Caving Underground Mining Methods (longwall, Sublevel caving, & Block caving)

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Topic 7: Underground Mining Methods
 Longwall
 Sublevel Caving
 Block Caving
Hassan Z. Harraz
hharraz2006@yahoo.com
2014- 2015

Outline of Topic 7:
 Longwall
 Longwall in coal
 Longwall in Hard Rock
 Sublevel Caving
 Characteristics of the ore body and mining method
 Development
 Production
 Equipments Used
 Block Caving
 Introduction
 Historical evolution of the method
 Condition deposit
 Principles of the method
 Methodology of block caving
 Basic issues of geomechanical to the black caving method:
1) Caveability
2) Mine design
3) Fragmentation and extraction control
4) Subsidence associated
 Advantages and Disadvantages of Block Caving
Prof. Dr. H.Z. Harraz Presentation
Caving methods
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We will explore all of the above in Topic 7.

Longwall (LW)
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Longwall (LW)
 The Longwall is a very old method, originated in
coal mines in Europe in the 7th century.
 The most important application of Longwall
relates to coal mining.
 Much of the production of coal from countries
like USA, Australia and China are obtained by
Longwall.
Conditions of applicability of the method:
 Stratiform tabular bodies, little thick, horizontal (tilt up to 20°);
 Uniform distribution of thicknesses / levels;
 High degree of continuity of the ore body;
 Geological discontinuities (e.g., faults) are highly detrimental to the method;
 Applicable in hard rock (metalliferous mines) and fragile (coal).
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Longwall Stoping
 Longwall stoping is applicable to bedded deposits of uniform shape, limited thickness and large horizontal extension (e.g., a coal seam, a potash
layer or the reef, the bed of quartz pebbles exploited by gold mines in South Africa).
 It is a caving method particularly well adapted to relatively flat-lying, thin, planar deposits or horizontal seams, usually coal, at some depth.
 It is suitable for tabular orebodies, with moderate dip (e.g., coal and stratiform hard-rock ores like diamond deposits).
 It is one of the main methods for mining coal. It recovers the mineral in slices along a straight line that are repeated to recover materials over a
larger area.
 Need to divide orebody to "face" or the "working face“.
 The collection of cuts, cross-cuts, and pillars all together make up a "panel" and all the equipment that goes together to operate in that panel is a
"unit or Longwall units".
 In this method, a face of considerable length (a long face or wall) is maintained, and as the mining progresses, the overlying strata are caved, thus
promoting the breakage of the coal itself.
 Applied to longer (~100 m) and longer diameter blastholes (i.e., thus requiring less drilling than sublevel stoping).
 Greater drilling accuracy is required.
 Need to a longwall machine (It's designed to let the roof fall behind it, and mines out big rooms in which the roof almost immediately collapses,
leaving only a small entryway and the metal barrier that protects the longwall unit).
 The space closest to the face in kept open while the hanging wall is allowed to collapse at a safe distance behind the miners and their equipment.
 Preparation for longwall mining involves the network of drifts required for access to the mining area and transport of the mined product to the
shaft. Since the mineralization is in the form of a sheet that extends over a wide area, the drifts can usually be arranged in a schematic network
pattern. The haulage drifts are prepared in the seam itself. The distance between two adjacent haulage drifts determines the length of the
longwall face.
 Continuous miner operations, and longwall units.
 Traditionally high production rates.
 Large openings with long open times.
 High ground support cost .
 Bottom up mining method.
 Non-selective mining.
 Not stress friendly.
 Many equipment types.
Top Gate
Bottom Gate
Face
Longwall mining method
includes drivage of two
long roadways in coal
and joining them at the
end by a perpendicular
drivage forming a face.
Longwall General Layout

Example: Longwall Mining of Coal
http://en.wikipedia.org/wiki/File:SL500_01.jpg
 Longwall mining is a highly mechanized underground mining system for mining coal.
 It set of longwall mining equipment consists of a coal shearer mounted on conveyor operating
underneath a series of self-advancing hydraulic roof supports.
 Almost the entire process can be automated.
 Longwall mining machines are typically 150-250 meters in width and 1.5 to 3 meters high.
 Longwall miners extract "panels" - rectangular blocks of coal as wide as the face the
equipment is installed in, and as long as several kilometers.
 A layer of coal is selected and blocked out into an area known as a panel (A typical panel
might be 3000 m long X 250 m wide).
 Passageways would be excavated along the length of the panel to provide access and to place
a conveying system to transport material out of the mine.
 Entry tunnels would be constructed from the passageways along the width of the panel.
 Extraction is an almost continuous operation involving the use of: self-advancing hydraulic
roof supports sometimes called shields, a shearing machine, and a conveyor which runs
parallel to the face being mined.
 Powerful mechanical coal cutters (Shearers) cut coal from the face, which falls onto an
armoured face conveyor for removal.
 The longwall system would mine between entry tunnels.
 Longwalls can advance into an area of coal, or more commonly, retreat back between
development tunnels (called "Gate roads")
 As a longwall miner retreats back along a panel, the roof behind the supports is allowed to
collapse in a planned and controlled manner.

