Methods of Supporting Excavation

TIMBERING; FILLING WITH WASTE; FILLING WITH BROKEN ORE; PILLARS
OF ORE; ARTIFICIAL PILLARS; CAVING SYSTEM.

Most stopes require support to be given to the walls and often to
the ore itself. Where they do require support there are five principal
methods of accomplishing it. The application of any particular method
depends upon the dip, width of ore-body, character of the ore and
walls, and cost of materials. The various systems are by:--

1. Timbering.
2. Filling with waste.
3. Filling with broken ore subsequently withdrawn.
4. Pillars of ore.
5. Artificial pillars built of timbers and waste.
6. Caving.

TIMBERING.--At one time timbering was the almost universal means of
support in such excavations, but gradually various methods for the
economical application of waste and ore itself have come forward,
until timbering is fast becoming a secondary device. Aside from
economy in working without it, the dangers of creeps, or crushing,
and of fires are sufficient incentives to do away with wood as
far as possible.

There are three principal systems of timber support to excavations,--by
stulls, square-sets, and cribs.

Stulls are serviceable only where the deposit is so narrow that
the opening can be bridged by single timbers between wall and wall
(Figs. 28 and 43). This system can be applied to any dip and is most
useful in narrow deposits where the walls are not too heavy. Stulls
in inclined deposits are usually set at a slightly higher angle than
that perpendicular to the walls, in order that the vertical pressure
of the hanging wall will serve to tighten them in position. The
"stull" system can, in inclined deposits, be further strengthened by
building waste pillars against them, in which case the arrangement
merges into the system of artificial pillars.

[Illustration: Fig. 28.--Longitudinal section of stull-supported
stope.]

[Illustration: Fig. 29.--Longitudinal section showing square-set
timbering.]

[Illustration: Fig. 30.--Square-set timbering on inclined ore-body.
Showing ultimate strain on timbers.]

Square-sets (Figs. 29 and 30), that is, trusses built in the opening
as the ore is removed, are applicable to almost any dip or width
of ore, but generally are applied only in deposits too wide, or to
rock too heavy, for stulls. Such trusses are usually constructed on
vertical and horizontal lines, and while during actual ore-breaking
the strains are partially vertical, ultimately, however, when the
weight of the walls begins to be felt, these strains, except in
vertical deposits, come at an angle to lines of strength in the
trusses, and therefore timber constructions of this type present
little ultimate resistance (Fig. 30). Square-set timbers are sometimes
set to present the maximum resistance to the direction of strain,
but the difficulties of placing them in position and variations in
the direction of strain on various parts of the stope do not often
commend the method. As a general rule square-sets on horizontal
lines answer well enough for the period of actual ore-breaking. The
crushing or creeps is usually some time later; and if the crushing
may damage the whole mine, their use is fraught with danger.
Reënforcement by building in waste is often resorted to. When done
fully, it is difficult to see the utility of the enclosed timber,
for entire waste-filling would in most cases be cheaper and equally
efficient.

[Illustration: Fig. 31.--"Cribs."]

There is always, with wood constructions, as said before, the very
pertinent danger of subsequent crushing and of subsidence in after
years, and the great risk of fires. Both these disasters have cost
Comstock and Broken Hill mines, directly or indirectly, millions of
dollars, and the outlay on timber and repairs one way or another
would have paid for the filling system ten times over. There are
cases where, by virtue of the cheapness of timber, "square-setting"
is the most economical method. Again, there are instances where the
ore lies in such a manner--particularly in limestone replacements--as
to preclude other means of support. These cases are being yearly
more and more evaded by the ingenuity of engineers in charge. The
author believes it soon will be recognized that the situation is
rare indeed where complete square-setting is necessarily without an
economical alternative. An objection is sometimes raised to filling
in favor of timber, in that if it become desirable to restope the
walls for low-grade ore left behind, such stopes could only be
entered by drawing the filling, with consequent danger of total
collapse. Such a contingency can be provided for in large ore-bodies
by installing an outer shell of sets of timber around the periphery
of the stope and filling the inside with waste. If the crushing
possibilities are too great for this method then, the subsequent
recovery of ore is hopeless in any event. In narrow ore-bodies
with crushing walls recovery of ore once left behind is not often
possible.

The third sort of timber constructions are cribs, a "log-house" sort
of structure usually filled with waste, and more fully discussed
under artificial pillars (Fig. 31). The further comparative merits
of timbering with other methods will be analyzed as the different
systems are described.

