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CHOOSING AN UNDERGROUND MINING METHOD
1. INTRODUCTION
Once an orebody has been probed and outlined, and sufficient information has been
collected to warrant further analysis, the important process of selecting the most
appropriate method or methods for mining can begin. At this stage, the selection is
preliminary only, serving as the basis for a project layout and feasibility study. Later, it may
be found necessary to revise the details, but the basic principles for the ore extraction
should remain a part of the final layout.
With respect to the basic principles employed, relatively few mining methods are used
today. For the purposes of this presentation, the principles are classified as follows:
1. For naturally supported stopes, the available methods are room-and-pillar mining
and sublevel stoping.
2. For artificially supported stopes, the available methods are shrinkage stoping, cutand-fill mining, square-set mining, and longwall mining.
3. For caving methods, the available techniques are sublevel caving and block caving.
Due to the uniqueness of each ore deposit, the variations on each of these methods are
nearly limitless. It is impossible to include even the major variations in this chapter; the
goal of this chapter is to present the principles as a guide to the mining engineer in
adapting the general methods to specific needs. The selection process described herein is
intended to supply the techniques by which the candidate methods available for a given
orebody can be reduced to one or two feasible approaches. The feasible approaches then
can be evaluated in detail, and the particular modifications can be investigated.
2. BASIC UNDERGROUND MINING METHODS
Before describing the selection of the method or methods applicable to the mining of a
particular orebody, the following paragraphs briefly summarize the characteristics of the
major mining methods in-part, this material has been adapted from the ''Brief Guide to
Mining Methods,'' published by Atlas Copco MCT AB (publication 15073a).
2.1. Definitions of Terms
To better understand the material presented herein, some of the more common mining
terms are defined in the following paragraphs; Fig. 2.1 further clarifies some of the terms.
adit - An adit is a horizontal or nearly horizontal entrance to a mine.
back - The back is the roof or overhead surface of an underground excavation.
chute - A chute is a loading arrangement that utilizes gravity flow to move material from a
higher level to a lower level.
cone - A cone is a funnel-shaped excavation located at the top of a raise, and it is used to
collect rock from the area above.
crosscut - A crosscut is a horizontal or nearly horizontal underground opening that is
driven to intersect an orebody.
dip - The dip is the angle at which an ore deposit is inclined from the horizontal.
drawpoint - A drawpoint is a place where ore can be loaded and removed. A drawpoint is
located beneath the stoping area, and gravity flow is used to transfer the ore to the loading
place.
drift - A drift is a horizontal or nearly horizontal underground opening.
finger raise - A finger raise is used for transferring ore. The usual arrangement is as a
system of several raises that branch together to the same delivery point.
footwall - The footwall is the wall or rock under the ore deposit (compare dip).
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grizzly - A grizzly is an arrangement that prevents oversize rock from entering an ore
transfer system. A grizzly usually consists of a steel grating for coarse screening or
scalping.
hanging wall - The hanging wall is the wall or rock above an ore deposit (compare dip).
Ievel - A level is a system of horizontal underground workings that are connected to the
shaft. A level forms the basis for excavation of the ore above or below.
manway - A manway is an underground opening that is intended for personnel access
and communication.
ore - An ore is a mineral deposit that can be worked at a profit under existing economic
conditions.
orepass - An orepass is a vertical or inclined underground opening through which the ore
is transferred.
prospect - A prospect is a mineral deposit for which the economic value has not yet been
proven.
raise - A raise is an underground opening that is driven upward from one level to a higher
level or to the surface; it may be either vertical or inclined (compare winze).
ramp - A ramp is an inclined underground opening that connects levels or production
areas; ramps are inclined to allow the passage of motorized vehicles. Ramps usually are
driven downward.
shaft - A shaft is a vertical or inclined underground opening through which a mine is
worked.
slot - A slot is a vertical or inclined ore section that is excavated to open up for further
stoping.
stope - A stope is an underground excavation that is made by removing ore from the
surrounding rock.
strike - The strike is the main horizontal course or direction of the mineral deposit.
sublevel - A sublevel is a system of horizontal underground workings; normally, sublevels
are used only within stoping areas where they are required for the ore production.
wall rock - The wall rock is the wall in which the ore deposit is enclosed.
waste - The waste is the barren rock or the rock of too low a grade to be mined
economically.
winze - A winze is a vertical or inclined underground opening that is driven downward from
one level to another level or from the surface to a level (compare raise).
