Cave Tracking Technology

Monitoring the material flow in block caves has not been possible previously, leading to poor control of block

cave operations and catastrophic outcomes. For more effective caving based mining methods, technologies must

be developed to better control the caving process in a manner that ensures good fragmentation and minimises ore

dilution.

The key to this is to measure the direction of flow of material within the caving zones as well as its degree of

fragmentation. This information is necessary in order to control the ingress of waste material from outside the ore

zone into the ore. It also provides necessary data to develop and validate caving models to improve cave designs

and ensure improved cave performance.

Our cave tracking technology uses unique 3D position systems to enable real-time monitoring of sensors in the block cave, which move in line with the ore. The proposed technique attempts to track the location of beacons

placed in the cave. Our approach uses a modulated magnetic carrier approach to track the beacon through the

caving rock. The cave tracking system has three major components:

Beacons Devices that are placed in the cave and tracked during caving; Detectors Detectors that measure the signal generated by the beacons; and Central Cave Tracking unit Collects measurements made by each detector to determine the 3D location of the

each beacon detected.

The system also requires a communication infrastructure to transmit the detector measurements to the central

cave tracking unit.

System benefits for the mining industry include:

Real-time mapping of caving material movement; Minimise dilution and maximise recovery from cave; Test validity of existing caving models and develop new models; and Design better cave layouts using improved models.

CRCMining established the concept for the Cave Tracker before inviting Newcrest Mining and Elexon

Electronics to develop the concept into a commercial cave tracking system. Commercial development of the

Cave Tracker technology remains on track with Rio Tinto recently joining the partnership between CRCMining,

Newcrest Mining and Elexon Electronics. This will allow continued progress towards cave deployment testing as

a first generation commercial product in multiple operational mines by mid 2014.

See the CRCMining Cave Tracker video

https://www.youtube.com/watch?feature=player_detailpage&v=cqvHBUxM3yc

CATERPILLAR has developed a new technology that is meant to improve the way in which ore is moved in

block caving mines from draw point to crusher. Cat Rock Flow is achieved through continuous caving, removal

from the drawpoint and conveying.

The Caterpillar Rock Flow system for block cave mines.

The Cat Rock Flow system comprises of several components, all centrally controlled from a remote location - the

Rock Feeder RF300 to remove ore from the drawpoint, the Rock Mover RM900 (a newly developed chain

conveyor) to feed ore into a crusher and an automation unit to control and smooth the production process.

Block cave mining is a mass-mining technique used in many underground mines around the world. The

innovative Cat Rock Flow System jointly developed with the Chilean mining company Codelco greatly improves productivity by making use of continuous haulage technology as compared to the conventional use of

LHDs. An automation system, featuring remote operation from surface, allows draw and quality control and records operational data.

Features of the Rock Flow system include:

Innovative continuous mass mining production system - Combination of Rock Feeder (RF300) and Rock

Mover (RM900) brings high productivity and improved equipment utilization for caving operations.

High-performance continuous ore handling system multiplies extraction rates - Improves traditional hard

rock block caving operation.

Highly automated system with Real Time Draw Control - Remote controlled extraction and haulage of

ore without vehicles or underground personnel.

System flexibility - Easy removal of Rock Feeder for maintenance and clearance work as well as panel

moves.

High level of health, safety and sustainability - Only electric drives, increasing safety, reducing cost and

improving underground climate and carbon footprint.

Caving Initiation, Propagation and Subsidence

Artists rendition of material movements into a fictional inclined cave, following slope failure. In this case the

failure is a similar scale to the cave.

To better estimate the likely performance of caves, a tool that properly accounted for the physical coupling of the

caved material to the un-caved rock mass and the draw schedule, driven by the known physics of both parts of

the problem was needed. No such tool was available.

After analysing a number of alternatives, a coupled Discontinuum Finite Element (DFE) Cellular Automata (LGCA, or Newtonian CA) scheme was developed, making use of our existing DFE capability and our partners cave simulation tools.

In this scheme, the CA part computes the particle movements within the cave and changes in airgap geometry,

while the DFE part computes a new solution for stress, deformation, damage and fault movement hundreds, or even thousands of explicit faults can be incorporated in the DFE models.

As a consequence of the draw and the solution for stress and strain in the rock mass, an unstable zone in the cave

back develops at each coupling step and sloughs into the cave, at which point these elements become available to

be drawn in the CA step. The process is repeated, following the draw schedule, and is able to simulate most cave

propagation phenomena including stalling and chimneying.

