Development of Mines

ENTRY TO THE MINE; TUNNELS; VERTICAL, INCLINED, AND COMBINED SHAFTS;
LOCATION AND NUMBER OF SHAFTS.

Development is conducted for two purposes: first, to search for
ore; and second, to open avenues for its extraction. Although both
objects are always more or less in view, the first predominates
in the early life of mines, the prospecting stage, and the second
in its later life, the producing stage. It is proposed to discuss
development designed to embrace extended production purposes first,
because development during the prospecting stage is governed by
the same principles, but is tempered by the greater degree of
uncertainty as to the future of the mine, and is, therefore, of
a more temporary character.

ENTRY TO THE MINE.

There are four methods of entry: by tunnel, vertical shaft, inclined
shaft, or by a combination of the last two, that is, by a shaft
initially vertical then turned to an incline. Combined shafts are
largely a development of the past few years to meet "deep level"
conditions, and have been rendered possible only by skip-winding. The
angle in such shafts (Fig. 2) is now generally made on a parabolic
curve, and the speed of winding is then less diminished by the
bend.

The engineering problems which present themselves under "entry"
may be divided into those of:--

1. Method.
2. Location.
3. Shape and size.

The resolution of these questions depends upon the:--

a. Degree of dip of the deposit.
b. Output of ore to be provided for.
c. Depth at which the deposit is to be attacked.
d. Boundaries of the property.
e. Surface topography.
f. Cost.
g. Operating efficiency.
h. Prospects of the mine.

[Illustration: Fig. 2.--Showing arrangement of the bend in combined
shafts.]

From the point of view of entrance, the coöperation of a majority
of these factors permits the division of mines into certain broad
classes. The type of works demanded for moderate depths (say vertically
2,500 to 3,000 feet) is very different from that required for great
depths. To reach great depths, the size of shafts must greatly
expand, to provide for extended ventilation, pumping, and winding
necessities. Moreover inclined shafts of a degree of flatness possible
for moderate depths become too long to be used economically from
the surface. The vast majority of metal-mining shafts fall into
the first class, those of moderate depths. Yet, as time goes on
and ore-deposits are exhausted to lower planes, problems of depth
will become more common. One thing, however, cannot be too much
emphasized, especially on mines to be worked from the outcrop, and
that is, that no engineer is warranted, owing to the speculation
incidental to extension in depth, in initiating early in the mine's
career shafts of such size or equipment as would be available for
great depths. Moreover, the proper location of a shaft so as to
work economically extension of the ore-bodies is a matter of no
certainty, and therefore shafts of speculative mines are tentative
in any event.

Another line of division from an engineering view is brought about
by a combination of three of the factors mentioned. This is the
classification into "outcrop" and "deep-level" mines. The former
are those founded upon ore-deposits to be worked from or close
to the surface. The latter are mines based upon the extension in
depth of ore-bodies from outcrop mines. Such projects are not so
common in America, where the law in most districts gives the outcrop
owner the right to follow ore beyond his side-lines, as in countries
where the boundaries are vertical on all sides. They do, however,
arise not alone in the few American sections where the side-lines
are vertical boundaries, but in other parts owing to the pitch of
ore-bodies through the end lines (Fig. 3). More especially do such
problems arise in America in effect, where the ingress questions
have to be revised for mines worked out in the upper levels (Fig.
7).

[Illustration: Fig. 3.--Longitudinal section showing "deep level"
project arising from dip of ore-body through end-line.]

If from a standpoint of entrance questions, mines are first classified
into those whose works are contemplated for moderate depths, and those
in which work is contemplated for great depth, further clarity in
discussion can be gained by subdivision into the possible cases arising
out of the factors of location, dip, topography, and boundaries.

MINES OF MODERATE DEPTHS.

Case I. Deposits where topographic conditions permit the
alternatives of shaft or tunnel.
Case II. Vertical or horizontal deposits, the only practical
means of attaining which is by a vertical shaft.
Case III. Inclined deposits to be worked from near the surface.
There are in such instances the alternatives of either
a vertical or an inclined shaft.
Case IV. Inclined deposits which must be attacked in depth,
that is, deep-level projects. There are the alternatives
of a compound shaft or of a vertical shaft, and
in some cases of an incline from the surface.

MINES TO GREAT DEPTHS.