Longwall (LW) in Coal
"As applied to longwall coal mining, is not maintained the
integrity of the immediate roof above the newly mined coal. This
ceiling should desplacar the main ceiling, separating into blocks and
falling into the void left behind the line automarchantes brackets. The
process of peeling is accompanied by swelling (about 50%). Ceiling
and immediately occupies the void left by coal mined, acting as a
natural bed against which converges the main ceiling. The greater role
of the immediate roof is desplacar and blistering, filling the void
mined and retaining the convergence of the main roof, maintaining its
integrity. "
Ref.: Brady & Brown, 1993, Rock mechanics for underground mining,
cap.12.4.6.
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LW in Coal
carvão
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Longwall in Coal
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Longwall (LW) in Coal
Preferred conditions (beyond those already
mentioned):
 immediate roof of coal consists of shales, siltstones or other brittle rocks,
enough to produce peeling fracturing;
 competent main roof, which can deform without breaking on the immediate
roof has collapsed.
 competent flooring to withstand the stress produced by the monkeys;
Situations in which there is an advantage in applying LW in relation to the
R & P:
 bad roof (fragile), preventing bolting ceiling;
 great depths (e.g., beyond 500m), causing much loss of coal pillars;
 reduced thickness of coal seams.
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Anatomy of a Coal Mine
Pennsylvania Department of Environmental Protection
Bureau of Deep Mine Safety

Types of Longwall
 Longwall advancing
 Longwall retreating
Longwall advancing
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Longwall retreating
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Longwall in Coal
Setup room: where the face of
longwall begins operation;
Recovery room: where the
longwall finishes and equipment
are removed from the panel;
Barrier pillar: pillars to protect
main and bleeder galleries axis.
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Advantages of LW in coal
 Greater than the recovery room and pillar panel;
 High rate of production and productivity - over 100 ton / man / shift
"face productivity" - the highest of underground methods;
 The lower production costs in underground mines (next to the block
caving);
 Ease of hand-to-work training.
 Adequate to poor roof;
 Coal generally produces better quality (lower dilution);
 Better able to control venting and elimination of gases and dust;
 Good control of subsidence.
 It is safer - the workers are all the time under the roof bracing.
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Disadvantagesof LW in coal
 Does not work well in layers of irregular thickness;
 Stops result in a large variation in production (high production / low
availability);
 Geological discontinuities (faulting or problems with the ceiling) can
cause long downtime;
 Dust control often difficult;
 Problems of methane under high production;
 Variability and intermittency in production between simultaneous
fronts cause overload in the discharge of mine system;
 Impact on the construction of the surface (subsidence);
 High initial investment in equipment;
 Significant development in the preparation of mining panels;
 Need for immediate ceiling collapse after the withdrawal of support
from apes;
 Long delay to exchange panel;
 Rock bursts: e.g. big problem in depth beyond 750m.
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Basic equipments Longwall (coal)
AFC = Armored Face Conveyor
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Equipment for Longwall method in coal
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Coal Mine
And NowThen
Mechanized cutting machine on a longwall coal-mining face:
Shearer Working at Longwall Face.
http://en.wikipedia.org/wiki/File:SL500_01.jpg
Pennsylvania Department of Environmental Protection
Bureau of Deep Mine Safety

Equipment for longwall method in coal (shearer)
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Longwall in Coal: operation
Longwall Mining Machine
The extraction is done with the
aid of rotary cutters will
fragmenting the carbon layer.
The coal falls on a channel of
transmission and is
transferred to a continuous
transport.

At the coalface....!!!
A virtual reality 3D creation of a working shaver on a longwall.
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• A very attractive
feature of this method
is the protection
system roof that
provides complete
safety to operators.
• The hydraulic
cylinders move as the
carbon layer is drawn,
creating an area
without support on
the back of falling
relieving stress on the
system.
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After completion of
mining in the panel, it
is necessary to change
the equipment.
This change takes 10 to
30 days to be
performed and is
performed, on average,
1 to 3 times a year.
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Equipment for Longwall method in coal (plough)
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Longwall Mining Machine
It's designed to let the roof fall behind it, and mines out
big rooms in which the roof almost immediately
collapses, leaving only a small entryway and the metal
barrier that protects the longwall unit.
http://upload.wikimedia.org/wikipedia/commons/thumb/
5/5d/Schildausbau.jpg/220px-Schildausbau.jpg
Figure shows Hydraulic chocks
http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Longwall_wit
h_hydraulic_chocks%2C_conveyor_and_shearer.jpg/220px-
Longwall_with_hydraulic_chocks%2C_conveyor_and_shearer.jpg
Figure shows Hydraulic chocks, conveyor and shearer

