UNIVERSITY OF IBADAN, NIGERIA
DEPARTMENT OF GEOLOGY
A TERM PAPER – GEY 719 (MINING GEOLOGY)
ON
THE TOPIC
“UNDERGROUND MINING METHODS”
BY
OLANIPEKUN TIMILEYIN AYOMIDE
MATRIC NO: 157828
MINERAL EXPLORATION (GEOCHEMISTRY OPTION)
LECTURER-IN-CHARGE: BOLARINWA, A.T (Ph.D)
MARCH, 2018
1.0 INTRODUCTION
Underground mines are used for accessing and exploiting ore bodies that are generally not exposed
on surface and due to technical or economic reasons cannot be mined by open pit methods. The
infrastructure of underground mines is more complex than that of the open pits. A typical layout
of an underground mine is shown on the Fig. 1. A main feature of an underground mine is a shaft,
which is a vertical or sub-vertical access to the underground workings or an inclined tunnel, called
a decline. Both types of developments, shaft and decline, can be present in the same mine (Fig. 1).
In the rugged terranes the concealed ore bodies can be accessed from the slope of the hills using
horizontal or gently declining tunnels, called an adit. The surface entrance to decline or adit is
called a portal.
After accessing the ore body, underground workings are continued on successive horizontal
planes, referred to as levels. Each level being a system of related underground workings located
on the same horizontal plane. The underground workings include production stopes and mine
development infrastructure for transportation of the broken ore to the surface for processing.
Between the main development levels, a mine can have sublevels, which are usually needed for
more effective drill and blast control. Levels and sublevels are connected by inclined underground
openings called ramps and also by the vertical opening, including raises and winzes. The main
elements of the underground infrastructure are shown on the Fig. 1 and explained below.
• Drive or drift is a horizontal or nearly horizontal underground opening developed on the
underground levels along the strike of ore body. The drives are subdivided into hanging wall
(located at the upper ore-waste contact, ‘hanging’ above the ore body) and footwall
(located at the lower ore-waste contact, at the ‘foot’ of the ore body) drives. A footwall drive is
also commonly called an ore drive.
• A crosscut is a horizontal underground tunnel intersecting the ore body across its strike. The
crosscuts are usually developed to connect the drives with the area in which stoping occurs.
• A raise is underground opening driven upward.
• A stope is an underground excavation made by removing ore from the host rocks. Development
of stopes often starts from blasting a slot, which is a steeply dipping to vertical excavation at one
boundary of the ore body. Mining then continues by blasting rings or slides of the orebody into the
slot.
• A pillar is block of ore or barren rock left intact in the mined-out stope or between two
stopes to act as a mean of support. It required to provide structural integrity to the stoping
process and prevent the stope walls from collapsing. Pillars may be removed after stopes are mined
out, but some pillars may be left in place permanently.
• A draw point is a place from which the ore is extracted from the stope and loaded onto trucks or
conveyors for further transportation.
• An ore pass is a steeply dipping underground opening for passing ore from one level to another
under gravity. The ore is loaded through the chutes, which are the loading arrangements that utilise
gravity. Important element of the ore loading and transportation system
is a coarse steel grating, called a grizzly, for screening out oversize rock fragments.
• A winze is a small vertical excavation which can be developed in underground mines by
driving it downward from one level to another, or it can be driven from a surface to a level.
Fig. 1: General layout of an underground mine, generalised after Hamrin (1982, 2001)
The specific nature of an underground mining environment has led to the development of
specialized machinery designed for effective operation in the tightly constrained spaces of the
underground mines. This includes specialised machinery used for drilling of the blast holes in the
underground development or production workings.
A common variety are Jumbo drilling rigs, which may have one or two beams (Fig. 2a). For very
selective mining, miners use hand held drilling equipment called air-legs (Fig. 2b).
Another standard underground machine is a load-haul-dump machine or simply LHD (Fig. 2c)
used for loading the broken ore from the draw points of the stopes and tramming (hauling) it to the
nearest ore pass or loading a truck. The crushed ore is transported to the
surface (hoisted) either by the underground dump trucks through the decline or by the shaft’s skips
(Fig. 1).