Example of 3D non-linear mine-scale block cave modelling results

The coupled DFE-LGCA simulation procedure enables rapid simulation of cave propagation, flow and induced

deformation driven by the cave draw schedule with a level of reliability shown to exceed any other available tool

in all comparative studies undertaken so far. The method can be calibrated directly using observations of cave

back location, grade and recovery, seismicity, tunnel damage, tomography or ground movement.

BE believes that this technique represents best practice for cave simulation in the world today. The key

ingredients are the coupling, the improved represantation of structures to a smaller scale, capturing of the

changing swell and flows and realistic discontinuous deformation.

Geotechnical factors

Geotechnical factors influencing selection of mining method

Contents [hide]

1 Geotechnical factors influencing selection of mining method 2 Mining Method Classification 3 Change of Mining Method in a Mining Operation 4 Thickness and Orientation of Mineralization 5 Ore and Country Rock Strength 6 Distribution of Mineralization within the Orebody 7 Depth of Mineralization and Surface Conditions 8 Geotechnical Environment 9 Geotechnical Factors of Underground Mining Methods

o 9.1 Pillar Supported 9.1.1 Room and Pillar Mining 9.1.2 Sublevel Open Stoping

o 9.2 Artificially Supported 9.2.1 Cut-and-Fill Mining

9.2.1.1 Case Study: Kristineberg Mine 9.2.2 Bench-and-Fill Stoping 9.2.3 Shrink Stoping

9.2.3.1 Case Study: Mouska Gold Mine 9.2.4 Vertical Crater Retreat (VCR) Stoping

o 9.3 Unsupported 9.3.1 Longwall Mining 9.3.2 Sublevel Caving 9.3.3 Block Caving

10 Summary 11 References

The characteristics of the orebody itself form the basis for geotechnical factors, including the thickness and

orientation of the mineralization, the ore and rock strength, the distribution of mineralization within the orebody,

the geotechnical environment, and the depth of mineralization and surface conditions. In some cases, these

conditions change in a single mining operation. If significant enough, a change in mining method in one ore

deposit can occur.

Geotechnical considerations when selecting a mining method are becoming increasingly important, due to the

increased dimensions and production rates required of mining operations in order to meet growing expectations

of profitability. Since these larger projects require a longer period of satisfactory performance in terms of ore

recovery and ground support, more formal and rigorous methodologies are necessary in mine design (Brady and

Brown, 2006). Geotechnical factors include in-situ mechanical properties of the orebody and country rocks, the

geological structure of the rockmass, the ambient state of stress and the hydrogeological considerations in the

zone of potential mining influence (Brady and Brown, 2006). The goals of geotechnical consideration in mine

design, regardless of the mining method, are to:

Ensure the overall stability of the complete mine structure, defined by the main orebody, mined voids, ore remnants (pillars) and adjacent country rock;

To protect the major service openings and infrastructure throughout their design life; To provide safe access and working places in and around the centres of ore production; and To preserve the mineable condition of unmined ore reserves (Brady and Brown, 2006).

Mining methods have evolved significantly in the last several decades as improvements have been made on

machinery used to extract the ore, understanding and experience with the behaviour of the rockmass and

underground stresses has developed, and as newly discovered ore bodies are located in increasingly difficult

conditions.

Mining Method Classification

From a geomechanical perspective, mining methods can be classified based on the type and degree of support

required in mining operations. Supported mining methods include open stoping and room-and-pillar mining,

where natural support is provided by ore remnants (e.g. pillars), or cut-and-fill mining and shrinkage stoping,

where support for the walls of the void remaining after ore extraction is provided by backfill or by fractured ore

temporarily retained in contact with mined stope walls. Cave mining methods include block caving and sublevel

caving, where no support is used because fragmented rock fills and flows through the stopes. A classification of

underground mining methods, subdivided based on pillar supported and unsupported groups, is shown in

Figure 1.

Figure 1: A hierarchy of underground mining methods and associated rockmass response to mining (Brady and Brown,

2006)

The distinction between these two broad categories of mining methods can be made by comparing the

displacements induced in the country rock and energy redistributions in the rockmass caused by mining

activities. Supported mining aims to restrict displacements in the country rock to elastic behaviour and prevent

failure of the rockmass. The success of these methods depends on the ability of the near-field rockmass to sustain

compressive stresses in order to maintain elastic behaviour. The mining issue therefore becomes prevention of

unstable energy releases (e.g. rockbursts) associated with increased near-field stress, which could cause failure of

support elements, sudden closure of stopes, or rapid fracture generation in the surrounding rock. A schematic of a

supported mining method (room-and-pillar) is shown in Figure 2.