Figure shows another continuous miner in an underground coal mine.
Fig 12a: Passageway
Figure 12b: A typical panel is 3000 m long by
250 m wide
Mechanized cutting machineon a
longwall coal-mining face.
Figure 12c: Longwall system in place.
http://wikimedia.org/wikipedia/common
s/thumb/1/19/SL500_01.jpg/

Typical dimensions of a panel longwall :
Extension panel: 900 - 5300m;
Width. the gal. Face: 2.4 - 3.6m;
Length of face: 200 - 360m;
Height: 0.9 - 4.5mm;
Cutting Thickness: 80 - 800mm;
Depth: 60 - 800m.
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Capital price/longwall:
 30 million $ for a face equipment;
 Need for large reserves … minimum of 50
million tons;
 Producing a front … 2-6 million tons / year;
 Employment of a shearer …. 200-500
minutes / day.
Compared with a front operating with
continuous miner ...
 Capital of 3-5 million $;
 Production 0300000-0800000 t/year;
 3 Continuous miners are needed in developing a
front LW;
 Continuous miner is flexible and can be easily
availed in other reserves.Prof. Dr. H.Z. Harraz Presentation
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The mine Kuhn-
town (Pensilvânia)
achieves a
production of up
46.000 t/d of iron
ore by plowing a
layer of 900mm
coal cutting at a
rate of 2.700 t/h.
Longwall: examples
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Mine in Colorado
operates production
until 4.500t / h,
reaching 22.700t /
day in a coal seam
thickness of 1.07m.
The power cutter has
cutting 1.100kW,
moving from 8 to 12
m / min along the
face.
Longwall: examples
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 Experimental longwall at Mine Leão I - Rio
Grande do Sul, 80s.
 Extension panel = 800m
 Face-width = 70m
 Height layer = 2m
 1 double drum cutter (300 hp), diameter 1,09m;
 Hydraulic cylinders 54 automarchantes type "chock" (6 legs capacity of
240t)
 1 panzer front 65hp, with 64m long and capac. 600t / h;
 Side galleries of the panel developed by Roadheader;
 Daily production of around 800t.
Longwall: exemplos
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Longwall:
Leão I
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Longwall in Hard Rock
 In this case, the method must maintain the integrity of the
floor and ceiling; cover and lapa must be composed of hard,
competent rock.
 Temporary support (near side) and permanent (eg uprights of
wood and / or concrete columns) are used to prevent
discontinuities in the stope.
 Used in metalliferous deposits; differs greatly for the Longwall
coal.
 During the work of the scraper, the roof is anchored with
temporary supports that are later replaced by permanent
concrete supports.
 Additional information about the LW method for coal on the Internet ...
Wollongong University-Austrália
www.ouw.edu.au/eng/current/longwall
------------------
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Protective Screen
In hard-rock minerals mining a
“scraper” is pulled down the
length of the stope face after
drilling and blasting, to collect
the fragmented ore rock.
In coal mining, a mechanized
cutting device is run along the
length of the coal face.
Temporary support
near the working
face: often
hydraulic props.
“Permanent” support,
often timber packs, will
remain in place after
mining. With time, these
become deformed or
completely crushed –as
part of the “controlled”
closure of the panel.
Schematic of Longwall Panel
(Hangingwall Stripped Away For Illustrative Purposes)
Figure from Hartman and Mutmansky, 2002.
The extraction proceeds
during the strike,
with the dismantling
of the face done
with the aid of
explosives.
The ore is
disassembled
collected with a
scraper and taken
to a orepass.
Longwall in Hard Rock

Work Face at South African Gold Mine
37

http://www.bullion.org.za/MiningEducation/Images/images/
CrossSectMine.jpg
Stopes (yellow):
on-reef
excavations
where the reef
(orebody) is
mined.
Stope face with temporary support
Deep level gold mining, South Africa

Sublevel
Caving
Topics
 Characteristics of the
ore body and mining
method
 Development
 Production
 Equipments Used
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Figure 7: Sublevel Caving
Sublevel Stoping

 It is used to mine large orebodies with steep dip tabular or massive deposit and continuation at depth
(Fig.17).
 The ore is extracted via sublevels which are developed in the orebody at regular vertical spacing.
 Each sublevel has a systematic layout of parallel drifts, along or across the orebody.
 Sublevel stoping recovers the ore from open stopes separated by access drifts each connected to a ramp.
 The orebody is divided into sections about 100 m high and further divided laterally into alternating stopes
and pillars.
 A main haulage drive is created in the footwall at the bottom, with cut-outs for draw-points connected to
the stopes above. The bottom is V-shaped to funnel the blasted material into the draw-points.
 Short blastholes are drilled from the access drifts in a ring configuration. The ore in the stope is blasted,
collected in the draw-points, and hauled away.
 Blasting on each sublevel starts at the hangingwall and mining then proceeds toward the footwall.
 Blasting removes support for the hangingwall, which collapses into the drift.
 As mining progresses downward, each new level is caved into the mine openings, with the ore materials
being recovered while the rock remains behind.
 Loading continues until it is decided that waste dilution is too high Work then begins on a
nearby drift heading with a fresh cave.
 As mining removes rock without backfilling, the hangingwall keeps caving into the void. Continued mining
results in subsidence of the surface, causing sink holes to appear. Ultimately, the ground surface on top of
the orebody subsides (Fig.18).
 However, the stopes are normally backfilled with consolidated mill tailings after being mined out (This
allows for recovery of the pillars of unmined ore between the stopes, enabling a very high recovery of the
orebody).
Sublevel Caving