One of the key responsibilities of the mine geologists is accurate mapping of all underground
openings. This is mainly done by mapping the overhead surface of the underground excavation,
called back of the drives and crosscuts. This is coupled with a systematic mapping of the drives
faces.
The natural strength of the rocks is not always sufficient for safe excavation of the rock mass
in the underground mines. In order to prevent a collapse of the rocks in the underground workings
they must be strengthened and reinforced using the common roof support techniques of rock
bolting and wire meshing (Fig. 2d).
In the case of extremely loose rocks the overhead surface and the side walls of the drives can
be reinforced by the spraying a fluid cement or cement fibre mixing onto the rock surface. A
method commonly referred to as shotcreting. A newly developed technique is to use thin sprayon
liners which prevent the small fragments from dislodging and thereby holding the larger fragments
in place.
Mining infrastructure significantly changes depending on the mining method. Mining techniques
are commonly subdivided into selective and bulk mining methods. Selective methods
generally, extract lower tonnages but allow greater selectivity between ore and waste, which
minimises dilution and mining loses and ensures the maximum ore recovery. Typically, selective
mining used for extracting the narrow vein-type reefs. Bulk mining methods are fully mechanized
allowing the excavation of large volumes of ore.
These techniques enable high production rates in underground mines, but are less effective in
separating ore and waste so are preferentially used in extracting massive ore bodies.
Fig. 2: Examples of the underground machineries and equipment: (a) two beams Jumbo; (b) air
leg; (c) LHD truck; (d) underground working supported by meshing and rock bolting (RB)
2.0 UNDERGROUND MINING METHODS
2.1 Selective mining methods
Narrow steeply dipping veins are usually mined using cut-and-fill and shrinkage stoping methods,
which allow highly selectively excavation of the ore while minimising dilution by the waste
material.
2.1.1 Cut-and-Fill Method
The principles of cut-and-fill mining are shown in Fig. 3. The method is designed for selective
mining of the steeply dipping veins, in particular for the narrow high-grade ore bodies. The method
removes the ore in horizontal slices, starting from the bottom undercut an advancing upward on a
backfilled base (Fig. 3). Every slice is mined by drilling and blasting
the face after which the broken ore (muck) is removed from the stope. The process is repeated
until the entire slice (lift) is excavated along the strike of ore body. When the lift is mined out
the resultant void is backfilled by waste rocks or, most commonly, by sand tailings or by sandcement mixture. The backfill acts to support the stope walls and it also used as a working platform
for equipment to mine the next slice (Fig. 3).
Drilling of the stope face can be done by either a Jumbo or an air-leg, depending on the
thickness of the ore-body and the permissible external dilution.
Fig. 3: Cut-and-fill mining procedure
2.1.2 Shrinkage Stoping
Shrinkage stoping is another highly selective mining method designed for mining narrow
steeply dipping veins. Similarly, to cut-and-fill method, shrinkage stoping starts from the bottom
of the ore body and advances upward excavating the ore in horizontal slices (Fig. 4). However,
the key difference is that the broken ore is not removed completely from the shrinkage stope.
Approximately 60% of the broken ore is left in the stope where it is used as a working platform
for mining the next slice of the ore. The broken ore remaining in the stope, also serves as a
support for the stope walls.
Shrinkage stoping is highly selective and cost-effective mining method. However, its application
is limited to the steeply dipping regular ore bodies in the very stable host rocks. Regular shape of
the ore body is also important for effective use of this technique.
Fig. 4: Shrinkage stoping
2.2 Bulk Mining Methods
Where the ore body is large, massive and has regular shape, selective mining is not necessary
nor desirable from a cost perspective. The ore body can be efficiently mined in large volumes
and with higher productivity using larger underground equipment and the bulk mining methods.
There are several methods for bulk underground mining, the most common being block caving,
long hole open stoping, sublevel open stoping, sublevel caving and vertical-crater-retreat.
2.2.1 Block Caving
Block caving is a large-scale production technique applicable to homogeneous ore bodies of
large dimensions and most commonly steep to vertical ore bodies. Block caving is regarded
as one of the most productive and lowest cost underground mining methods and as such is often
applied to low grade ore bodies.
