,000 tons
per day. A similar shaft, under ordinary working conditions, with
an engine speed of 2,000 feet per minute, should from, say, 3,000
feet have a capacity of about 400 to 600 tons daily.

It is desirable to inquire at what stages the size of shaft should
logically be enlarged in order to attain greater capacity. A
considerable measure of increase can be obtained by relieving the
main hoisting engine of all or part of its collateral duties. Where
the pumping machinery is not elaborate, it is often possible to
get a small single winding compartment into the gangway without
materially increasing the size of the shaft if the haulage compartments
be made somewhat narrower (Fig. 10). Such a compartment would be
operated by an auxiliary engine for sinking, handling tools and
material, and assisting in handling men. If this arrangement can
be effected, the productive time of the main engine can be expanded
to about twenty hours with an addition of about two-thirds to the
output.

Where the exigencies of pump and gangway require more than two
and one-half feet of shaft length, the next stage of expansion
becomes four full-sized compartments (Fig. 11). By thus enlarging the
auxiliary winding space, some assistance may be given to ore-haulage
in case of necessity. The mine whose output demands such haulage
provisions can usually stand another foot of width to the shaft,
so that the dimensions come to about 21 feet to 22 feet by 7 feet
to 8 feet outside the timbers. Such a shaft, with three- to four-ton
skips and an appropriate engine, will handle up to 250 tons per
hour from a depth of 1,000 feet.

The next logical step in advance is the shaft of five compartments
with four full-sized haulage ways (Fig. 13), each of greater size
than in the above instance. In this case, the auxiliary engine
becomes a balanced one, and can be employed part of the time upon
ore-haulage. Such a shaft will be about 26 feet to 28 feet long
by 8 feet wide outside the timbers, when provision is made for
one gangway. The capacity of such shafts can be up to 4,000 tons a
day, depending on the depth and engine. When very large quantities
of water are to be dealt with and rod-driven pumps to be used,
two pumping compartments are sometimes necessary, but other forms
of pumps do not require more than one compartment,--an additional
reason for their use.

For depths greater than 3,000 feet, other factors come into play.
Ventilation questions become of more import. The mechanical problems
on engines and ropes become involved, and their sum-effect is to
demand much increased size and a greater number of compartments.
The shafts at Johannesburg intended as outlets for workings 5,000
feet deep are as much as 46 feet by 9 feet outside timbers.

It is not purposed to go into details as to sinking methods or
timbering. While important matters, they would unduly prolong this
discussion. Besides, a multitude of treatises exist on these subjects
and cover all the minutić of such work.

SPEED OF SINKING.--Mines may be divided into two cases,--those
being developed only, and those being operated as well as developed.
In the former, the entrance into production is usually dependent
upon the speed at which the shaft is sunk. Until the mine is earning
profits, there is a loss of interest on the capital involved, which,
in ninety-nine instances out of a hundred, warrants any reasonable
extra expenditure to induce more rapid progress. In the case of
mines in operation, the volume of ore available to treatment or
valuation is generally dependent to a great degree upon the rapidity
of the extension of workings in depth. It will be demonstrated
later that, both from a financial and a technical standpoint, the
maximum development is the right one and that unremitting extension
in depth is not only justifiable but necessary.

Speed under special conditions or over short periods has a more
romantic than practical interest, outside of its value as a stimulant
to emulation. The thing that counts is the speed which can be maintained
over the year. Rapidity of sinking depends mainly on:--

_a_. Whether the shaft is or is not in use for operating the
mine.
_b_. The breaking character of the rock.
_c_. The amount of water.

The delays incident to general carrying of ore and men are such that
the use of the main haulage engine for shaft-sinking is practically
impossible, except on mines with small tonnage output. Even with a
separate winch or auxiliary winding-engine, delays are unavoidable
in a working shaft, especially as it usually has more water to contend
with than one not in use for operating the mine. The writer's own
impression is that an average of 40 feet per month is the maximum
possibility for year in and out sinking under such conditions. In
fact, few going mines manage more than 400 feet a year. In cases
of clean shaft-sinking, where every energy is bent to speed, 150
feet per month have been averaged for many months. Special cases
have occurred where as much as 213 feet have been achieved in a
single month. With ordinary conditions, 1,200 feet in a year is
very good work. Rock awkward to break, and water especially, lowers
the rate of progress very materially. Further reference to speed
will be found in the chapter on "Drilling Methods."