On the other hand, cave mining purposefully induces large displacements following fragmentation of the

rockmass, resulting in energy dissipation in the caving rockmass. The success of this method depends on

exploiting the discontinuous behaviour of a rockmass when confining stresses are relaxed

(Brady and Brown, 2006). The mining issue here is to maintain steady displacement of the fragmented orebody

so to prevent the development of unstable voids. The rate of slip and fragmentation of the rockmass must be

proportional to the rate of ore extraction. A schematic of a caving mining method (block caving) is shown in

Figure 3.

Figure 2: Schematic of a supported (room-and-pillar)

method of mining (after Hamrin, 2001)

Figure 3: Schematic of a mechanized block caving operation

method of mining at the El Teniente Mine, Chile (after

Hamrin, 2001)

Change of Mining Method in a Mining Operation

In practice, it is possible for a mining operation to utilize different mining methods, which can even be classified

by different geomechanical concepts, at different stages of orebody extraction. The transition between methods

of different geomechanical behaviour can have significant consequence on the stability of permanent openings,

which lends to the importance of defining a mine plan for the entire mine life that will be able to successfully

accommodate the necessary mining methods and induced behaviours of the rockmass. However, in underground

mines, a complete change of mining method once operations have begun is uncommon. Slight variations in the

method occur when faced with minor changes to the ore body and mining environment, but a complete overhaul

of the initial method requires a change that significantly impacts the mine output and its overall profitability. In

general, mines that have experienced continuous problems are more willing to adopt new mining techniques to

improve their operations with a changing mining situation (Laubscher, 1994).

Thickness and Orientation of Mineralization

Orebodies can occur in a variety of geometries, related to the deposits geological origin. Tabular or stratabound deposits are of sedimentary origin and are extensive in two dimensions (horizontally if in unmetamorphosed

sedimentary rock). Veins, lenses and lode deposits are also generally extensive in two directions but are formed

by hydrothermal and/or metamorphic processes. Massive deposits have a more regular orebody shape that are

controlled less by geologically imposed boundaries (Brady and Brown, 2006). The details of preferred orebody

shapes are discussed in the section for each mining method. A detailed discussion of oil and gas deposits is not

included in this article.

Ore and Country Rock Strength

The strength of the ore and the surrounding rock is one of the most significant geotechnical factors in mining

method selection. The strength and related stability of the rockmasses are important for all types of excavation,

and are assessed using rockmass classification systems throughout a mine. Common rockmass classification

systems include Bieniawskis Rock Mass Rating (RMR) system (Bieniawski, 1989), the Modified Rock Mass Rating (MRMR) system developed by Laubscher (1990), the Q system developed by Barton (Barton et al.,

1974), and the Geological Strength Index (GSI) by Hoek and Brown (1997). Ground support requirements within

the guidelines of a mining method are determined based on the strength of the rockmass as well as the intended

use of the excavation (e.g. permanent service excavations versus stopes) and the related risk of failure to the

mine.

The RMR system was developed to assess the stability of an excavation, while the MRMR system addresses

cavability of an orebody being mined using caving methods. Although the MRMR system has been used

successfully for the weaker and larger orebodies for which it was first developed, more recent experience in

stronger, smaller and isolated or constrained orebodies has not provided satisfactory results (Brown, 2003).

The mineralization in a rockmass that defines the orebody changes the geomechanical properties of that

rockmass. The intact rock strength as well as the amount and quality of joint sets and fractures of an ore body are

usually different from the surrounding rock. The type and grade of mineralization affects the sharpness of the

contact between the rock types, which is an important control on dilution during mining.

Distribution of Mineralization within the Orebody

The variation of ore grade through the volume of an orebody influences the mining strategy. The critical

parameters are average grade, cut-off grades, and grade distribution. The average grade determines the degree of

flexibility for method selection as related to the operating costs and current market conditions that define the

monetary value of the deposit. The amount of dilution of ore expected during extraction is also related to the

value per unit weight of ore. For deposits with a lower average grade, there is a higher economic sensitivity to

the effects of dilution.