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Figure 2.1 Sample layout of an underground mine, identifying various mining operations
and terms.
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2.2. Stopes with Natural Support
Naturally supported stopes may be mined either by room-and-pillar mining or by sublevel
stoping.
Room-and-Pillar Mining. In the method known as room-and-pillar mining, the orebody is
excavated as completely as possible, leaving parts of the ore as pillars to support the
hanging wall. The dimensions of the stopes and pillars depend upon factors such as the
stability of the hanging wall, the stability of the ore, the thickness of the deposit, and the
rock pressure. Generally, the objective is to extract the ore as completely as possible
without jeopardizing working conditions or personnel safety. Typically, the pillars are
arranged in a regular pattern, and they can be circular, square, or shaped as longitudinal
walls that separate the stopes.
Although some of the ore left in the pillars can be extracted by ''robbing'' as a final
operation in the mine, the ore in the pillars usually is regarded as non-recoverable.
Applications. The normal applications of room-and-pillar mining include orebodies with a
horizontal or flat dip, normally having an inclination that does not exceed 0.52 rad (30o). To
use room-and-pillar mining, the hanging wall and the ore must be relatively competent.
In this respect, the stability of the ore and the hanging wall is a flexible concept. Increasing
the number of pillars and reducing the stope width can compensate for poor ground
conditions. However, ore recovery is sacrificed, since a larger portion of the orebody is left
to support the back. Another method of increasing the stability of the hanging wall utilizes
roof bolting, and this method is used extensively in room-and-pillar mining.
Room-and-pillar mining is the only feasible method of mining flat deposits of limited
thickness. To a great extent, the method is used for mining bedded deposits of
sedimentary origin, including copper-mineralized shales, coal, and many industrial
minerals such as limestone, salt, and potash.
Three different systems of room-and-pillar mining may be used. The first and most
common system is applicable to horizontal or nearly horizontal deposits. lt also is
applicable to inclined deposits of greater thickness, with the floor arranged for a moderate
slope that allows the use of mobile equipment.
The second system is applicable to orebodies that are inclined from 0.35 to 0.52 rad (20°
to 30o). The stoping proceeds upward along the dip. The steep gradient of the stope
precludes the use of mobile equipment with this system.
The third system adapts horizontal stoping to an inclined orebody. A special layout of the
stopes and a special sequence of ore extraction result in working areas that have
moderately sloping floors and that allow mobile equipment to be used in the inclined
orebody.
Horizontal Mining. Figure 2.2 illustrates the general approach to room-and-pillar mining of
flat or nearly horizontal ore deposits.
Development. In a flat or nearly horizontal ore deposit, minimum development is required
prior to mining. Roadways are required for transporting, the broken ore and for access
between working areas. Often, the required development can be combined with the actual
mining operation, and certain mined-out stopes can be utilized as transport routes.
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Figure 2.2. Room-and-pillar mining of a flat orebody.
Production. In horizontal orebodies, the drilling and. blasting operation can be compared
to drifting and slashing, where the room width and height correspond to the drift
dimensions. The available space and access to the rooms determine the mechanized
equipment that can be used. Generally, both space and access are ample, allowing the
use of large drill jumbos for high productivity.
In thick orebodies, the total thickness often is not excavated in a single step. Normally,
extraction starts with the top section; during this phase, roof control (e.g., roof bolting) is
accomplished. The remainder of the ore is recovered by benching in one or more steps.
Crawler rigs can be used for drilling vertical or inclined holes; the bench also can be drilled
from the front with standard drill jumbos.
Ore Handling. The broken ore is loaded directly at the working faces, usually by means of
diesel-powered loaders. Various transport systems may be used, depending upon the
space available and the distance to be traversed. With sufficient height in the stopes,
standard dump trucks of suitable size provide economical ore transportation. For low back
heights, special low-profile ore carriers are available. For even thinner orebodies, loadhaul-dump (LHD) units may be used.
Because the ore has a large horizontal extent with production at a number of working
faces spread over a large area, mining equipment with high mobility is required (drilling,
loading, and haulage machinery). On the other hand, the mining layout allows access
roads to be arranged easily in the mined-out stopes for rapid movement from one location
to another.
Comments. Room-and-pillar mining of a flat or nearly horizontal orebody favors the use of
mechanized equipment. Usually, the mine layout is schematic, several production areas
can be arranged, and communication and access are simple. These factors allow high
utilization of both personnel and machinery in a very efficient mining process.