TUNNEL ENTRY.--The alternative of entry to a mine by tunnel is
usually not a question of topography altogether, but, like everything
else in mining science, has to be tempered to meet the capital
available and the expenditure warranted by the value showing.

In the initial prospecting of a mine, tunnels are occasionally
overdone by prospectors. Often more would be proved by a few inclines.
As the pioneer has to rely upon his right arm for hoisting and
drainage, the tunnel offers great temptations, even when it is
long and gains but little depth. At a more advanced stage of
development, the saving of capital outlay on hoisting and pumping
equipment, at a time when capital is costly to secure, is often
sufficient justification for a tunnel entry. But at the stage where
the future working of ore below a tunnel-level must be contemplated,
other factors enter. For ore below tunnel-level a shaft becomes
necessary, and in cases where a tunnel enters a few hundred feet
below the outcrop the shaft should very often extend to the surface,
because internal shafts, winding from tunnel-level, require large
excavations to make room for the transfer of ore and for winding
gear. The latter must be operated by transmitted power, either
that of steam, water, electricity, or air. Where power has to be
generated on the mine, the saving by the use of direct steam, generated
at the winding gear, is very considerable. Moreover, the cost of
haulage through a shaft for the extra distance from tunnel-level
to the surface is often less than the cost of transferring the
ore and removing it through the tunnel. The load once on the
winding-engine, the consumption of power is small for the extra
distance, and the saving of labor is of consequence. On the other
hand, where drainage problems arise, they usually outweigh all
other considerations, for whatever the horizon entered by tunnel,
the distance from that level to the surface means a saving of
water-pumpage against so much head. The accumulation of such constant
expense justifies a proportioned capital outlay. In other words,
the saving of this extra pumping will annually redeem the cost of
a certain amount of tunnel, even though it be used for drainage
only.

In order to emphasize the rapidity with which such a saving of
constant expense will justify capital outlay, one may tabulate the
result of calculations showing the length of tunnel warranted with
various hypothetical factors of quantity of water and height of lift
eliminated from pumping. In these computations, power is taken at
the low rate of $60 per horsepower-year, the cost of tunneling at
an average figure of $20 per foot, and the time on the basis of
a ten-year life for the mine.

Feet of Tunnel Paid for in 10 Years with Under-mentioned Conditions.

=============================================================
Feet of | 100,000 | 200,000 | 300,000 | 500,000 |1,000,000
Water Lift | Gallons | Gallons | Gallons | Gallons | Gallons
Avoided |per Diem |per Diem |per Diem |per Diem |per Diem
-----------|---------|---------|---------|---------|---------
100 | 600 | 1,200 | 1,800 | 3,000 | 6,000
200 | 1,200 | 2,400 | 3,600 | 6,000 | 12,000
300 | 1,800 | 3,600 | 5,400 | 9,000 | 18,000
500 | 3,000 | 6,000 | 9,000 | 15,000 | 30,000
1,000 | 6,000 | 12,000 | 18,000 | 30,000 | 60,000
=============================================================

The size of tunnels where ore-extraction is involved depends upon
the daily tonnage output required, and the length of haul. The
smallest size that can be economically driven and managed is about
6-1/2 feet by 6 feet inside the timbers. Such a tunnel, with single
track for a length of 1,000 feet, with one turn-out, permits handling
up to 500 tons a day with men and animals. If the distance be longer
or the tonnage greater, a double track is required, which necessitates
a tunnel at least 8 feet wide by 6-1/2 feet to 7 feet high, inside
the timbers.

There are tunnel projects of a much more impressive order than those
designed to operate upper levels of mines; that is, long crosscut
tunnels designed to drain and operate mines at very considerable
depths, such as the Sutro tunnel at Virginia City. The advantage
of these tunnels is very great, especially for drainage, and they
must be constructed of large size and equipped with appliances
for mechanical haulage.