General grade distribution in an orebody may be uniform, uniformly variable, or irregular. Uniform ore grades

are found in massive ore deposits, uniformly variable grades exhibit a spatial trend in ore grade, and irregular ore

grades are found in deposits with high local concentrations of ore minerals, including vein, lens, and nugget

deposits.

Depth of Mineralization and Surface Conditions

The depth of the mineralization and surface conditions are the main considerations when choosing between an

open pit and underground mining method. As a brief discussion of open pit mining, the ore deposit must be

shallow enough to maintain an economic ore grade with construction of the pit walls. The angle of the pit walls is

controlled by the strength and stability of the rockmass. In general, a less stable rockmass requires a shallower pit

slope angle, while a more stable rock enables steeper slope angles to be used. More stable rock is preferred so the

excavation is more focused on the ore body, which results in less excavation of waste rock. An open pit mine has

a much larger footprint at surface, and therefore must be located further away from existing infrastructure and

population than an underground mine.

The depth of an underground mining operation can impact the mine or the surrounding ground in both shallow

and deep conditions. In shallow conditions, the effects of subsidence in the mine can extend to surface and

impact nearby surface infrastructure. Supported mining methods frequently have no visible subsidence effect at

surface; however, ground subsidence has been known to occur in unsupported mining methods, including

longwall mining of coal (Brady and Brown, 2006). Caving methods generally have a more pronounced impact at

surface through subsidence (Brady and Brown, 2006). In sublevel caving of a massive ore body near surface,

subsidence is included in the mine design and is controlled by the rate of ore extraction. The most effective mine

closure plan for this scenario is to flood the mine and create a lake at surface.

In addition to ore and rock strength, the in-situ stresses associated with deep mining are a significant factor in

mining method selection. This is important to consider for pillar supported methods, since pillar size correlates to

stress conditions. Pillar size affects the overall profitability of the mine since a major design focus of pillar

supported mining methods is to minimize pillar size in order to maximize the amount of ore extracted. In low

stress environments, pillars are small, which results in a higher ore recovery. For more information on pillar

design, see the pillar design article.

Geotechnical Environment

The geotechnical environment of an ore deposit is characterized by the intact rock and rockmass properties, in-

situ stress in the host rock, and chemical properties of the ore (Brady and Brown, 2006). Intact rock properties

include strength, deformation characteristics and weathering characteristics. Rockmass properties are defined by

the influence of joint sets, faults, shear zones, and other penetrative discontinuities.

Adverse chemical properties of ore rock may prohibit caving methods of mining, which generally require the ore

to be chemically inert (Brady and Brown, 2006). For example, a sulphide ore subject to rapid oxidation by

fragmentation of the orebody may create difficult ventilation conditions in working areas and even break down

into smaller pieces of rock after predicted primary and secondary fragmentation have occurred. These smaller

rock fragments could reduce the effectiveness of the height of draw in the stope and the transport and handling

facilities for the ore.

There are certain cases where a pervasive geological feature can be influential enough to control the entire

mining method selection and mine plan, including large fault or shear zone systems, highly fractured rock linking

to an aquifer, and the local tectonic setting (Brady and Brown, 2006). Faults and shear zones may separate the

orebody into multiple sections that would otherwise be able to be mined using large scale caving. Similarly,

aquifers existing in or near the zone of mining influence where large fractures occur in the rockmass may provide

hydraulic connections to other water sources during mining. An active tectonic setting would be problematic for

large voids left by mined stopes, due to the possibility of local instability induced by a seismic event. Also,

mining activity in a stope near an existing fault has the potential to cause a local seismic event. In addition to the

direct dangers of structural failure of the rockmass in the stope, an indirectly caused an air blast is a

consequential risk for mine safety.

Geotechnical Factors of Underground Mining Methods

The discussion of specific underground mining methods is organized based on the type and degree of support

required in mining operations: pillar supported, artificially supported, and unsupported. Case studies are

presented on mining methods at the Kristeneberg Mine (cut-and-fill) and the Mouska Gold Mine (shrink

stoping).

Pillar Supported

The successful performance of a pillar supported system is related to both the dimensions of the individual pillars

and their geometric location in the orebody (Brady and Brown, 2006). A good understanding of in-situ stress

conditions is necessary for a successful pillar supported mine design. If there is a high horizontal maximum

stress in a particular direction, the orientation of both the room advance through the orebody and rectangular

pillars should be planned in order to maximize support in that direction. Many very shallow room-and-pillar

operations may have very little horizontal stress such that the orientation of the rooms and pillars has a minimal

effect. However, very deep operations in high stress environments may have rockburst issues. As such, the

sequence of extraction in addition to pillar orientation is important (Bullock and Hustrulid, 2001). A more

detailed discussion on pillar design can be found here.