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Inclined Mining. Figure 2.3 illustrates the general approach to room-and-pillar mining of
inclined ore deposits.
Figure 2.3. Room-and-pillar mining of an inclined orebody.
Development. Developing an inclined orebody usually requires the development of
several horizontal levels at specified vertical intervals. On each level, a haulage drift is
prepared, following the footwall of the orebody. These haulage drifts serve both for access
to the production areas and for transporting the broken ore to the hoisting shaft.
Production. Mining starts from one of the horizontal levels and proceeds upward, along
the dip, until the next level is encountered. Normally, drilling is performed with air-leg drills,
and the stope is advanced with holes arranged in a slashing pattern. The inclined and
uneven surface precludes the use of mechanized equipment, requiring considerably more
manual labor and physical effort than many other mining methods.
Ore Handling. Because the floor of the stope slopes, slushers are the predominant
system used to bring the broken ore down to a point where it can be transferred into mine
cars for haulage to the shaft.
Comments. The unfavorable dip results in a labor-demanding operation, with little or no
possibility for mechanization. Although various attempts have been made to develop
systems by which the efficiency can be improved, most have been unsuccessful. However,
step mining is one variant that has been used successfully, replacing the conventional
method of inclined room-and-pillar mining.
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Step Mining. Figure 2.4 illustrates the general approach to step mining in an inclined ore
deposit.
Figure 2.4. Room-and-pillar mining, using step mining of an inclined orebody.
Development. Step mining utilizes trackless mobile equipment instead of the rail-bound
haulage vehicles that are used in the inclined mining system. The trackless vehicles are
capable of traveling up and down graded ramps and floors, so they are not restricted to
almost horizontal haulage levels. Therefore, inclined drifts for access to the mining areas
can be oriented to traverse the orebody. if the orebody has a high degree of regularity, the
access drifts can be assigned directions that match the climbing ability of the equipment.
Production. Ore extraction starts with horizontal drifts that branch out from the access
drifts. These stope drifts follow the ore boundary (hanging wall) as closely as possible.
Successive sequences start similar drifts or slashing operations adjacent and parallel to
the preceding drifts, at levels immediately below the last level. This procedure is repeated
regularly, with the extraction of the orebody advancing downward, one step at a time.
During the mining operation, part of the ore must be left as pillars, and this may complicate
the procedure to some extent. However, ore extraction is accomplished in stopes having
reasonably level floors, interconnected by the inclined access drifts. These conditions
allow the use of mobile mechanized equipment and a subsequent reduction in manual
labor requirements. Drilling can be performed conveniently with regular drifting jumbos.
Ore Handling. The ore handling method for step mining is the same as that for horizontal
room-and-pillar mining; the ore is picked up at the face and is carried to the shaft or other
transfer point. The available space and headroom determine the equipment used, and the
equipment may range from trucks to LHD vehicles.
Comments. The concept of step mining is a comparatively recent development in the
industry. Although its applicability to inclined orebodies cannot be generalized completely,
the approach has significant potential as an efficient modem approach to mining orebodies
where the dip normally would be considered an inconvenience.
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Sublevel Stoping (Blasthole Stoping). Figure 2.5 illustrates the general approach to
sublevel stoping, showing ring drilling as the primary means of breaking the ore. Sublevel
stoping is a mining method in which ore is extracted, and the stope is left empty. In this
case, the stope is often very large, with the largest dimension being in the vertical
direction. The sublevel stoping method applies only to vertical or steeply dipping
orebodies.
Figure 2.5. Sublevel stoping with ring drilling as the primary means of breaking ore.
To prevent the stope walls from collapsing, large orebodies normally are divided into two
or more separate stopes. Between the stopes, part of the ore is not mined; instead, it is left
in place to serve as the support for the hanging wall. These pillars can be designed as
both vertical and horizontal separations, sometimes being of substantial thickness.
In some cases, the pillars can be recovered, either partially or fully. Normally, the pillar
recovery takes place during the final stage of the mining operation, when the eventual
collapse of the surrounding rock no longer poses a hazard to mining personnel and
equipment and would no longer affect normal mining activities.
In sublevel stoping, the mining is accomplished from levels at predetermined vertical
intervals. Sublevels may be developed between the main levels, and the ore is attacked
from both the main levels and the sublevels. The ore is drilled and blasted from drifts on
the sublevels. The ore, broken in large vertical slices, falls to the bottom of the empty
stope, where it can be recovered for transport out of the mine.