Case V. Vertical or horizontal deposits, the only way of reaching
which is by a vertical shaft.
Case VI. Inclined deposits. In such cases the alternatives are
a vertical or a compound shaft.

CASE I.--Although for logical arrangement tunnel entry has been
given first place, to save repetition it is proposed to consider
it later. With few exceptions, tunnels are a temporary expedient
in the mine, which must sooner or later be opened by a shaft.

CASE II. VERTICAL OR HORIZONTAL DEPOSITS.--These require no discussion
as to manner of entry. There is no justifiable alternative to a
vertical shaft (Fig. 4).

[Illustration: Fig. 4.--Cross-sections showing entry to vertical
or horizontal deposits. Case II.]

[Illustration: Fig. 5.--Cross-section showing alternative shafts
to inclined deposit to be worked from surface. Case III.]

CASE III. INCLINED DEPOSITS WHICH ARE INTENDED TO BE WORKED FROM
THE OUTCROP, OR FROM NEAR IT (Fig. 5).--The choice of inclined or
vertical shaft is dependent upon relative cost of construction,
subsequent operation, and the useful life of the shaft, and these
matters are largely governed by the degree of dip. Assuming a shaft
of the same size in either alternative, the comparative cost per
foot of sinking is dependent largely on the breaking facilities
of the rock under the different directions of attack. In this,
the angles of the bedding or joint planes to the direction of the
shaft outweigh other factors. The shaft which takes the greatest
advantage of such lines of breaking weakness will be the cheapest
per foot to sink. In South African experience, where inclined shafts
are sunk parallel to the bedding planes of hard quartzites, the cost
per foot appears to be in favor of the incline. On the other hand,
sinking shafts across tight schists seems to be more advantageous
than parallel to the bedding planes, and inclines following the
dip cost more per foot than vertical shafts.

An inclined shaft requires more footage to reach a given point
of depth, and therefore it would entail a greater total expense
than a vertical shaft, assuming they cost the same per foot. The
excess amount will be represented by the extra length, and this
will depend upon the flatness of the dip. With vertical shafts,
however, crosscuts to the deposit are necessary. In a comparative
view, therefore, the cost of the crosscuts must be included with
that of the vertical shaft, as they would be almost wholly saved
in an incline following near the ore.

The factor of useful life for the shaft enters in deciding as to
the advisability of vertical shafts on inclined deposits, from the
fact that at some depth one of two alternatives has to be chosen.
The vertical shaft, when it reaches a point below the deposit where
the crosscuts are too long (_C_, Fig. 5), either becomes useless,
or must be turned on an incline at the intersection with the ore
(_B_). The first alternative means ultimately a complete loss of
the shaft for working purposes. The latter has the disadvantage
that the bend interferes slightly with haulage.

The following table will indicate an hypothetical extreme case,--not
infrequently met. In it a vertical shaft 1,500 feet in depth is taken
as cutting the deposit at the depth of 750 feet, the most favored
position so far as aggregate length of crosscuts is concerned. The
cost of crosscutting is taken at $20 per foot and that of sinking
the vertical shaft at $75 per foot. The incline is assumed for two
cases at $75 and $100 per foot respectively. The stoping height
upon the ore between levels is counted at 125 feet.

Dip of | Depth of | Length of |No. of Crosscuts| Total Length
Deposit from | Vertical | Incline | Required from | of Crosscuts,
Horizontal | Shaft | Required | V Shaft | Feet
-------------|-------------|-------------|----------------|---------------
80° | 1,500 | 1,522 | 11 | 859
70° | 1,500 | 1,595 | 12 | 1,911
60° | 1,500 | 1,732 | 13 | 3,247
50° | 1,500 | 1,058 | 15 | 5,389
40° | 1,500 | 2,334 | 18 | 8,038
30° | 1,500 | 3,000 | 23 | 16,237
==========================================================================
Cost of |Cost Vertical| Total Cost | Cost of Incline|Cost of Incline
Crosscuts $20| Shaft $75 | of Vertical | $75 per Foot | $100 per Foot
per Foot | per Foot |and Crosscuts| |
-------------|-------------|-------------|----------------|---------------
$17,180 | $112,500 | $129,680 | $114,150 | $152,200
38,220 | 112,500 | 150,720 | 118,625 | 159,500
64,940 | 112,500 | 177,440 | 129,900 | 172,230
107,780 | 112,500 | 220,280 | 114,850 | 195,800
178,760 | 112,500 | 291,260 | 175,050 | 233,400
324,740 | 112,500 | 437,240 | 225,000 | 300,000