Room and Pillar Mining

Room and pillar mining generates ore pillars as remnants as extraction progresses, in order to control the stability

of the roof rock and the global response of the surrounding rockmass (see Figure 2). Regular patterns of pillars

are typically developed in order to simplify planning, design, and operation. The roof may or may not be

artificially supported for worker safety, depending on the competency of the rockmass. Pillars can either remain

intact upon mine closure or extracted at the end of mine life, allowing the stope to collapse afterward (Brady and

Brown, 2006).

Room and pillar mining is best suited for tabular deposits that also must be relatively shallow to limit the size of

the ore pillars. Examples of tabular ore bodies or host rocks include copper shale, coal, salt and potash,

limestone, and dolomite (Hamrin, 2001). There are three typical variations of room and pillar mining that

account for changes in dip of the ore body.

1. Classic room and pillar mining is used for horizontal deposits with moderate to thick beds, as well as inclined deposits with thicker beds. Mining progresses downward from the hangingwall in slices and the required ground support is installed in the hangingwall.

2. Post room and pillar mining is used for thick, inclined deposits that have a dip between 20 and 55. The mining sequence here begins from the bottom and advances upward. Backfill is used to increase the support capacity of the pillars in the mined out areas, and to create a platform from which to mine the next section of ore.

3. Step room and pillar mining is used for ore deposits that are only 2 to 5 m thick and have a dip angle between 15 and 30. In this case, mining advances downward from the hanging wall (Hamrin, 2001).

A suitable geomechanical setting for room and pillar mining requires a strong, competent orebody and near-field

rockmass, with a low frequency of cross jointing in the immediate roof rockmass (Brady and Brown, 2006).

Mississippi Potash Inc.s underground operations are a good example of room and pillar mining of soft rock. The salt ore is surrounded and intruded by clay seams which form zones of weakness that are controlled by rock bolts

or cribs. Since the stability of the salt layers is much easier to control with ground support, the roof of the mine

excavations are designed to be developed in salt (Herne and McGuire, 2001).

Sublevel Open Stoping

Sublevel open stoping requires extensive development in and around the orebody during preproduction. Stope

faces and side walls remain unsupported during ore extraction, while support for the country rock is developed as

pillars are generated by stoping (Brady and Brown, 2006). The pillars may be left in place or extracted at a later

time (Bullock and Hustrulid, 2001). Bighole stoping is a larger scale variant of sublevel open stoping that uses

longer blast holes. This results in vertical spacings between sublevels of up to 60 m instead of 40 m for sublevel

open stoping. Sublevel open stoping is applied in massive or steeply dipping stratiform orebodies. For an

inclined orebody, the inclination of the stope footwall must exceed the angle of repose of the fragmented rock in

order to promote free flow of rock through the stope to the extraction horizon. Since stopes in these methods are

unsupported, the strength of the orebody and surrounding rockmass must be sufficient to provide stable walls,

faces, and crown for stope excavations. Additionally, the orebody boundary must be regular to minimize dilution.

Due to the blast hole drilling and blasting technique, the minimum orebody width for open stoping is

approximately 6 m (Brady and Brown, 2006). Pillar recovery is a common practice in open stoping, made

possible by the use of backfill placed into primary stope voids. The backfill replaces the support provided by the

ore pillar, allowing for pillar extraction.

Artificially Supported

Artificial support in mine openings is intended to control both local, stope wall behaviour and near-field

displacements. There are two main categories of artificial support for ground control: mechanized support (e.g.

rockbolts) and backfill. Potentially unstable rock near an excavation boundary may be reinforced with rockbolts.

Backfill is used to fill stope voids and can prevent the progressive disintegration of near-field rockmasses in low

stress conditions (Brady and Brown, 2006). Artificially supported methods include bench-and-fill stoping, cut-

and-fill stoping, shrink stoping, and vertical crater retreat (VCR).

Cut-and-Fill Mining

Cut-and-fill mining is a very versatile method that can be adapted to an orebody with any shape (Bullock and

Hustrulid, 2001). It is a very selective mining method that most commonly advances up-dip in an inclined

orebody. Mining costs are relatively high compared to other methods; recovery is also high, and dilution is low.