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Applications. Normally, sublevel stoping is used in orebodies having the following
characteristics:
1. The orebody must have a steep dip; that is, the inclination of the footwall must
exceed the angle of repose.
2. The surrounding rock must provide a strong hanging wall and a strong footwall.
3. The orebody must be competent.
4. The boundaries of the orebody must be regular.
Development. Comprehensive development is needed for sublevel stoping, including the
following:
1. A haulage drift must be developed at the main level below the bottom of the stope.
2. Raises must be developed to provide access to sublevels and subsequent
development of those sublevels.
3. Drilling drifts must be driven through the ore on the sublevels.
4. An undercut must be made at the bottom of the stope.
5. A loading drawpoint system must be developed to allow the ore at the bottom of the
stope to be recovered safely.
6. A slot raise must be developed at the end of the stope; subsequently, the slot raise
is enlarged to a full slot to open up the area for blasting.
Several variations of the undercut drawpoint development are possible. To avoid the
complicated and time-consuming preparation of raises, the procedure can be simplified by
locating a drilling drift on the drawpoint level, as shown in Fig. 2.5.
Production. Today, production drilling in sublevel stoping operations is accomplished
exclusively by means of longhole drilling, using extension drill steels to achieve the
appropriate hole depth.
When ring drilling is used, the entire cross section of the stope is drilled with holes that
radiate from the drilling drifts. The drilling pattern is matched to the shape of the orebody
and the location of the drift. The pattern may consist of holes arranged as a part of a circle
or as a complete circle. In narrow ore-bodies, parallel holes or holes in a fan-shaped
pattern may be feasible and may be preferable to a ring pattern.
Two principal drilling systems are used. The traditional and most common system entails
the use of special long-hole rock drills, equipped with extension drill steels in 1.2 to 1.8 m (
4 to 6 ft) sections. The bit diameter is approximately 51 mm (2 in.). The hole length varies
with the hole pattern, but normally does not exceed 24 m (80 ft). Longer holes are
possible, but they may cause problems as a result of deflection from the intended
direction.
Recently, the application of down-the-hole hammers has become common in sublevel
stoping. These hammers provide a hole size between 102 and 178 mm (4 and 7 in.); the
Iarger holes accommodate a much greater burden with wide spacing between the holes.
Consequently, the drilling requirements are reduced substantially. The Iarger holes are
reported to provide improved-fragmentation of the ore, and, to date, no adverse side
effects have been experienced as a result of using heavy explosive charges in the Iarge
holes. This method offers another advantage in that the down-the-hole technique
minimizes hole deflection, allowing the hole Iength to be increased to 45 to 60 m (150 to
200 ft). The Ionger hole Iengths reduce the development effort by allowing an increase in
the vertical spacing between the sublevels. Figure 2.6 illustrates a generalized sublevel
stoping method that employs the down-the-hole drilling technique.
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Figure 2.6. Sublevel stoping with large-hole drilling and blasting.
In sublevel stoping, the drilling often can be done considerably in advance of the actual
mining operation. Large sections of the ore can be drilled, left in place, and blasted later as
required. Because the drilling can be accomplished as a separate operation, with a large
area drilled from a single drift, this method favors the use of special mechanized
equipment that provides controlled and accurate hole alignment.
Ore Handling. For further handling of the ore, the lower part of the stope is designed and
developed to match one of the following systems:
1. The ore may be loaded through chutes and into mine cars. Boulders can be
frequent in sublevel stoping, and they can make this system inconvenient. Because
blasting (secondary breaking) in the chutes is quite complicated, the presence of
boulders in the ore lowers the overall production capacity.
2. Slushers may be used to draw the ore from the drawpoints and into the mine cars.
This system provides good access to oversize rock and makes handling of boulders
much simpler.
3. Overhead loaders may be used for drawpoint loading, placing the ore into the mine
cars. This approach is practiced in many large mining operations, whereas LHD
loaders are used with a trackless mining layout.
Comments. With the development of extension steels, special long-hole rock drills, and
large-hole blasting techniques, sublevel stoping has grown in popularity throughout the
mining industry. The complicated and comprehensive development is a drawback, but
efficient production operations are the compensation. Drilling, blasting, and loading are
operations that can be performed practically independent of each other, offering the
potential for high utilization of mechanized equipment, high output with few units, and
minimal personnel. High productivity can be obtained from a concentrated area within the
mine.