FILLING WITH WASTE.--The system of filling stope-excavations completely
with waste in alternating progress with ore-breaking is of wide
and increasingly general application (Figs. 32, 33, 34, 35).

Although a certain amount of waste is ordinarily available in the
stopes themselves, or from development work in the mine, such a
supply must usually be supplemented from other directions. Treatment
residues afford the easiest and cheapest handled material. Quarried
rock ranks next, and in default of any other easy supply, materials
from crosscuts driven into the stope-walls are sometimes resorted
to.

In working the system to the best advantage, the winzes through
the block of ore under attack are kept in alignment with similar
openings above, in order that filling may be poured through the
mine from the surface or any intermediate point. Winzes to be used
for filling should be put on the hanging-wall side of the area to
be filled, for the filling poured down will then reach the foot-wall
side of the stopes with a minimum of handling. In some instances,
one special winze is arranged for passing all filling from the
surface to a level above the principal stoping operations; and
it is then distributed along the levels into the winzes, and thus
to the operating stopes, by belt-conveyors.

[Illustration: Fig. 32.--Longitudinal section. Rill stope filled
with waste.]

[Illustration: Fig. 33.--Longitudinal section. Horizontal stope
filled with waste.]

[Illustration: Fig. 34.--Longitudinal section. Waste-filled stope
with dry-walling of levels and passes.]

In this system of stope support the ore is broken at intervals
alternating with filling. If there is danger of much loss from
mixing broken ore and filling, "sollars" of boards or poles are
laid on the waste. If the ore is very rich, old canvas or cowhides
are sometimes put under the boards. Before the filling interval,
the ore passes are built close to the face above previous filling
and their tops covered temporarily to prevent their being filled
with running waste. If the walls are bad, the filling is kept close
to the face. If the unbroken ore requires support, short stulls
set on the waste (as in Fig. 39) are usually sufficient until the
next cut is taken off, when the timber can be recovered. If stulls
are insufficient, cribs or bulkheads (Fig. 31) are also used and
often buried in the filling.

[Illustration: Fig. 35.--Cross-section of Fig. 34 on line _A-B_.]

Both flat-backed and rill-stope methods of breaking are employed in
conjunction with filled stopes. The advantages of the rill-stopes
are so patent as to make it difficult to understand why they are
not universally adopted when the dip permits their use at all. In
rill-stopes (Figs. 32 and 34) the waste flows to its destination
with a minimum of handling. Winzes and ore-passes are not required
with the same frequency as in horizontal breaking, and the broken
ore always lies on the slope towards the passes and is therefore
also easier to shovel. In flat-backed stopes (Fig. 33) winzes must
be put in every 50 feet or so, while in rill-stopes they can be
double this distance apart. The system is applicable by modification
to almost any width of ore. It finds its most economical field
where the dip of the stope floor is over 45°, when waste and ore,
with the help of the "rill," will flow to their destination. For
dips from under about 45° to about 30° or 35°, where the waste
and ore will not "flow" easily, shoveling can be helped by the
use of the "rill" system and often evaded altogether, if flow be
assisted by a sheet-iron trough described in the discussion of
stope transport. Further saving in shoveling can be gained in this
method, by giving a steeper pitch to the filling winzes and to the
ore-passes, by starting them from crosscuts in the wall, and by
carrying them at greater angles than the pitch of the ore (Fig.
36). These artifices combined have worked out most economically
on several mines within the writer's experience, with the dip as
flat as 30°. For very flat dips, where filling is to be employed,
rill-stoping has no advantage over flat-backed cuts, and in such
cases it is often advisable to assist stope transport by temporary
tracks and cars which obviously could not be worked on the tortuous
contour of a rill-stope, so that for dips under 30° advantage lies
with "flat-backed" ore-breaking.

[Illustration: Fig. 36.--Cross-section showing method of steepening
winzes and ore passes.]

On very wide ore-bodies where the support of the standing ore itself
becomes a great problem, the filling system can be applied by combining
it with square-setting. In this case the stopes are carried in
panels laid out transversally to the strike as wide as the standing
strength of the ore permits. On both sides of each panel a fence
of lagged square-sets is carried up and the area between is filled
with waste. The panels are stoped out alternately. The application
of this method at Broken Hill will be described later. (See pages
120 and Figs. 41 and 42.) The same type of wide ore-body can be
managed also on the filling system by the use of frequent "bulkheads"
to support the ore (Fig. 31).