As such, it is an appropriate method for high grade orebodies (Bullock and Hustrulid, 2001). Cut-and-fill is a

very controlled cycle of mining that is repeated many times in a single deposit. The simplified steps are:

1. Drilling and blasting, where a 3 m thick slice of rock is stripped from the crown of the stope; 2. Scaling and support, where loose rock is removed from the stope crown and walls and lightweight support is

installed; 3. Ore loading and transport, where ore is mechanically transported in the stope to an ore pass; and 4. Backfilling, where a layer of backfill with a depth equal to the thickness of the ore slice is placed on the stope floor

(Brady and Brown, 2006).

The success of this method depends on continued stability of the rockmass surrounding the work area where

miners work continuously. This is achieved through controlled blasting, application of local rock support, and

more general ground control using backfill. Cut-and-fill stoping is applied in veins, inclined tabular orebodies

and massive deposits. When mining a large enough orebody, mining can be divided into multiple sections

separated by vertical pillars. This method is suitable for orebodies dipping 35-90 degrees in either shallow or

deep locations. The backfill allows for a weaker country rock, but the orebody itself must be a competent

rockmass (Brady and Brown, 2006). However, if the orebody strength is very poor, a variation on cut-and-fill,

underhand cut-and-fill, may be used (Bullock and Hustrulid, 2001). The ore grade must be sufficiently high to

withstand dilution from backfill, but the grade can also be variable since lenses below the cut-off grade can be

left unmined (Brady and Brown, 2006).

Case Study: Kristineberg Mine

Cut-and-fill mining is the primary mining method at the Kristineberg Mine, which is located in northern Sweden,

approximately 130 km west of Skellefte. The orebody is a typical vein structure with a dip varying between 45

and 80. The host rock is a schistose sericitic quartzite and the rock immediately adjacent to the orebody is often

highly altered, frequently very weak, talcy sericitic schists that vary in thickness between 0 and 3 m. The rock

strength of both wall rocks and ore decreases from the hangingwall to the footwall as a result of metamorphic

folding and faulting. Ground control problems, including roof collapse and wall slabbing, arose from a

combination of variable rock quality and high in-situ stresses. Ore in the roof of the backfilled stope is subjected

to large horizontal stresses, which results in the failure of both the roof and sidewall. In response to the resulting

decline of ore production in the 1980s, extensive investigations of the feasibility of cut-and-fill mining at greater

depth under these conditions were undertaken. The continuation of successful mining at greater depths is a result

of the following findings:

Dense support is necessary to maintain stability, and efficient, mechanized support is necessary to reduce costs and maintain reasonable production capacity; and

The combined effect of changes in support strategy, more efficient support capability, and reduced level intervals has resulted in increased production reliability and capacity, with improved mining costs (Krauland et al., 2001).

Bench-and-Fill Stoping

Bench-and-fill stoping is a more productive alternative to cut-and-fill where geotechnical conditions permit.

Here, initial drilling and excavation drives are mined along the length and width of the orebody. Mining

advances by the sequential blasting of production rings into the advancing void and the ore is mucked remotely

from the extraction horizon (Villaescusa, 1996). Following mining, the stopes are backfilled to provide support

for the stope walls. An example of bench-and-fill stoping geometry is shown in Figure 4. This method may be

used at several scales and variants. In some cases it has become the preferred method of narrow vein mining

(Brady and Brown, 2006).

Figure 4: Bench-and-fill stoping geometry in the Lead Mine, Mount Isa Mines, Queensland, Australia; (a) longitudinal

section, and (b) cross-section (after Villaescusa, 1996)

Shrink Stoping

Shrink (or shrinkage) stoping involves vertical or subvertical advance of mining in a stope, where the fragmented

ore provides both a working platform and temporary support for the stope walls, as shown in Figure 5. This

method is similar to cut-and-fill stoping, where the fragmented ore fulfills a similar function to backfill used in

cut-and-fill. It is generally applied to very narrow extraction blocks that have traditionally not been suitable for a

high degree of mechanization (Bullock and Hustrulid, 2001). The suitable orebody type, orientation,

geomechanical properties and setting for shrink stoping are virtually the same as those for cut-and-fill. However,

the chemical properties of the ore become more important for shrink stoping where the rock must be completely

chemically inert. The ore rock must also be competent and resistant to crushing during draw in order to maintain

flow through the stope (Brady and Brown, 2006). Shrink stoping remains one of the few methods that can be

practiced effectively with a minimum investment in machinery but is still not entirely dependent on manual

labour (Hamrin, 2001).