Characteristics of the method ...
Sublevel Caving in the process of fragmentation of the ore is done by
explosives (induced caving) and the ore is detonated with drilling in
ascending fans. The sterile overlying should crumble as the ore is
removed.
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Characteristics of the orebody and enclosing
 massive and/or tabular (varying inclinations are allowed);
 Diving > 50o case is thin;
 competent body with mineral rock wall rock (cover) fractured;
 stable development of the footwall to access;
 the method requires minimal stability to the ore body, because the
galleries sublevel should be self-supporting piece and can receive routine
bolting;
 significant dilution, very little sensitive to fragmentation;
 likely surface subsidence;
 rock cover must accompany the ore in a continuous felling,
producing subsidence at the surface. The ideal condition is that the
enclosing fragment into larger blocks which ore disassembled to
facilitate flow separation at the extraction drift.

Design alternatives for dips and varying thickness of
the ore body
Tabular ore body and thick makes all production galleries are always in
the ore, avoiding fans incomplete perforation (loss of ore), open galleries in
sterile (the roof support problems and expenses), losses along the footwall
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Design alternatives for dips and varying thickness of
the ore body
Losses ore
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Design:
transverse sublevel caving – for thick bodies
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Figure shows Continued mining results in
subsidence of the surface, causing sink holes
to appear.
Ultimately, the ground surface on top of the
orebody subsides.
Figure shows sublevel caving is used to
mine large orebodies with steep dip and
continuation at depth.