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Sublevel stoping requires a straightforward layout of the stopes and ore boundaries. Inside
a stope, everything qualifies as ore, and there is no chance of recovering small
mineralizations in the wall rock. Key factors for a successful application of the sublevel
stoping method include a thorough knowledge of the geology, a knowledge of the ore
boundaries, and careful control of the hole alignment in the long-hole pattern
2.3. Methods With Filling
The mining methods that use filling techniques include shrinkage stoping, cut-and-fill
mining, square-set mining, and longwall mining.
Shrinkage Stoping. Figure 2. 7 illustrates the general approach to shrinkage stoping. In
this method, the ore is excavated in horizontal slices, starting at the bottom of the stope
and advancing upward. Part of the broken ore is left in the mined-out stope, serving as a
working platform while mining the ore above and supporting the stope walls.
Figure 2.7. Shrinkage stoping in a large vertical orebody.
The blasted ore increases its occupied volume by about 70%. Therefore, 30 to 40% of the
ore must be drawn off continuously to keep a suitable working distance between the back
and the top of the broken ore. When the stoping has advanced to the upper limit of the
planned stope, drilling and blasting are discontinued, and the remaining 60 to 70% of the
ore can be recovered.
Small orebodies can be mined as a complete shrinkage stope, but Iarge ore-bodies
usually are divided into separate stopes with intermediate pillars that stabilize the hanging
wall. Generally, the pillars are recovered when the reguIar mining operations have been
completed.
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Applications. Shrinkage stoping can be used in orebodies having the following
characteristics:
1. The orebody must have a steep dip that exceeds the angle of repose.
2. The orebody must be firm and relatively competent.
3. The hanging wall and footwall must be stable.
4. The orebody must have regular boundaries.
5. The ore must not be affected by storage in the stope. Certain sulfide ores oxidize
and decompose when exposed to air, making them unsuitable candidates for
shrinkage stoping.
Development. Shrinkage stoping requires the following development effort:
1. A haulage drift must be developed along the bottom of the stope.
2. Crosscuts must be made into the ore under the stope.
3. Finger raises and cones must be developed from the crosscuts to the undercut.
4. The stope must be undercut, taking a complete bottom slice at a distance of 5 to 10
m (15 to 30 ft) above the haulage drift.
5. A raise must be provided, passing from the haulage drift, through the undercut, and
to the main level above. This raise provides both access and ventilation to the stope.
The development of the bottom section can be simplified in the same manner as the
bottom level for sublevel stoping; the finger raises are eliminated, and the crosscut is used
for the drawpoint loading.
Production. Drilling and blasting are performed as overhead operations. The rough
surface of the pile of ore in the stope prevents the use of mechanized equipment, so the
standard practice utilizes air-Ieg rock drills and stopers. Occasionally, some horizontal
long-hole drilling is practiced.
Ore Handling. In shrinkage stoping, the conventional method of handling the ore entails
direct loading into rail cars from chutes at the bottom of the finger raises. Today, Ioaders
are more effective when they are used in a drawpoint Ioading system.
Comments. When labor-saving machines were Iittle used in underground mining,
shrinkage stoping was a common and important mining method. The ore could be dropped
directly into cars from the chutes, eliminating the hand Ioading requirements of other
methods.
Today, this is of little importance, and the shrinkage stoping method has been largely
replaced by other approaches. Its drawbacks include the fact that it is a Iabor-intensive
method, with difficult and comparatively dangerous working conditions and with limited
production capacity. Furthermore, the bulk of the ore remains tied up in the mine for a long
period of time. Under the same conditions, sublevel stoping, sublevel caving, and cut-andfill mining usually can be practiced with a considerable economic advantage over
shrinkage stoping.
However, shrinkage stoping has not been totally eliminated from the mining industry. It is
still practiced where a small-scale operation is undertaken with a minimum investment in
costly machinery.
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Cut-and-Fill Mining. As illustrated in Fig.2.8, cut-and-fill mining excavates the ore in
horizontal slices, starting at the bottom of the stope and advancing upward. The broken
ore is loaded and completely removed from the stope. When a full slice has been
excavated, the vacated volume is filled with waste material that supports the walls and
provides a working platform while the next ore slice is mined.
Figure 2.8. Cut-and-fill mining in a large vertical orebody.
The fill material can consist of waste rock, such as that from development work in the
mine; the waste rock is distributed mechanically over the stoping area. However, In
modem cut-and-fill mining, the common practice is to use hydraulic filling methods. The
filling material consists of fine-grained tailings from the mill, mixed with water and
transported into the mine for distribution through pipelines. When the water is drained off,
a competent fill with a smooth surface is produced. In some cases, the material is mixed
with cement to provide a harder and more durable surface with improved support
characteristics.