The method uses gravity force in conjunction with internal rock stress which leads to fracturing
of the rocks and eventually breaks them into small enough pieces which can be extracted from
draw points by LHD’s (Fig. 5). The term ‘block’ refers to the entire volume of the ore
which is prepared for excavation as a single block (Fig. 5).
Caving is induced by undercutting of a block. This is made by blasting the narrow ore slice
located underneath of a block and removing of the blasted ore (Fig. 5). This creates a
void beneath the block (undercut) and leaves the overlaying rock mass unsupported. As a
consequence of removal this support the rocks fracturing, which was induced by the undercut
blasting, starts spreading through the entire block due to the gravity force. The rock mass in a block
breaks into small pieces which falls to the bottom of a stope (cave) where they are extracted
through draw points (Fig. 5).
Application of the block caving technique is limited to the large ore bodies preferably of a shape
of a vertical cylinder and characterised by a favourable hydrolic radius (Brady and Brown
2004). The success of the method hinges on the understanding of the fracturing process and
capability of the broken ore to flow to the extraction
points. In particular, it is essential that hanging wall of a stope is allowed to subside. Another
condition for the ore body to be amenable for block cave mining is the capability of the rock
mass for breaking into small pieces by the rock stress and gravity. All three conditions are rarely
met therefore the method is used only for certain type of deposits, most commonly these are
copper and molybdenum porphyry deposits and diamond-bearing kimberlite pipes (Table 1).
The block caving is also used for mining some large iron-ore deposits.
Fig. 5: Block caving, generalised after Hamrin (1982, 2001)
Table 1: Block cave mines
2.2.2 Sublevel Open Stoping
When block caving is not suitable for mining the ore body but its large size is still favourable for
large scale production the methods like bighole open stoping and sublevel open stoping (SLOS)
can be considered. Both methods subdivide the ore body into several large stopes. Each of the
stopes are mined either using sublevels (sublevel open stoping) or as a single large void (bighole
open stoping). The latter is merely a scaled-up variant of the sublevel open stoping and used when
geometry of the ore body and the rock conditions allows to use the larger underground
openings.
The basic principles of this group of the methods is explained using the sublevel open stoping
(SLOS) as an example (Fig. 6). The SLOS stopes are distributed along the strike of ore body
with the pillars left between the stopes to support the hanging wall. Pillars are normally shaped as
vertical beams and distributed across the entire ore body. Horizontal sections of the ore, known as
crown pillars, are also left to support the mine workings above the producing stope.
The stopes are developed using the sublevel drives which are prepared inside the ore body between
the main levels (Fig. 6). The blast holes are drilled from the sublevel drives distributed
as a tight fan pattern covering the whole stope. The ore is broken by firing the blast holes on the
same cross-sections, usually referred to a firing ring. The SLOS mining advances in a horizontal
direction, usually along the strike of the ore body, by drilling of the next firing ring, charging the
blast holes and blasting them (Fig. 6).
The broken ore is removed by LHD from the draw points distributed along the stope bottom
(Fig. 6). The recovery of the ore from the stopes is facilitated by trough shaped bottom
of the stope (Fig. 6). This technique creates large open voids, in particular in a vertical direction.
In order to prevent their collapsing after the ore was recovered the stopes are normally
backfilled.
The method is used for steep dipping ore bodies and require stable rocks in the hanging wall
and footwall of the stope and also competent ore. Irregular shape of the ore body and uneven
contacts are undesirable because this can lead to excessive dilution of the ore. The same comments
are related to internal waste distributed inside of the ore body. The SLOS method doesn’t allow to
separate internal waste from the valuable material therefore everything inside the drilled pattern is
recovered as an ore. Therefore, geologists reporting the ore reserves must make a correction for
the internal and external dilution.
Fig. 6: Sublevel open stoping, generalised after Hamrin (1982, 2001)
2.2.3 Sublevel Caving
Sublevel caving is another method available for mining of the large steeply dipping ore bodies
(Fig. 7). This technique uses sublevels developed through the entire ore body at regular intervals.