Design: transverse sublevel caving
- this case, the galleries of production (drifts) are perpendicular to the
strike of the ore;
- Mining recoveries are greater than the longitudinal layout.
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Design:
longitudinal sublevel caving – for narrow bodies
and sharp dip
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Development in Sublevel Caving
 The method requires significant development, being part of ore (in high producing mines, 6% of the
total ore mined comes from developing).
 The cost / ton of ore in the development is several times higher than in production. Should
maximize production and minimize development.
 The ore body is divided into panels whose height varies from 50 to 250 m in height, depending on
the scale of production and reserves per vertical meter.
 Each panel is divided into sublevels spaced 20-30 m (increasing the spacing between sublevels
minimizes the development) which will be issued successively downward.
 The lower level of the panel is characterized by a main gallery of transportation that serves all
"orepasses", connecting the premises of the extraction well.
 Access to sublevels is accomplished by a ramp situated between the ends of the ore body. The
ramp is linked to gallery transporting each sublevel. This gallery of transporting each sublevel must
accompany the footwall contact at a distance 15-20 m.
 In transverse sublevel, crosscuts traverse the deposit, going to the hanging wall; development is in
the footwall. Starting in the transport gallery, galleries are open from distant production center-to-
center 15-25 m, parallel to each other, extending to the contact with the footwall. Have dimensions
(width x height) 5x4 m; 6x5 m; 7x5 m.
 Sublevel of the galleries, just above the ore is drilled and drilling with longholes in range.
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Production in Sublevel Caving
 The production begins when the galleries of a sublevel reach the subsequent
contact.
 These galleries do not come into general production while, but should not retreat in a
un-organized way. In some mines, adopts the recoil so that its edges are coarsely
contained in a plane.
 In other cases, the recoil is done so that the galleries production of more distant
"orepasses" are the first to reach the main gallery.
 Equipment used for production drilling (ascending): carts with two spears, with
sectioned stems and crowns of up to 115 mm. Currently, the drills used are electro-
hydraulic and DTH's.
 Drilling targeting the production is made in the form of irradiated fans of the galleries
of the sublevel. The holes made are long (up to 50 m long) and is used in this drilling
process "longholes".
 Charging is done by pneumatic devices. ANFO explosive is the most common. In the
case of explosive in cartridge, it uses a similar device, equipped with blades to break
through the cartridge.
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One way to start is
dismantling create a free face
(slot relief) with a hole pattern
up to an inclination of approx.
80 to 90.
Another way is to open the
slot from a raise.
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 LHD's perform the loading, transport and unloading of ore in the end "orepasses" being
sublevels designed for better efficiency of loaders.
 Drilling operations and loading are performed independently and at different levels.
 Due to the large number of galleries sublevel there are many fronts production
 The explosive consumption is high due to the fact the dismantling be carried out
against the mass of fragmented rock.
 Must be careful in removing the ore from drawpoints (you need to control levels at the
point of load):
 removal of the material causes little lower recovery;
 removal of too much material causes excessive dilution.
 There is a cut-off grade given below which do not remove more ore in crosscut and
should detonate new range.
Production
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Equipment for development and production
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In the design of production drifts, the Sublevel Caving uses the
principles of "gravitational flow" ore dismantled, with subsequent
collapse of the host rocks.
Approaches used to issue a gravitational flow:
A) The physical-scale models
B) field experiments in real scale
C) models mathematical / numerical.
The solutions found so far are not fully satisfactory from the
standpoint of optimization of the gravitational flow of the ore.
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These models are the oldest, made with particles (e.g., sand),
leaking containers of small size.
A) Example test with physical scale models:
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Two major factors related dilution and ore recovery (*)
 Width (c) extraction of the gallery;
 Clearance (V) between ranges of production
(*) View Article : Theory and pratice of very-large-scale Sublevel Caving.
Underground Min. Methods: Eng. Fundamentals and International Case
Studies, 2001, W.A.Hustrulid & R.Bullock. Chapter 46, p.381-384.
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Width (c):
 A gallery of production should be as wide as possible;
 When the ceiling of the gallery is concave, the flow of
ore is very centralized and inefficient sides, requiring
closer galleries.
 The amount of ore away from LHD's increases with
the increase of the height of the gallery production.
Therefore, the height should be as low as possible.
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Examples of ancient settings production galleries...
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Are directly executed in underground mines,
where they put up markers (markers) numbered, to be
retrieved and counted after detonation.
A recent experiment shows that the
flow of ore is mainly formed by the
material above drawPoint, but he is not
very predictable.
(Quinteiro, C R, Larsson, L and Hustrulid,
W A., 2001. Theory and practice of very
large scale Sublevel caving. Underground
Min. Methods- Eng. fundam. and
Intern. Case Studies SME)
B) Real Scale Experiments:
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Experiments on real scale:
 Recently, Sublevel Caving mines have increased lateral spacing interval
drifts and sublevels.
 This has increased production, but also increased dilution.
 There are still several outstanding questions about the best production
design in this method.
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There are several numerical models being investigated to explain the influence
of the main variables of the method.
Among the principal's PFC - numerical code developed by Itasca group since
the 1990s PFC = Particle Flow Code.
C) Mathematical / numerical models:
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Principles of choice of the configuration of production galleries:
 the idea of minimization of production costs leads to employ the maximum vertical spacing
between sublevels (currently around 30m);
 essential for maximum spacing between sublevels ... ability to drill and carry long, straight holes
and large diameter factor;
 the largest possible diameter hole, which allows drilling and loading, is ideal (the maximum is
now 115mm);
 large galleries (7x5m, for example) allow the use of drill pipe and longer rectilinear holes (few
rods provide increased rigidity to the drillstring). There are projects of fans with holes up to 50m
in length;
 distance between planes of fans (B):
 depends on the hole diameter (D) and the type of explosive.
 Initial estimate for ANFO ... B = 20 D.
 For higher energy explosives ... B = 25 D.
 D = 115mm and assuming emulsion as an explosive has been B ≈ 3m.
 Number of holes in the array:
 should follow the S / B ratio ≈ 1.3; where S is the distance between the ends of
neighboring holes in the same range.
 In this case (B = 3m), S is 4m.
 Interval between sublevels:
 is chosen based on the maximum drilling capacity and the ability to maintain satisfactory
alignment of the holes.
 As an example, assume 25m.
 Lateral spacing between production galleries:
 makes an angle of 70o between the upper gallery of the reference and the midpoint of
the gallery just above the sublevel (this is the angle of minimum theoretical expected to
drive the ore detonated). The center-to-center spacing resulting from side galleries is
approx. 22m (see Figure A1 in the next slide).
 This initial configuration (Figure A1) was adapted in the 1990s to become more practical
operational point of view (Figure A2).
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Figura A1 Figura A2
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Principles of choice of the configuration of
production galleries:
 For adaptation:
 effecting up to 55 lateral holes of inclination. Function of these holes ...
ore fragment lying in the slope of approximately 70 and reduce the
length of (longer) central holes of the fan.
 Holes smaller than 55o inclination are difficult to load with explosives,
due to the angle of repose of the ore in drawPoint.
 The fans may be vertical or inclined (α generally uses up 70 to 80o to the
horizontal). The steepening improves the stability of the roof drawPoint and
easy access for loading the holes with explosive.
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 High production rate;
 Many And efficient fronts of simultaneous mining;
 Possibility of high degree of mechanization;
 Method Safe for operators.
advantages
disadvantages
 Dilution may be high (15-30%) and moderate recoveries (75-85%);
 subsidence on the surface;
 high consumption of explosives;
 high cost of development;
 intensive drilling and disassemble to generate a suitable granular product to flow ore;
 controlling the cut-off level can result in low recovery of ore.
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Block Caving
Block
CavingProf. Dr. H.Z. Harraz Presentation
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Introduction:
Block Caving* is method in which volumes of
rock are left without support and suffer rebate
under its own weight; the overlying rock
fragments-along with the ore. The fracturing and
the disposal of ore are obtained by the action of
gravity and efforts resulting from tectonic and
lithostatic stress.
* Translation: Allowance for blocks.
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Block Caving
 Block-caving method is employed generally for steeply dipping ores, and thick sub-
horizontal seams of ore. The method has application, for example in sulfide deposits
and underground kimberlite (diamond) mining.
 It is most applicable to :-
o A large-scale or bulk mining method that is highly productive, low in cost,
and used primarily on massive steeply dipping orebodies that must be
mined underground.
o Weak or moderately strong orebodies that readily break up when caved.
o Large, deep (>2 km deep), low-grade deposits with high friability (Fig.19).
 It is often done to continue mining after open pit mining becomes uneconomic or
impossible. However, some mines start as block cave operations (e.g., There are
several of these in Chile. Rio Tinto is considering a deep at the Resolution deposit to
the east of Phoenix).
 A grid of tunnels is driven under the orebody The rock mass is then
undercut by blasting.
 Ideally the rock will break under its own weight Broken ore is then taken
from draw points.
 There may be hundreds of draw points in a large block cave operation (Fig.20).