Applications. Cut-and-fill mining can be applied in steeply dipping ore-bodies having
reasonably firm ore. Compared with sublevel stoping and shrinkage stoping used with
similar orebodies, the cut-and-fill method offers the advantage of selectivity. The method
can be adapted to irregular and discontinuous orebodies, extracting the high-grade ore
and leaving the low-grade material behind in the fill.
Development. The development required for cut-and-fill mining consists of the following:
1. A haulage drift must be developed along the orebody at the main level.
2. Short raises and manways to an undercut must be provided.
3. The stope area must be undercut, usually 5 to 10 m (15 to 30 ft) above the haulage
drift.
4. Raises for ventilation and fill transport must be provided to the Ievel above.
Production. As shown in Fig.2.9, two different systems may be used for drilling and
blasting. The most common method is to drill vertical or inclined holes upward into the
back, and then bring down a slice of ore in a manner that resembles inverted benching.
Large sections of the roof can be drilled without interruption, and large rounds can be
blasted. Simple lightweight wagon drills are available for the roof drilling.
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Figure 2.9. Comparison of cut-and-fill drilling systems.
One drawback to this method is that the headroom in the stope is increased to about 7 m
(23 ft) when the ore is removed. Blasting creates a ragged back that is difficult to control
and is potentially dangerous to the miners, unless the surface is trimmed by smoothblasting prior to mucking.
As an alternative to overhead drilling, horizontal holes may be drilled, and the ore may be
blasted with a breasting technique. The stope is filled as completely as possible with
hydraulic fill, and only a narrow slot remains between the previous back and the surface of
the fill. The drilling can be accomplished with regular drifting jumbos. The size of the round
is limited to what can be drilled from the face, and it is much smaller than the round
resulting from vertical holes. With modern mobile drilling and loading equipment, the round
has little influence on the stoping efficiency.
Face drilling offers several advantages over vertical drilling. First, the holes are horizontal,
and the back can be blasted to leave a more even surface. After the ore has been
removed, the headroom is only on the order of 4 m (14 ft) making the back easy to control.
Face drilling also provides selectivity, because the low-grade material can be left in place
to an extent. It also is possible to adjust the outline of the stope to extract mineralizations
found in the walls.
Ore Handling. Some version of the LHD technique commonly is applied to the handling
and transport of the ore. The surface of the hydraulic fill is smooth and suitable for rubbertired equipment. The travel distance is limited because transportation usually consists of
bringing the ore to an orepass within the stope.
Comments. Cut-and-fill mining has a broad range of applications, as a direct result of
selective mining, good recovery, and practicability under rather weak and unpredictable
rock conditions. Hydraulic fill has improved the economical and technical aspects of this
method, and cut-and-fill mining often has replaced other methods.
With modern mobile equipment, complete and efficient mechanization can be achieved.
Where mobile equipment is used, inclines or ramps commonly are provided for access to
the stopes. The equipment can be moved to other stopes during interruptions for filling,
and good equipment utilization can be achieved.
It is characteristic of cut-and-fill mining that the ore production from the, stopes is
discontinuous. This is attributed to the interruptions in the production cycles as the fill is
distributed. However, hydraulic filling reduces the interruptions to comparatively short
periods.
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Square-set Mining. The traditional method of square-set mining is of limited importance
today. Its main advantage is that it may be applied to almost any orebody shapes and
ground conditions. However, it is extremely labor intensive, and it requires a steady supply
of timber.
The method is based on a timber-support system, where precut modular pieces are
assembled in a regular square pattern throughout the excavated area. The space between
the timbers normally is filled with waste material, leaving only certain sections that serve
as drifts, manways, and orepasses. Extraction is an entirely manual operation, and
handling the timbers and erecting the supports are laborious operations.
Square-set mining is suitable for small orebodies of high-grade material, where good
recovery is important and where the grade justifies the high operating costs.
Longwall Mining. Figure 2.10 illustrates the general approach to longwall mining in hard
rock, and Fig. 2.11 illustrates longwall mining in coal. The ore is excavated in slices,
usually along a straight working face. The excavated area close to the face is supported to
provide space for drilling and ore removal. At some distance from the face, the roof may
be allowed to cave.
Figure 2.10. Longwall mining in a hard-rock environment.
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Figure 2.11. Longwall mining of a coal deposit.