On each sublevel a system of the drives and crosscuts is developed following a geometrically
systematic layout. The drives are developed along the footwall and hanging wall of the ore body
and joined by series of the parallel and regularly distributed crosscuts (Fig. 7).
Production of the sublevel caves is made by drilling the long blast holes into the hanging wall from
the sublevel crosscuts. The long blast holes are charged and blasted to generate a controlled
caving when hanging wall fractures and collapses following the cave. Ground on the top of the ore
body must be permitted to subside.
The ore body can be mined by retreating from the hanging wall to footwall, which is referred
to as transverse sublevel caving (Fig. 7), or, conversely, retreating the stopes along the strike of
ore body.
The latter approach is used if the thickness of the ore body is not suitable for transverse sublevel
caving technique. The rock mass must be stable enough to allow the sublevel drives to remain
open with a limited support, usually a local rock bolting and meshing of the most unstable areas.
It general, the method can be used instead of the SLOS technique if the rock mass competence is
insufficient for open stoping.
Fig. 7: Sublevel caving, generalised after Hamrin (1982, 2001)
2.2.4 Vertical Crater Retreat
Vertical crater retreat method (VCR) is used at the deposits that have competent steeply dipping
ore hosted by the competent wall rocks (Fig. 8). The ground conditions in general is similar to
that of the SLOS method, however the VCR is technically simpler and allows to achieve a large
production rate commonly at lower production costs.
The method is based on the crater blasting which is made by firing the large diameter blast
holes drilled downward from the overcut developed on the top of a stope (Fig. 8). The
blast holes are charged by explosives which is placed into the short sections of the holes which
is referred to as crater charge. The explosive is positioned in each hole at the same distance above
the open surface (Fig. 8). The blasting loosens the ore slice of ore creating a crater
which is vertically retreated. Ore is continuously extracted from the draw points together with
rigorous recording of the blasting progress in each hole.
Fig. 8: Vertical crater retreat mining, generalized after Hamrin (1982, 2001)
2.3 Mining of the Gently Dipping Ore Bodies
The methods which were described above are designed for steeply dipping ore bodies. Mining
of the horizontal and gently dipping ore bodies require different approach, which includes
continues support for the large overhead surfaces exposed during mining of the flat stopes. The
different methods are available for mining of the flat-bedded deposits. Two of them, room-andpillar and longwall mining, are described in the section below.
2.3.1 Room-and-pillar Method
The room-and-pillar method is used for mining flat bedded deposits using the flat open stopes.
Hanging wall of such stopes extends for large areas and therefore have to be prevented from
collapsing by leaving pillars, which support the hanging wall of the stopes (Fig. 9). In order to
minimise the mining losses the pillars are usually left where low-grade material or internal waste
present in the ore. In case if the ore grade material has been left in the pillars it is unrecoverable
and therefore mine geologists have to exclude these volumes from the ore reserves. The flat ore
body and large open areas allows to establish several production areas with an easy communication
between different sites. These factors make the room-and-pillar method a highly efficient system
for recovery ore from the flat beds.
Fig. 9: Room-and-pillar methods, generalised after Hamrin (1982, 2001)
2.3.2 Longwall Mining
Narrow gently dipping ore bodies of a uniform thickness can also be mined using longwall mining
technique. The ore is extracted from a long straight front using system of the long flat stopes (Fig.
10a). The method was found in particular efficient for mining of the coal seams hosted in the soft
sedimentary rocks. Excavation of such deposits does not require blasting and can be fully
mechanised. The large length of the working faces allows to use conveyor belts for haulage of the
ore. The method also allows the hanging wall to collapse at some distance behind the working face
which decreases the mining costs In South Africa the technique was adapted
for mining reef-type gold deposits, where gold is distributed in the thin beds of the quartz
conglomerates, usually less than 1 m thick (Fig. 10b). Some of the mines are deep and therefore
roof is supported by the pillars made of concrete or timber.