Figure 5: Layout for shrink stoping (after Hamrin, 2001)

Case Study: Mouska Gold Mine

Shrink stoping has been implemented at the Mouska gold mine, located 80 km west of Val-dOr and 20 km east of Rouyn-Noranda. 72% of the ore is produced in shrink stopes, 20% from longhole mining, and the remaining

8% from development work of the mine infrastructure. The minimum width of the shrink stopes is 1.6 m, and

three major joint sets are present in the rockmass. Most of the ground control problems can be attributed to (i)

brittle failure of the diorite country rock under high stress; (ii) unstable blocks formed by the intersection of

major joints; and (iii) major changes in orientation of the ore veins. The choice of shrink stoping at Mouska is

based on the following factors:

It is a selective method that allows daily assessment of the orientation of the vein being mined; It is a flexible method that permits better recovery of the ore in the extremities of the stopes; Ore inventory left in the stopes during the mining phase provides additional wall support; and Pillars can occasionally be left inside the stope, improving ground stability and mining grades (Marchand et al.,

2001).

Vertical Crater Retreat (VCR) Stoping

Vertical Crater Retreat (VCR) stoping is a larger scale variant of shrink stoping, made possible by advancements

in large-diameter blasthole drilling technology and explosive design. It is applicable in many places where shink

stoping is feasible, except for orebodies that are narrow in width (less than approximately 3 m). It is particularly

suitable for orebodies where sublevel development is impossible (Brady and Brown, 2006).

Unsupported

Unsupported mining methods include longwall mining and caving mining. These methods are distinguished from

other mining methods because the near-field rock undergoes, by design, large displacements where mined voids

become self-filling. Longwall mining is classically used in the deep mines of South Africa, where the near-field

rock is usually strong and in-situ stresses are high (Brady and Brown, 2006). Several caving mining methods are

generally well suited for massive ore bodies, including iron ore, low-grade copper, molybdenum deposits, other

massive sulphide deposits, and diamond-bearing kimberlite pipes (Hamrin, 2001). These include block caving,

sublevel open stoping and bighole stoping, and sublevel caving. Cave mining refers to all mining operations

where the orebody caves naturally after undercutting and the fragmented material is recovered through

drawpoints. Cave mining has the lowest cost for underground mining, provided the drawpoint size and handling

facilities are appropriate for the caved material in a given mine (Laubscher, 1994).

Longwall Mining

Longwall mining is best suited for thin ore deposits that have a large horizontal extent. Ground support is used to

maintain the excavation opening near the face, while the hangingwall behind the excavation can be allowed to

subside. Hydraulic props, cribs, and pillars of timber or concrete are common ground support systems (Hamrin,

2001). This method can be used for both hard rock mining of metal ore and coal mining in soft rock. In both

cases, the method maintains continuous behaviour in the far-field rock. An orebody must be dipping less than 20

and have a relatively uniform grade distribution. Additionally, any displacement along a fault must be less than

the thickness of the orebody. In hard rock mining, longwall mining aims to maintain near-continuous behaviour

of the near-field rockmass, which requires a strong and competent hangingwall and footwall rockmass. A generic

layout of longwall mining in hard rock is shown in Figure 6. In cases where only a single pass of the orebody is

mined, movement and closure of the hangingwall and footwall occur as mining advances. Once the hangingwall

and footwall are in contact, the ground stresses at that location are invariant with further mining. This allows for

the use of lighter rockmass support in the vicinity of mining activity (Brady and Brown, 2006).

Figure 6: Schematic of longwall mining in hard rock (after Hamrin, 2001)

Sublevel Caving

Sublevel caving is a true caving technique that seeks to induce free displacement of the country rock overlying

an orebody. Mining progresses downwards in an orebody where each sublevel is extracted as mining proceeds,

as shown in Figure 7. Since gravitational flow of the fragmented ore rock controls the ultimate yield,

development of the caving rockmass and the setup of the drill headings are the important aspects of the mining

method. Generally, sublevel caving is suitable only for steeply dipping orebodies, with reasonably strong

orebody rock enclosed by weaker overlying and wall rocks. The average grade must be high enough to sustain

dilution to amounts that are perhaps greater than 20%. This method results in a significant disturbance of the

ground surface, limiting its application to areas with suitable local topography and hydrology. Close control of

draw is required to limit dilution of the ore stream. Geomechanics issues are prone to arising in production

headings as a result of high concentration of field stresses in the lower abutment of the mining zone (Brady and

Brown, 2006). This method has been most commonly applied to mining magnetic iron ores that can be easily and

inexpensively separated from the waste rock (Bullock and Hustrulid, 2001).