From the above examples it will be seen that the cost of crosscuts
put at ordinary level intervals rapidly outruns the extra expense
of increased length of inclines. If, however, the conditions are
such that crosscuts from a vertical shaft are not necessary at so
frequent intervals, then in proportion to the decrease the advantages
sway to the vertical shaft. Most situations wherein the crosscuts
can be avoided arise in mines worked out in the upper levels and
fall under Case IV, that of deep-level projects.

There can be no doubt that vertical shafts are cheaper to operate
than inclines: the length of haul from a given depth is less; much
higher rope speed is possible, and thus the haulage hours are less
for the same output; the wear and tear on ropes, tracks, or guides
is not so great, and pumping is more economical where the Cornish
order of pump is used. On the other hand, with a vertical shaft
must be included the cost of operating crosscuts. On mines where
the volume of ore does not warrant mechanical haulage, the cost of
tramming through the extra distance involved is an expense which
outweighs any extra operating outlay in the inclined shaft itself.
Even with mechanical haulage in crosscuts, it is doubtful if there
is anything in favor of the vertical shaft on this score.

[Illustration: Fig. 6.--Cross-section showing auxiliary vertical
outlet.]

In deposits of very flat dips, under 30°, the case arises where the
length of incline is so great that the saving on haulage through
direct lift warrants a vertical shaft as an auxiliary outlet in
addition to the incline (Fig. 6). In such a combination the crosscut
question is eliminated. The mine is worked above and below the
intersection by incline, and the vertical shaft becomes simply a
more economical exit and an alternative to secure increased output.
The North Star mine at Grass Valley is an illustration in point. Such
a positive instance borders again on Case IV, deep-level projects.

In conclusion, it is the writer's belief that where mines are to
be worked from near the surface, coincidentally with sinking, and
where, therefore, crosscuts from a vertical shaft would need to be
installed frequently, inclines are warranted in all dips under 75°
and over 30°. Beyond 75° the best alternative is often undeterminable.
In the range under 30° and over 15°, although inclines are primarily
necessary for actual delivery of ore from levels, they can often
be justifiably supplemented by a vertical shaft as a relief to a
long haul. In dips of less than 15°, as in those over 75°, the
advantages again trend strongly in favor of the vertical shaft. There
arise, however, in mountainous countries, topographic conditions
such as the dip of deposits into the mountain, which preclude any
alternative on an incline at any angled dip.

CASE IV. INCLINED DEPOSITS WHICH MUST BE ATTACKED IN DEPTH (Fig.
7).--There are two principal conditions in which such properties
exist: first, mines being operated, or having been previously worked,
whose method of entry must be revised; second, those whose ore-bodies
to be attacked do not outcrop within the property.

The first situation may occur in mines of inadequate shaft capacity
or wrong location; in mines abandoned and resurrected; in mines
where a vertical shaft has reached its limit of useful extensions,
having passed the place of economical crosscutting; or in mines in
flat deposits with inclines whose haul has become too long to be
economical. Three alternatives present themselves in such cases: a
new incline from the surface (_A B F_, Fig. 7), or a vertical shaft
combined with incline extension (_C D F_), or a simple vertical
shaft (_H G_). A comparison can be first made between the simple
incline and the combined shaft. The construction of an incline from
the surface to the ore-body will be more costly than a combined
shaft, for until the horizon of the ore is reached (at _D_) no
crosscuts are required in the vertical section, while the incline
must be of greater length to reach the same horizon. The case arises,
however, where inclines can be sunk through old stopes, and thus
more cheaply constructed than vertical shafts through solid rock;
and also the case of mountainous topographic conditions mentioned
above.

[Illustration: Fig. 7.--Cross-section of inclined deposit which
must be attacked in depth.]