Compared with timbering methods, filling has the great advantage
of more effective support to the mine, less danger of creeps, and
absolute freedom from the peril of fire. The relative expense of
the two systems is determined by the cost of materials and labor.
Two extreme cases illustrate the result of these economic factors
with sufficient clearness. It is stated that the cost of timbering
stopes on the Le Roi Mine by square-sets is about 21 cents per
ton of ore excavated. In the Ivanhoe mine of West Australia the
cost of filling stopes with tailings is about 22 cents per ton
of ore excavated. At the former mine the average cost of timber
is under $10 per M board-measure, while at the latter its price
would be $50 per M board-measure; although labor is about of the
same efficiency and wage, the cost in the Ivanhoe by square-setting
would be about 65 cents per ton of ore broken. In the Le Roi, on the
other hand, no residues are available for filling. To quarry rock
or drive crosscuts into the walls might make this system cost 65
cents per ton of ore broken if applied to that mine. The comparative
value of the filling method with other systems will be discussed
later.

FILLING WITH BROKEN ORE SUBSEQUENTLY WITHDRAWN.--This order of support
is called by various names, the favorite being "shrinkage-stoping."
The method is to break the ore on to the roof of the level, and by
thus filling the stope with broken ore, provide temporary support
to the walls and furnish standing floor upon which to work in making
the next cut (Figs. 37, 38, and 39.) As broken material occupies 30
to 40% more space than rock _in situ_, in order to provide working
space at the face, the broken ore must be drawn from along the level
after each cut. When the area attacked is completely broken through
from level to level, the stope will be full of loose broken ore,
which is then entirely drawn off.

A block to be attacked by this method requires preliminary winzes
only at the extremities of the stope,--for entry and for ventilation.
Where it is desired to maintain the winzes after stoping, they
must either be strongly timbered and lagged on the stope side,
be driven in the walls, or be protected by a pillar of ore (Fig.
37). The settling ore and the crushing after the stope is empty
make it difficult to maintain timbered winzes.

[Illustration: Fig. 37.--Longitudinal section of stope filled with
broken ore.]

Where it can be done without danger to the mine, the empty stopes
are allowed to cave. If such crushing would be dangerous, either
the walls must be held up by pillars of unbroken ore, as in the
Alaska Treadwell, where large "rib" pillars are left, or the open
spaces must be filled with waste. Filling the empty stope is usually
done by opening frequent passes along the base of the filled stope
above, and allowing the material of the upper stope to flood the
lower one. This program continued upwards through the mine allows
the whole filling of the mine to descend gradually and thus requires
replenishment only into the top. The old stopes in the less critical
and usually exhausted territory nearer the surface are sometimes
left without replenishing their filling.

The weight of broken ore standing at such a high angle as to settle
rapidly is very considerable upon the level; moreover, at the moment
when the stope is entirely drawn off, the pressure of the walls
as well is likely to be very great. The roadways in this system
therefore require more than usual protection. Three methods are
used: (_a_) timbering; (_b_) driving a sublevel in the ore above
the main roadway as a stoping-base, thus leaving a pillar of ore
over the roadway (Fig. 39); (_c_) by dry-walling the levels, as in
the Baltic mine, Michigan (Figs. 34 and 35). By the use of sublevels
the main roadways are sometimes driven in the walls (Fig. 38) and in
many cases all timbering is saved. To recover pillars left below
sublevels is a rather difficult task, especially if the old stope
above is caved or filled. The use of pillars in substitution for
timber, if the pillars are to be lost, is simply a matter of economics
as to whether the lost ore would repay the cost of other devices.

[Illustration: Fig. 38.--Cross-section of "shrinkage" stope.]

Frequent ore-chutes through the level timbers, or from the sublevels,
are necessary to prevent lodgment of broken ore between such passes,
because it is usually too dangerous for men to enter the emptying
stope to shovel out the lodged remnants. Where the ore-body is
wide, and in order that there may be no lodgment of ore, the timbers
over the level are set so as to form a trough along the level;
or where pillars are left, they are made "A"-shaped between the
chutes, as indicated in Figure 37.

[Illustration: Fig. 39.--Cross-section of "shrinkage" stope.]

The method of breaking the ore in conjunction with this means of
support in comparatively narrow deposits can be on the rill, in order
to have the advantage of down holes. Usually, however, flat-back
or horizontal cuts are desirable, as in such an arrangement it
is less troublesome to regulate the drawing of the ore so as to
provide proper head room. Where stopes are wide, ore is sometimes
cut arch-shaped from wall to wall to assure its standing. Where
this method of support is not of avail, short, sharply tapering
stulls are put in from the broken ore to the face (Fig. 39). When
the cut above these stulls is taken out, they are pulled up and
are used again.