Figure 7: Schematic of transverse sublevel caving (after Hamrin, 2001)

Block Caving

In block caving, disintegration of the ore and country rock takes advantage of the natural fractures in the

rockmasses, the stress distribution around the boundary of the cave domain, the limited strength of the

rockmasses, and the tendency of the gravitational field to displace unstable blocks from the cave boundary. A

schematic of block caving is shown in Figure 3. This method is distinct from all others discussed thus far as the

primary fragmentation is achieved by natural mechanical processes. Block caving is a mass mining method,

capable of high, sustained production rates at relatively low cost per tonne. It can only be applied to large

orebodies where the height exceeds approximately 100 m. Productive caving in an orebody is prevented if the

advancing cave boundary spontaneously stabilizes into, for example, an arched crown of blocks. Important

geotechnical factors to consider when evaluating the caving potential of an orebody include the pre-mining state

of stress, the frequency and surface condition of joints and other fractures in the rockmass, and the strength of the

intact rock material. The most favourable rockmass structural condition for caving is one that contains at least

two prominent subvertical joint sets, plus a subhorizontal set (Brady and Brown, 2006). It should be noted that in

high stress fields, it has been observed that too rapid a draw can result in the creation of rockbursting conditions

(Bullock and Hustrulid, 2001).

Summary

A summary of the geotechnical factors for each underground mining method, including the suitable orebody

geometries, orebody grades, orebody and country rock strengths, and depths are shown in Table 1.

Table 1: Summary of geotechnical factors for each underground mining method

Method

Class Method

Relative

magnitude of

displacements

in country rock

Strain

energy

storage in

near field

rock

Suitable

orebody

geometry

Suitable

orebody

grade

Suitable orebody,

country rock

strength

Suitable

depth

Pillar

supported Room-and-pillar Very low Very high

Tabular,

maximum dip

55

High

Both strong and

competent, low

frequency of cross

jointing in roof

Shallow

Pillar

supported

Sublevel open

stoping Very low Very high

Massive or

steeply

dipping

stratiform,

regular

boundary

Moderate

Must be sufficient

to provide stable

walls, faces, and

crown for stopes

Variable

Artificially

supported Cut-and-fill Low High

Veins,

inclined

tabular,

massive; 35-

90 dip

High; variable

with lenses is

acceptable

Competent

orebody, can be

weaker country rock

Shallow

or deep

Artificially

supported Bench-and-fill Low High

Narrow vein

mining High

Competent

orebody, can be

weaker country rock

Shallow

or deep

Artificially

supported Shrink stoping Moderate Moderate

Very narrow

extraction

blocks; veins,

inclined

tabular,

massive

High; variable

with lenses is

acceptable

Competent orebody

(and resistant to

crushing), can be

weaker country rock

Shallow

or deep

Artificially

supported VCR stoping Moderate Moderate

Mininum 3 m

width

orebody;

veins,

inclined

tabular,

massive

High; variable

with lenses is

acceptable

Competent orebody

(and resistant to

crushing), can be

weaker country rock

Shallow

or deep

Unsupported Longwall mining High Low

Thin with

large

horizontal

extent, less

than 20 dip

Uniform

grade

distribution

Hard (e.g. gold) and

soft ore (e.g. coal)

rock; hard ore rock

requires strong and

competent

hangingwall and

footwall rockmass

Shallow

(soft) or

deep

(hard)

Unsupported Sublevel caving High Low

Steeply

dipping

orebodies

High enough

to sustain

dilution

(perhaps

>20%)

Reasonably strong

orebody rock

enclosed by weaker

overlying and wall

rocks

From

shallow

to deep

Unsupported Block caving Very high Very low

Large

orebodies

where height

>100 m

High enough

to sustain

dilution

Rockmass of limited

strength, containing

at least two

prominent

subvertical and one

subhorizontal joint

set

Shallow

or deep

References

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Written by J.J. Day, Dept. of Geological Sciences and Geological Engineering, Queen's University