 An undercut with haulage access is driven under the orebody, with "drawbells"
excavated between the top of the haulage level and the bottom of the undercut. The
drawbells serve as a place for caving rock to fall into.
 The orebody is drilled and blasted above the undercut, and the ore is removed via the
haulage access.
 Due to the friability of the orebody the ore above the first blast caves and falls into the
drawbells. As ore is removed from the drawbells the orebody caves in providing a
steady stream of ore[3].
 If caving stops and removal of ore from the drawbells continues, a large void may form,
resulting in the potential for a sudden and massive collapse and potentially catastrophic
windblast throughout the mine.[4]
 Where caving does continue, the ground surface may collapse into a surface depression
(such as those at the Climax and Henderson molybdenum mines in Colorado. Such a
configuration is one of several to which miners apply the term "glory hole“).
 Orebodies that do not cave readily are sometimes preconditioned by hydraulic
fracturing, blasting, or by a combination of both. Hydraulic fracturing has been applied
to preconditioning strong roof rock over coal longwall panels, and to inducing caving in
both coal and hard rock mines.
 Essentially block caving creates an underground 'inverted open pit'. Surface subsidence
can be a problem….???.

Figure shows application of the
Block caving to large, deep, low-
grade deposits
Figure shows hundreds of draw
points to take broken ore in a
large block cave operation
www.ivanhoe-mines.com/s/Mongolia_ImageGallery
Figure: Models of block caving (Brown, 2003).

An undercut tunnel is driven under the orebody,
with "drawbells“ excavated above. Caving rock falls
into the drawbells. The orebody is drilled and
blasted above the undercut to initiate the “caving”
process. As ore is continuously removed from the
drawbells, the orebody continues to cave
spontaneously, providing a steady stream of ore. If
spontaneous caving stops, and removal of ore from
the drawbells continues, a large void may form,
resulting in the potential for a sudden and
massive collapse and a potentially catastrophic
windblast throughout the mine (e.g., the
Northparks Mine disaster, Australia).
TOP OF OREBODY
Surface
OREBODY
74
Mining Geology, Mining Methods

Block-cave mining: Mud-rushes –an under-reported hazard
Mud-rushes are sudden inflows of mud from ore drawpoints (or other
underground openings), in block-cave mines that are open to the surface.
Considerable violence, in the form of an airblast, is often associated with
mud-rushes. Mud-rushes are (under-reported) hazardous occurrences that
have occurred frequently in mines in South Africa, as well as in Chile and
Western Australia, and have caused fatalities (Butcher et al., 2005).
Mud is produced by the breakdown
of rock in the near-surface
muckpile in the presence of
rainwater.
Kimberlite rock on diamond mines
is particularly susceptible to
weathering by rainwater.
SCHEMATIC CUT-AWAY VIEW OF SUB-LEVEL BLOCK-CAVE MINE

Figure shows Effect of Mineral extraction upon displacement of country rock and
surface as well as rock displacement in mining.
The rock displacement zone include:- "a
caving zone" within which the displacement
is accompanied by the fault and destruction
of layers and the separation of lumps and
blocks from the solid;
"a cracking zone" which is an area of rock
discontinuity and cracks;
"a smooth-displacement zone" wherein rock
features plastics deformation without
discontinuities.
The earth's surface area which experienced
displacement is called a "trough".
Effect of Mineral extraction upon displacement of country rock and surface
This phenomenon is called "Displacement of rock". Displacement causes smooth
subsidence of the earth's surface without ruptures, or abrupt subsidence with
considerable movements, caving and collapses.
Workings and voids formed after extraction of mineral gets filled with time by the
caving rock so that the rock over the deposit may deformed and subside.