This method of stoping is only applicable when:--

1. The deposit dips over 60°, and thus broken material will freely
settle downward to be drawn off from the bottom.

2. The ore is consistently payable in character. No selection can be
done in breaking, as all material broken must be drawn off together.

3. The hanging wall is strong, and will not crush or spall off waste
into the ore.

4. The ore-body is regular in size, else loose ore will lodge on
the foot wall. Stopes opened in this manner when partially empty
are too dangerous for men to enter for shoveling out remnants.

The advantages of this system over others, where it is applicable,
are:--

(_a_) A greater distance between levels can be operated and few
winzes and rises are necessary, thus a great saving of development
work can be effected. A stope 800 to 1000 feet long can be operated
with a winze at either end and with levels 200 or 220 feet apart.

(_b_) There is no shoveling in the stopes at all.

(_c_) No timber is required. As compared with timbering by stulling,
it will apply to stopes too wide and walls too heavy for this method.
Moreover, little staging is required for working the face, since
ore can be drawn from below in such a manner as to allow just the
right head room.

(_d_) Compared to the system of filling with waste, coincidentally
with breaking (second method), it saves altogether in some cases
the cost of filling. In any event, it saves the cost of ore-passes,
of shoveling into them, and of the detailed distribution of the
filling.

Compared with other methods, the system has the following disadvantages,
that:

_A_. The ore requires to be broken in the stopes to a degree of
fineness which will prevent blocking of the chutes at the level.
When pieces too large reach the chutes, nothing will open them but
blasting,--to the damage of timbers and chutes. Some large rocks
are always liable to be buried in the course of ore-breaking.

_B_. Practically no such perfection of walls exists, but some spalling
of waste into the ore will take place. A crushing of the walls
would soon mean the loss of large amounts of ore.

_C_. There is no possibility of regulating the mixture of grade
of ore by varying the working points. It is months after the ore
is broken before it can reach the levels.

_D_. The breaking of 60% more ore than immediate treatment demands
results in the investment of a considerable sum of money. An equilibrium
is ultimately established in a mine worked on this system when a
certain number of stopes full of completely broken ore are available
for entire withdrawal, and there is no further accumulation. But,
in any event, a considerable amount of broken ore must be held in
reserve. In one mine worked on this plan, with which the writer
has had experience, the annual production is about 250,000 tons
and the broken ore represents an investment which, at 5%, means
an annual loss of interest amounting to 7 cents per ton of ore
treated.

_E_. A mine once started on the system is most difficult to alter,
owing to the lack of frequent winzes or passes. Especially is this
so if the only alternative is filling, for an alteration to the
system of filling coincident with breaking finds the mine short
of filling winzes. As the conditions of walls and ore often alter
with depth, change of system may be necessary and the situation
may become very embarrassing.

_F_. The restoping of the walls for lower-grade ore at a later
period is impossible, for the walls of the stope will be crushed,
or, if filled with waste, will usually crush when it is drawn off
to send to a lower stope.

The system has much to recommend it where conditions are favorable.
Like all other alternative methods of mining, it requires the most
careful study in the light of the special conditions involved. In many
mines it can be used for some stopes where not adaptable generally.
It often solves the problem of blind ore-bodies, for they can by
this means be frequently worked with an opening underneath only.
Thus the cost of driving a roadway overhead is avoided, which would
be required if timber or coincident filling were the alternatives.
In such cases ventilation can be managed without an opening above,
by so directing the current of air that it will rise through a
winze from the level below, flow along the stope and into the level
again at the further end of the stope through another winze.

[Illustration: Fig. 40.--Longitudinal section. Ore-pillar support
in narrow stopes.]

SUPPORT BY PILLARS OF ORE.--As a method of mining metals of the
sort under discussion, the use of ore-pillars except in conjunction
with some other means of support has no general application. To
use them without assistance implies walls sufficiently strong to
hold between pillars; to leave them permanently anywhere implies
that the ore abandoned would not repay the labor and the material
of a substitute. There are cases of large, very low-grade mines
where to abandon one-half the ore as pillars is more profitable
than total extraction, but the margin of payability in such ore must
be very, very narrow. Unpayable spots are always left as pillars,
for obvious reasons. Permanent ore-pillars as an adjunct to other
methods of support are in use. Such are the rib-pillars in the
Alaska Treadwell, the form of which is indicated by the upward
extension of the pillars adjacent to the winzes, shown in Figure
37. Always a careful balance must be cast as to the value of the ore
left, and as to the cost of a substitute, because every ore-pillar
can be removed at some outlay. Temporary pillars are not unusual,
particularly to protect roadways and shafts. They are, when left
for these purposes, removed ultimately, usually by beginning at
the farther end and working back to the final exit.