Applications. Longwall mining is applicable only to thin-bedded deposits of uniform
thickness, normally occurring as a flat seam of large horizontal extent. The method can be
used for hard or soft rock conditions, because the working area can be supported well.
The South African gold mines are examples of longwall mining in hard rock, whereas softrock longwall mining occurs in many coal mines.
Development. Longwall mining development mainly consists of providing a system of
haulage drifts for handling the extracted material and for access to the production areas.
The mining is performed along a face or front that connects two haulage drifts.
With a regular seam or with mineralization continuing over a large area, haulage drifts can
be arranged in a pattern that provides longwall faces or panels of the same length.
Production. In hard rock, the ore is drilled and blasted with side-slashing holes. Because
the headroom is limited, it usually is necessary to use machines that are operated
manually.
In contrast, longwall mining of coal and other soft materials is a process that can be
mechanized almost completely. The soft material does not require drilling and blasting, but
can be cut or broken mechanically with plows, drum shearers, and other machines that are
specially designed for this purpose.
Ore Handling. Under hard-rock conditions, scraping is the normal means of bringing the
ore from the face to the haulage drift for further transport. Low headroom and inclined
stopes preclude the use of other machinery. The handling of soft material is more
convenient and efficient; the special production machines usually are integrated with
material handling systems that bring the broken material out from the longwall face. Chain
conveyors are quite common for material handling in soft-rock mining.
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2.4. Caving Methods
All caving methods are based on fracturing the mineralized rock and the surrounding
waste rock under more or less controlled conditions. The fractured material fills the voids
left by ore extraction, creating a caved area on the surface over the orebody. Complete
and continuous caving is essential, because large empty spaces underground can
collapse suddenly with severe after-effects on the mining operation.
Sublevel Caving. Figure 2.12 illustrates the general approach to sublevel caving. The
orebody is divided into sublevels with fairly close vertical spacing, normally 8 to 15 m (25
to 50 ft) apart. Each sublevel is developed with a regular network of drifts that penetrate
the complete ore section. In wide ore-bodies, the drifts are laid out as crosscuts from a
footwall drift; in narrow deposits, the drifts are parallel along the strike.
From the sublevel drifts, the ore section immediately above is drilled with long holes in a
fan-shaped pattern. The drilling is done ahead of the blasting, and several sublevels can
be drilled completely before blasting and production are started.
Blasting on one level starts at the hanging wall or the far end of the orebody, retreating
along the footwall. Ore extraction follows along an approximately even front, allowing
several drifts to be operated on the same sublevel. The sublevels below and above also
are included in the production sequence.
Figure 2.12. Sublevel caving in a large and steeply dipping orebody.
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The blasting of one fan breaks the ore, causing it to cave into the drift. From the drift, the
ore is loaded and transported to orepasses. The waste rock in the hanging wall
continuously breaks and fills the void as the ore is removed. In the drift, this is realized as
a gradually increasing waste dilution of the loaded material. At some point, the dilution
reaches such a proportion that the loading is stopped and the next fan is blasted. Some of
the ore remains in the caved area and cannot be recovered. The dilution of the ore with
waste can vary between 10% and 35%, with the ore losses varying between 10% and
20%.
Applications. Sublevel caving is used in steeply dipping orebodies and other deposits
having large vertical dimensions. A minimum requirement for the stability of the ore is that
the sublevel drifts be largely self-supporting, requiring only occasional reinforcement. The
hanging wall must follow the ore extraction in a continuous cave, and surface conditions
must allow subsidence.
Dilution with waste material and ore losses are factors that influence the practical
application of sublevel caving techniques. The method is preferred for ores from which the
waste rock and the paying mineral are separated easily (e.g., by a simple magnetic
process). However, this situation is different where the ore boundaries are determined by a
vague cutoff grade, and where the surrounding rock is mineralized.
Development. The major part of the development effort consists of drifting to prepare the
sublevels. This is a comprehensive operation, and as much as 20% of the ore can be
extracted during the development. In addition to the sublevel drifts, orepasses and inclined
drifts are required to connect the sublevels with the main levels.
The amount of development work in sublevel caving is substantial. However, the drifting is
a repetitive and regular operation that can be organized and performed efficiently with
modem equipment.
Production. Drilling is performed in a fan-shaped pattern that radiates upward from the
sublevel drift. Specially designed fan-drilling rigs are used, making longhole drilling a
highly efficient mechanized operation.