Fig. 10: Longwall mining, generalised after Hamrin (1982, 2001): (a) plan showing general layout
of the longwall
stope; (b) gold reef mined by longwall method in South Africa
2.4 Unconventional Mining
Extraction of minerals from the host rocks not always made by digging of the open pits or sinking
shafts. Some minerals are extracted by dissolving them directly in their host rocks (in situ)
therefore the exploitation technique is referred to as in situ leach (ISL). This unconventional
mining approach is broadly used for extraction of the dissolvable minerals hosted in the water
permeable rocks. In particular, this is the main production method for the sandstone hosted
uranium deposits (Abzalov, 2012).
Another technique, included into the group of unconventional mining method is dredging,
which is also not a mining technology senso stricto. In the mining industry, this method is used
for exploitation of the mineral sand deposits. A brief description of these methods is given below.
2.4.1 In situ Leach (ISL) Technique
In situ leach (ISL) technology is used for exploitation different types of the deposits, however, the
most sophisticated version of the ISL technology was developed for mining uranium
from the weakly lithified sandstones. The method is based on dissolving the uranium minerals
directly in their host rocks by reactive solutions injected through the drill holes distributed along
the regular pattern (Abzalov, 2012). Solutions dissolve uranium minerals directly in the host
rocks and are then pumped to the surface through the extraction drill holes (Fig. 11a). The pregnant
solutions are collected on the surface and supplied to the processing plant where uranium is
extracted.
A significant advantage of the ISL operations over conventional mining is their low capital
and production costs. The favourable economics of the ISL technology coupled with a specific
technical characteristics of the method allow to use it for mining the low-grade deposits hosted
in the unconsolidated sands at the depth up to 600 m below surface, which cannot be mined by
conventional methods (Abzalov, 2012).
The method is based in its entirety on the drilling and therefore the surface disturbance at the ISL
operations is minimal (Fig. 11b). This is another advantage of the ISL technologies in comparison
with the conventional mining methods.
All these features have made the ISL technology a favourable option for exploitation of
the sandstone hosted uranium deposits (Abzalov, 2012) and for mining some industrial minerals
(e.g. salt, potash).
Fig. 11: In situ leach uranium mining: (a) principles of the technique; (b) production field at the
Budenovskoe mine,
Kazakhstan
2.4.2 Dredging of the Mineral Sands
Dredging is a process of excavation of the bottom sediments from the water basins. The prime
objective of the method was not mining as, primarily, the technique was designed for maintaining
the waterways navigable. Later, the technique found its application in mining, where it has become
used for excavation diamond from the off-shore marine placer deposits, gold placers at the river
beds and for mining the mineral sands deposits. The latter requires the mineralized strata, to be, at
least partially, below the water table (Fig. 12). In that case the mineral sand dunes can be mined
by dredging using the system of the artificial basins (dredge ponds). The excavated sands are
pumped in slurry to concentrator floating at the same pond where the dredge is. The heavy minerals
are separated from the barren sand and clay which are back filled to the pond behind the dredge.
Fig. 12: Dredging at the Richards Bay titanium sands deposit, South Africa
3.0
CONCLUSION
In the preceding sections, different underground mining methods has been described. From
observations, the variations in ore deposits are great and the state of mining technology so
dynamic. Every ore body is unique. The successful application of a mining method requires the
practical reasoning of the mining geologist and engineers to choose suitable method for different
mineral deposits (since every deposit is unique) putting into considerations, among others, the
following: the peculiarities of the ore body in terms of geometry, the safety of the operation,
economic considerations in terms of cost and benefits and the technological feasibility.
REFERENCES
Abzalov, M.Z., 2012, Sandstone hosted uranium deposits amenable for exploitation by in-situ
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Brady, B.H.G., and Brown, E.T., 2004, Rock mechanics for underground mining, Kluwer
Academic Publishing, New York, p. 628
Hamrin, H., 1982, Choosing an underground mining method, In: Hustrulid, W.A. (ed),
Underground mining methods handbook. AIME, New York, pp. 88–112
Hamrin, H., 2001, Underground mining methods and applications. In: Hustrulid, W.A., Bullock,
R.L (eds), Underground mining methods: engineering fundamentals and international case studies,
Society for Mining Metallurgy and Exploration, Littleton, pp. 3–14