[Illustration: Fig. 41.--Horizontal plan at levels of Broken Hill.
Method of alternate stopes and ore-pillars.]

[Illustration: Fig. 42.--Longitudinal section of Figure 41.]

A form of temporary ore-pillars in very wide deposits is made use
of in conjunction with both filling and timbering (Figs. 37, 39,
40). In the use of temporary pillars for ore-bodies 100 to 250
feet wide at Broken Hill, stopes are carried up at right angles
to the strike, each fifty feet wide and clear across the ore-body
(Figs. 41 and 42). A solid pillar of the same width is left in the
first instance between adjacent stopes, and the initial series of
stopes are walled with one square-set on the sides as the stope is
broken upward. The room between these two lines of sets is filled
with waste alternating with ore-breaking in the usual filling method.
When the ore from the first group of alternate stopes (_ABC_, Fig.
42) is completely removed, the pillars are stoped out and replaced
with waste. The square-sets of the first set of stopes thus become
the boundaries of the second set. Entry and ventilation are obtained
through these lines of square-sets, and the ore is passed out of
the stopes through them.

[Illustration: Fig. 43.--Cross-section of stull support with waste
reënforcement.]

ARTIFICIAL PILLARS.--This system also implies a roof so strong
as not to demand continuous support. Artificial pillars are built
in many different ways. The method most current in fairly narrow
deposits is to reënforce stulls by packing waste above them (Figs.
43 and 44). Not only is it thus possible to economize in stulls by
using the waste which accumulates underground, but the principle
applies also to cases where the stulls alone are not sufficient
support, and yet where complete filling or square-setting is
unnecessary. When the conditions are propitious for this method, it
has the comparative advantage over timber systems of saving timber,
and over filling systems of saving imported filling. Moreover,
these constructions being pillar-shaped (Fig. 44), the intervals
between them provide outlets for broken ore, and specially built
passes are unnecessary. The method has two disadvantages as against
the square-set or filling process, in that more staging must be
provided from which to work, and in stopes over six feet the erection
of machine-drill columns is tedious and costly in time and wages.

[Illustration: Fig. 44.--Longitudinal section of stull and waste
pillars.]

In wide deposits of markedly flat, irregular ore-bodies, where a
definite system is difficult and where timber is expensive, cribs
of cord-wood or logs filled with waste after the order shown in
Figure 31, often make fairly sound pillars. They will not last
indefinitely and are best adapted to the temporary support of the
ore-roof pending filling. The increased difficulty in setting up
machine drills in such stopes adds to the breaking costs,--often
enough to warrant another method of support.

[Illustration: Fig. 45.--Sublevel caving system.]

CAVING SYSTEMS.--This method, with variations, has been applied
to large iron deposits, to the Kimberley diamond mines, to some
copper mines, but in general it has little application to the metal
mines under consideration, as few ore-bodies are of sufficiently
large horizontal area. The system is dependent upon a large area of
loose or "heavy" ground pressing directly on the ore with weight,
such that if the ore be cut into pillars, these will crush. The
details of the system vary, but in general the _modus operandi_
is to prepare roadways through the ore, and from the roadways to
put rises, from which sublevels are driven close under the floating
mass of waste and ore,--sometimes called the "matte" (Fig. 45).
The pillars between these sublevels are then cut away until the
weight above crushes them down. When all the crushed ore which
can be safely reached is extracted, retreat is made and another
series of subopenings is then driven close under the "matte." The
pillar is reduced until it crushes and the operation is repeated.
Eventually the bottom strata of the "matte" become largely ore,
and a sort of equilibrium is reached when there is not much loss
in this direction. "Top slicing" is a variation of the above method
by carrying a horizontal stope from the rises immediately under the
matte, supporting the floating material with timber. At Kimberley
the system is varied in that galleries are run out to the edge of
the diamond-iferous area and enlarged until the pillar between
crushes.

In the caving methods, between 40 and 50% of the ore is removed
by the preliminary openings, and as they are all headings of some
sort, the average cost per ton of this particular ore is higher
than by ordinary stoping methods. On the other hand, the remaining
50 to 60% of the ore costs nothing to break, and the average cost
is often remarkably low. As said, the system implies bodies of large
horizontal area. They must start near enough to the surface that
the whole superincumbent mass may cave and give crushing weight,
or the immediately overhanging roof must easily cave. All of these
are conditions not often met with in mines of the character under
review.