Ore Handling. In the production area, ore handling consists of loading the ore in the
sublevel drift, transporting it, and dumping it into an orepass. These conditions favor the
use of LHD equipment. Often, the sublevels are designed for transportation distances
matched to a particular size of loader. High efficiency is achieved in the loading and
transport phases, because a loader can be kept in continuous operation by moving from
one sublevel drift to another.
Comments. Sublevel caving is systematic and utilizes regular layout and working
procedures. Development, production drilling, and loading are carried out on separate
levels, and each is a continuous operation independent of the others. A number of working
faces always are available for the operations. These factors make sublevel caving a
method in which mechanization can be introduced throughout, thus maintaining an
efficient mining process. The mining operation can be compared to an industrial process.
Waste dilution and ore losses are drawbacks that cannot always be accepted. To minimize
these factors, an extensive research program has been undertaken, involving both model
and full-scale tests. The experience gained from these investigations, combined with a
follow-up of practical results, now has developed sublevel caving into a method with high
technical standards.
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Block Caving. Figure 2.13 illustrates the general approach to block caving. The ore
fractures and breaks by itself as a result of internal stresses and forces. A minimum of
drilling and blasting is needed in the production operation. The mining plan is divided into
large sections or blocks, usually with a square horizontal cross section and an area of
more than 1000 m2 (11,000 sq ft). Each block is undercut completely by a horizontal slot
that is excavated in the Iower part of the block. Gravity forces on the order of millions of
tons act on the rock masses, and successive fracturing occurs, affecting the entire block.
As the rock pressure increases at the bottom of the block, the ore is crushed to a
fragmentation that allows removal at drawpoints or through other arrangements.
Figure 2.13. Block caving in a massive orebody.
Applications Block caving is used for large and massive orebodies having the following
characteristics:
1. The orebody must exhibit a steep dip. In a massive orebody, there must be a large
vertical extension.
2. After undercutting, the rock must be capable of caving and breaking into suitable
fragments.
3. The surface conditions must allow subsidence of the caved area.
These rather specific conditions limit the application of the block-caving method. Common
applications are found in the mining of iron ore and the mining of low-grade disseminated
ores (e.g., copper or molybdenum).
Development The development efforts for block caving normally include the following:
1. A pattern of loading and haulage drifts are developed underneath each block.
2. Orepasses or finger raises are provided up to a grizzly level.
3. A grizzly level is developed where fragmentation is controlled and secondary
blasting is performed.
4. Another set of finger raises are widened into cones at the undercut level.
5. An undercut is developed underneath the block.
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By arranging the two sets of finger raises similar to the branches of a tree, the ore from a
larger area on the undercut level is funneled together to one point on the haulage level.
The layout described is for a conventional mine plan, where the ore is loaded into mine
cars through chutes. In a mine using a trackless layout, drawpoint loading is common, and
development is simplified by omitting the grizzly level and the second set of finger raises.
All excavations underneath the block are subject to high rock pressure. Therefore, the
cross sections of the drifts and other excavations are kept as small as possible. Heavy
reinforcement with concrete often is necessary. Overall, the development stage is a timeconsuming and complicated process, usually requiring several years to bring a block to
production.
Production. Once the undercut is completed, the ore should start falling down the finger
raises, continuing to do so as the material is removed on the haulage level. Theoretically,
no production drilling is required. In actual practice, it sometimes is necessary to start or
assist the fracturing by drilling and blasting longholes in a widely spaced pattern.
Secondary drilling of oversize rock is a frequent operation in block caving.
Ore Handling. The conventional block-caving system, with a grizzly level and finger
raises, utilizes gravity forces to deliver the material into rail cars. However, the close
control of fragmentation that is required for loading through chutes can make the grizzly
treatment a labor-intensive operation, as well as a bottleneck in the production cycle. With
the layout shown in Fig. 2. 14 for trackless mining and drawpoint loading, the handling of
oversize material is simplified greatly, and development work is reduced substantially.
Figure 2.14. Block caving in a massive orebody, showing the layout of trackless mining.
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To obtain the best ore recovery from a block, it is important that the upper caved surface
be kept even. Therefore, the volume of ore drawn from different positions within the block
must be controlled and must follow a planned schedule.
Comments. Under favorable conditions, block caving is an economical mining method. Its
drawbacks are the comprehensive development required and the time lag before
production reaches full capacity. Also, there are certain hazards, because caving and
fragmentation are events that cannot be predicted or controlled fully. Unexpected hangups
and large boulders can cause serious problems that are difficult to tackle in material that is
already fractured.