Block caving
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Historical evolution of the method ...
 End of the century. xix: block caving applied to iron ore mines in
Michigan, USA;
 Beginning of the century. xx: block caving applied in the USA for iron ore
and copper states in the west side;
 20s: block caving starts in Canada and Chile;
 50s: block caving starts in South Africa, diamond mines and asbestos;
 Beginning of the 60s: LHD vehicles developed for underground mining;
 1970: LHD's used with block caving mine in El Salvador, Chile;
 1981: panel mechanized caving introduced in the primary ore of El
Teniente, Chile;
 90: Planning new generation of mines with greater height block and ore
bodies more resistant (eg Northparkes, Palabora).
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Operating mines closed and planned using the method …
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Some data using block caving mines...
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Condition deposit
 Resistance ore: weak to moderate, preferably soft or friable ore with intense
fracturing;
 Resistance the wall rock: similar to the ore, distinct interface between /
barren ore;
 Diving: vertical is better, but can be flat if the deposit is thick;
 shape: large areal extent and thick (> 30 meters);
 uniform and homogeneous distribution of levels (suitable method at low
levels);
 Depth: moderate (> 500m and <1200m).
Principles of the method ...
 In block caving method, the ore is moved by subsidence (caving) to a
cavity formed almost always without the use of drilling and blasting. Drilling
and disassemble are used in establishing the initial "enhancement".
 The base of the ore is excavated by removing their support, this results in
fracturing of the ore which migrates to the enhanced vacuum and which is
then removed.
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Methodology of block caving:
 develop below the panel to be mined, a layout extraction of ore;
 above the level of extraction, a horizon of "undercut" (highlight) will provide free
face below the ore body, causing the collapse;
 temporary pillars in the "undercut" horizon are removed and the collapse of the ore
starts;
 ore haggard blister and fills the void of the "undercut";
 remove fragmented material in the extraction horizon, inducing flow of ore and ore
loss of support has not beaten down that is also subject to collapse;
 vertical progress of "caving" is related to the extraction of fragmented ore and its
blistering.
 During the flow of the fragmented ore is reduced the size of the blocks.
 Primary fragmentation is done by natural mechanical process, advantageously in
terms of cost. Sometimes explosive is used in production, making long and spaced
holes to induce fracturing.
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Basic issues of geomechanical to the black caving method:
1) Caveability
2) Mine design
3) Fragmentation and extraction control
4) Subsidence associated
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1) Caveability:
 The process is still not well understood, but it is known that the main factors
involved.
 The caving is principally defined by the rock quality (RMR, Q, etc.) and its
hydraulic radius - RH.
 The basic requirement for the method to work is that the rebate (caving) occurs.
The slump of the ore is the result of the action of gravity, being influenced by:
 pattern of fracturing the middle ...
for good fracturing at least two families cross sub-vertical joints between
themselves and one horizontal family;
 stress distribution in the area to be mined.
It is not easy to predict whether the resulting fragmentation which occurs rebate or.
 A rule of thumb: For an ore body be subject to abatement, approx. 50% of the fragments should have a maximum size of
1.5m.
There are several geomechanical classifications to forecast caving and fragmentation. The
most commonly used: RMR, Q system, classification of Laubscher (1981) system.
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Geomechanical parameters observed in some mines:
 Palabora Mine (copper) in South Africa ... MRMR between 57-70, which is
among the highest values for block caving (this method is not advised when
MRMR >50).
 Henderson Mine (molybdenum) in Denver- USA ... with RMR 27-60.
 Northparkes (copper-gold) in Australia ... features RMR between 33-54, for Lift
1 (upper part of the ore body).
 El Teniente, Chile ... MRMR between 55-74, for various lithologies of the mining
area (andesites, diorites, breccias).
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Note .:
 RMR classification
 Developed by Bieniawski
 Ranges 0-100
 Main parameters:
 RQD (Rock Quality Designation)
 Spacing between discontinuities
 Uniaxial compressive strength of rock
 Quality of discontinuities
 Presence of water in the rock mass
 Orientation of discontinuities relative orientation of the
excavation
Rating MRMR (dev by Laubscher,. Page 413 SME book.)
Similar to RMR, but includes stress induced by mining and blasting in the calculation of
MRMR parameter.
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2) Mine Design :
Key elements in establishing the layout of mine ...
2.1) is possible division of the area to be mined :
 division separated by pillars of security blocks sequentially to be
mined;
 division into blocks without pillars, with continuous mining.
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2.2) Selection of extracting ore system :
 The extraction system is complex, time consuming and costly preparation. Usually
requires several years of work to be put into production.
a) traditional by gravity system;
b) by slusher system;
c) by LHD's system.
a) Traditional gravity system - ideal for very fragmented ores:
 loading and transportation system developed under each block
 orepasses are open and finger raises with grids
 the level of fragmentation is controlled louvers
 Finally enhancement is done in the block which begins the fragmentation and
migration of the ore and racks through the undercut up to the level of transport.
b) By slusher system- for medium or little fragmented ores:
 development is simplified by omitting grids level. The cones migration ore bind
directly to points of discharge.
 the high wear on the pillars discharge requires very resistant concrete coating.
c) By LHD's system - more modern system, to little fragmented ore:
 Provides greater productivity drawPoint simpler design eliminates the need for a
orepass for each drawPoint but need area larger tube (strut costs!) Due to the
size of the equipment.
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Systems ore extraction by gravity
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Systems ore extraction by gravity
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Systems ore extraction
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Ore extraction system for LHD's
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Isometric view of a system of extraction of ore per LHD`s :
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Plan view of the level of
extraction of ore to a
system with LHD`s:
Drawpoints
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2.3) Location of permanent facilities
The block caving is generally used in low resistance of rocks, but
the developments and openings production (drawpoints, etc.)
must be kept in places where the rock has better quality.
2.4) Other important aspects ...
drawPoint size, spacing (small fragments implies closer drawpoints),
geometry of the pillars, sequence and direction of mining.
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Typical design parameters …
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Typical design parameters …
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3) Fragmentation and extraction control:
 Fragmentation is difficult to predict and influence
the choice of ore extraction system and spacing
drawpoints.
 The rate of ore extraction affects fragmentation:
very rapid extraction :
very rapid extraction can create voids near the surface
enhancement; fragments of ore become larger because they suffer
fewer burdens on the mass of collapsed overlying materials; for
better fragmentation stack height of caved ore must be maximized;
very slow extraction :
very slow extraction can cause compaction of the ore and restore
locally stable structures.
 Modes of observation of progress caving: see SME Min. Eng. Handbook, 1992 pg.1820.
 Accident Northparkes (1999) by sudden collapse of the ore in the abatement process.
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Examples of fragmentation obtained in some mines…
Mina
Tamanho do
fragmento
médio D50mm
Espaçamento
entre os pontos de
drenagem (m)
Grace 1.500 6,0 x 9,0
Urad 700 9,0 x 9,0
Clímax 350 10 x 10
El Salvador Minério fino
Bell Mine
Com rastelamento
Com Trav Carregamento
7,6 x 7,6
12,2 x 12,2
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Examples of fragmentation inferred from borehole...
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Types of fragmentation :
• In situ ... represented by the blocks that are
naturally present in the rock before mining
activities;
• Primary ... represented by blocks in the
vicinity of the cavity abatement and
separate themselves from the massive
intact when the rebate is started;
• Secondary ... occurs when blocks of
primary fragmentation move for
drawpoints.
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Examples of fragmentation observed in drawpoints :
Northparkes E26, Austrália Esmeralda Sector, El Teniente Mine, Chile
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Cumulative distribution
of fragmentation observed in
Premier Mine, South Africa
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4) Subsidence associated :
 subsidence is discontinuous and affects large
areas of the surface;
 the final geometry of the subsidence area is quite
varied, depending on ...
 resistance ore;
 resistance overburden;
 presence of significant structural features (e.g., dykes, faults);
 depth of mining;
 natural slope of the surface.
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Examples of subsidence - kimberlites in West Africa
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Examples of subsidence - Mine El Salvador, Chile
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Examples of subsidence - San Manuel Mine, USA
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 higher production rate than any other method in underground-coal
 High productivity
 lower production cost of underground methods, side of the Longwall (e.g., production
costs of $ 6.0 / t in the Premier Mine-South Africa)
 High recovery (90% or more), but with significant dilution
 production (not development) runs for abatement; i.e. there is no need for drilling and
blasting
 can be highly mechanized
 good ventilation and security for workers
Advantages of Block Caving
Disadvantages of Block Caving
 subsidence and collapse in large scale
 high dilution
 control resumption is critical to the success of the method
 development is slow
 costly operations of support
 reduction and fragmentation difficult to predict and control
 method with little flexibility and no selective
 possibility of oxidation of the ore due to the long time of percolation water
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 the International Caving Study (ICS) Stages I and II (ICS II sponsors:
Codelco Chile, De Beers Consolidated Mines, LKAB, Newcrest Mining
Limited, Northparkes Mines, Rio Tinto Technical Services, Sandvik
Tamrock, WMC Resources Limited);
 the ICS I monograph, Block Caving Geomechanics, published by the
JKMRC, 2003;
 Proceedings, MassMin 2000, Brisbane;
 Proceedings, MassMin 2004, Santiago, and PowerPoint presentations
made to that conference;
 Rock Mechanics for Underground Mining, 3rd edition, by B H G Brady and
E T Brown, 2004;
 Block Caving Geomechanics by E T Brown;
 Individual works of GP Chitombo, BA Eadie, GE Flores, NJ Harries, E
Hoek, The Karzulovic, DH Laubscher, The Logan, The Moss, IA and I
Oñederra Ross.
Important References …
Caving methods
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Caving Underground Mining Methods (longwall, Sublevel caving, & Block caving)

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