From an operating point of view, the bend in combined shafts (at
_D_) gives rise to a good deal of wear and tear on ropes and gear.
The possible speed of winding through a combined shaft is, however,
greater than a simple incline, for although haulage speed through
the incline section (_D F_) and around the bend of the combined
shaft is about the same as throughout a simple incline (_A F_), the
speed can be accelerated in the vertical portion (_D C_) above that
feasible did the incline extend to the surface. There is therefore an
advantage in this regard in the combined shaft. The net advantages
of the combined over the inclined shaft depend on the comparative
length of the two alternative routes from the intersection (_D_)
to the surface. Certainly it is not advisable to sink a combined
shaft to cut a deposit at 300 feet in depth if a simple incline
can be had to the surface. On the other hand, a combined shaft
cutting the deposit at 1,000 feet will be more advisable than a
simple incline 2,000 feet long to reach the same point. The matter
is one for direct calculation in each special case. In general, there
are few instances of really deep-level projects where a complete
incline from the surface is warranted.

In most situations of this sort, and in all of the second type
(where the outcrop is outside the property), actual choice usually
lies between combined shafts (_C D F_) and entire vertical shafts (_H
G_). The difference between a combined shaft and a direct vertical
shaft can be reduced to a comparison of the combined shaft below
the point of intersection (_D_) with that portion of a vertical
shaft which would cover the same horizon. The question then becomes
identical with that of inclined _versus_ verticals, as stated in Case
III, with the offsetting disadvantage of the bend in the combined
shaft. If it is desired to reach production at the earliest date,
the lower section of a simple vertical shaft must have crosscuts
to reach the ore lying above the horizon of its intersection (_E_).
If production does not press, the ore above the intersection (_EB_)
can be worked by rises from the horizon of intersection (_E_).
In the use of rises, however, there follow the difficulties of
ventilation and lowering the ore down to the shaft, which brings
expenses to much the same thing as operating through crosscuts.

The advantages of combined over simple vertical shafts are earlier
production, saving of either rises or crosscuts, and the ultimate
utility of the shaft to any depth. The disadvantages are the cost
of the extra length of the inclined section, slower winding, and
greater wear and tear within the inclined section and especially
around the bend. All these factors are of variable import, depending
upon the dip. On very steep dips,--over 70°,--the net result is in
favor of the simple vertical shaft. On other dips it is in favor
of the combined shaft.

CASES V AND VI. MINES TO BE WORKED TO GREAT DEPTHS,--OVER 3,000
FEET.--In Case V, with vertical or horizontal deposits, there is
obviously no desirable alternative to vertical shafts.

In Case VI, with inclined deposits, there are the alternatives
of a combined or of a simple vertical shaft. A vertical shaft in
locations (_H_, Fig. 7) such as would not necessitate extension in
depth by an incline, would, as in Case IV, compel either crosscuts
to the ore or inclines up from the horizon of intersection (_E_).
Apart from delay in coming to production and the consequent loss of
interest on capital, the ventilation problems with this arrangement
would be appalling. Moreover, the combined shaft, entering the deposit
near its shallowest point, offers the possibility of a separate
haulage system on the inclined and on the vertical sections, and
such separate haulage is usually advisable at great depths. In
such instances, the output to be handled is large, for no mine of
small output is likely to be contemplated at such depth. Several
moderate-sized inclines from the horizon of intersection have been
suggested (_EF_, _DG_, _CH_, Fig. 8) to feed a large primary shaft
(_AB_), which thus becomes the trunk road. This program would cheapen
lateral haulage underground, as mechanical traction can be used
in the main level, (_EC_), and horizontal haulage costs can be
reduced on the lower levels. Moreover, separate winding engines
on the two sections increase the capacity, for the effect is that
of two trains instead of one running on a single track.

SHAFT LOCATION.--Although the prime purpose in locating a shaft
is obviously to gain access to the largest volume of ore within
the shortest haulage distance, other conditions also enter, such
as the character of the surface and the rock to be intersected,
the time involved before reaching production, and capital cost.
As shafts must bear two relations to a deposit,--one as to the
dip and the other as to the strike,--they may be considered from
these aspects. Vertical shafts must be on the hanging-wall side
of the outcrop if the deposit dips at all. In any event, the shaft
should be far enough away to be out of the reach of creeps. An
inclined shaft may be sunk either on the vein, in which case a
pillar of ore must be left to support the shaft; or, instead, it
may be sunk a short distance in the footwall, and where necessary
the excavation above can be supported by filling. Following the
ore has the advantage of prospecting in sinking, and in many cases
the softness of the ground in the region of the vein warrants this
procedure. It has, however, the disadvantage that a pillar of ore
is locked up until the shaft is ready for abandonment. Moreover, as
veins or lodes are seldom of even dip, an inclined shaft, to have
value as a prospecting opening, or to take advantage of breaking
possibilities in the lode, will usually be crooked, and an incline
irregular in detail adds greatly to the cost of winding and maintenance.
These twin disadvantages usually warrant a straight incline in the
footwall. Inclines are not necessarily of the same dip throughout,
but for reasonably economical haulage change of angle must take
place gradually.

[Illustration: Fig. 8.--Longitudinal section showing shaft arrangement
proposed for very deep inclined deposits.]

In the case of deep-level projects on inclined deposits, demanding
combined or vertical shafts, the first desideratum is to locate
the vertical section as far from the outcrop as possible, and thus
secure the most ore above the horizon of intersection. This, however,
as stated before, would involve the cost of crosscuts or rises and
would cause delay in production, together with the accumulation
of capital charges. How important the increment of interest on
capital may become during the period of opening the mine may be
demonstrated by a concrete case. For instance, the capital of a
company or the cost of the property is, say, $1,000,000, and where
opening the mine for production requires four years, the aggregate
sum of accumulated compound interest at 5% (and most operators
want more from a mining investment) would be $216,000. Under such
circumstances, if a year or two can be saved in getting to production
by entering the property at a higher horizon, the difference in
accumulated interest will more than repay the infinitesimal extra
cost of winding through a combined shaft of somewhat increased
length in the inclined section.

The unknown character of the ore in depth is always a sound reason
for reaching it as quickly and as cheaply as possible. In result,
such shafts are usually best located when the vertical section
enters the upper portion of the deposit.

The objective in location with regard to the strike of the ore-bodies
is obviously to have an equal length of lateral ore-haul in every
direction from the shaft. It is easier to specify than to achieve
this, for in all speculative deposits ore-shoots are found to pursue
curious vagaries as they go down. Ore-bodies do not reoccur with
the same locus as in the upper levels, and generally the chances
to go wrong are more numerous than those to go right.

NUMBER OF SHAFTS.--The problem of whether the mine is to be opened
by one or by two shafts of course influences location. In metal
mines under Cases II and III (outcrop properties) the ore output
requirements are seldom beyond the capacity of one shaft. Ventilation
and escape-ways are usually easily managed through the old stopes.
Under such circumstances, the conditions warranting a second shaft
are the length of underground haul and isolation of ore-bodies or
veins. Lateral haulage underground is necessarily disintegrated by
the various levels, and usually has to be done by hand. By shortening
this distance of tramming and by consolidation of the material
from all levels at the surface, where mechanical haulage can be
installed, a second shaft is often justified. There is therefore
an economic limitation to the radius of a single shaft, regardless
of the ability of the shaft to handle the total output.

Other questions also often arise which are of equal importance
to haulage costs. Separate ore-shoots or ore-bodies or parallel
deposits necessitate, if worked from one shaft, constant levels
through unpayable ground and extra haul as well, or ore-bodies may
dip away from the original shaft along the strike of the deposit
and a long haulage through dead levels must follow. For instance,
levels and crosscuts cost roughly one-quarter as much per foot as
shafts. Therefore four levels in barren ground, to reach a parallel
vein or isolated ore-body 1,000 feet away, would pay for a shaft
1,000 feet deep. At a depth of 1,000 feet, at least six levels
might be necessary. The tramming of ore by hand through such a
distance would cost about double the amount to hoist it through
a shaft and transport it mechanically to the dressing plant at
surface. The aggregate cost and operation of barren levels therefore
soon pays for a second shaft. If two or more shafts are in question,
they must obviously be set so as to best divide the work.

Under Cases IV, V, and VI,--that is, deep-level projects,--ventilation
and escape become most important considerations. Even where the
volume of ore is within the capacity of a single shaft, another
usually becomes a necessity for these reasons. Their location is
affected not only by the locus of the ore, but, as said, by the time
required to reach it. Where two shafts are to be sunk to inclined
deposits, it is usual to set one so as to intersect the deposit at
a lower point than the other. Production can be started from the
shallower, before the second is entirely ready. The ore above the
horizon of intersection of the deeper shaft is thus accessible from
the shallower shaft, and the difficulty of long rises or crosscuts
from that deepest shaft does not arise.