gravity flow of broken rock in sublevel caving (slc ...990302/fulltext05.pdf · swebrec - swedish...

Report 2010:P1 ISSN 1653-5006

Swedish Blasting Research CentreMejerivägen 1, SE-117 43 Stockholm

Luleå University of TechnologySE-971 87 Luleå www.ltu.se

Gravity flow of broken rock in sublevel caving (SLC) – State-of-the-art

Gravitationsflöde hos sprängda bergmassor i skivrasbrytning – dagens teknik läge

Matthias Wimmer, Swebrec

Universitetstryckeriet, L

uleå

Swebrec Report 2010:P1

Gravity flow of broken rock in sublevel caving (SLC) – State-of-the art

Gravitationsflöde hos sprängda bergmassor i skivrasbrytning – dagens teknik läge

Matthias Wimmer, Swebrec

Luleå November 2010, revised December 8, 2010

Swebrec - Swedish Blasting Research Centre

Luleå University of Technology

Department of Civil and Environmental Engineering • Division of Rock Engineering

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Gravity flow of broken rock in SLC Swebrec Report 2010:P1

SUMMARY

This report surveys the state-of-the art of gravity flow in sublevel caving (SLC). The principles of

SLC operations are firstly explained as well as factors influencing gravity flow behavior of blasted

and caved rock.

Thereafter, flow of broken rock is highlighted from a modeling perspective. The traditional

ellipsoid approach and its later modifications are firstly reviewed. Common modern modeling

approaches are surveyed. Small- and full-scale experimental studies are presented in detail with

respect to their actual performance and outcomes.

The difficulties in simulating SLC relate to the physical scale as well as the broad range of time-

scales involved: this results in a vast number of unknowns and uncertainties. It is commonly

accepted that blasting greatly influences the subsequent flow phase. Various attempts have been

made to increase understanding of gravity flow even though the initial situation after blasting is

somewhat obscure. Interest is great as ore recovery, dilution, and flow disturbance are direct

consequences of flow behavior.

Several conceptual flow models have, therefore, been developed based on small- and full-scale

experiments. Of these, the disturbed flow models, and in particular the phenomenon of shallow

draw is the subject of special attention since observations of it recently have been made in large-

scale, modern SLC geometries.

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SAMMANFATTNING

Denna rapport ger en översikt över kunskapsläget inom gravitationsflöden inom skivrasbrytning.

Till att börja med gås grundförutsättningarna för skivrasbrytningen och möjliga påverkansfaktorer

igenom.

Sedan belyses flödet eller rörelsen av den sprängda malmen från modelleringssynpunkt, med

början i den traditionella ellipsoidmodellen och dess successiva förbättringar fram till modernare

modelleringsansatser. Modell- och fullskaleförsök presenteras med fokus på hur de fungerat och

vilka resultat de gett.

Svårigheterna med att simulera rasflöden har dels med skalan att göra, dels med de olika tidsskalor

som är inblandade. Dessa ger i sin tur ett stort antal okända parametrar och förhållanden.

Det antas vanligen att sprängningen kraftigt påverkar rasflödet. Även om förhållandena direkt efter

sprängning är höljda i dunkel, så har olika försök att öka kunskapen om rasflöden gjorts. Den

praktiska drivkraften bakom detta är att såväl malmutbytet som gråbergsinblandningen är en följd

av hur rasflödet fungerar.

Därför har flera olika s.k. konceptuella modeller förslagits med grund i sådana modell- och

fullskaleförsök. Av dessa bör modeller med ojämnt flöde och särskilt den med grund drag

(”shallow draw” phenomenon) uppmärksammas då observationer av detta fenomen nyligen gjorts i

storskaliga, moderna skivrasgruvor.

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CONTENTS

1 SUBLEVEL CAVING (SLC) .................................................................................... 1

1.1 Basic considerations ................................................................................................................ 1

1.2 Factors influencing flow behavior ........................................................................................... 2

2 GRAVITY FLOW OF BROKEN ROCK ................................................................... 4

2.1 Modeling gravity flow ............................................................................................................. 4

2.1.1 Ellipsoid Theories.................................................................................................................... 4

2.1.2 Newer modeling approaches ................................................................................................... 9

2.2 Experimental work ................................................................................................................ 12

2.2.1 Small-scale experiments ........................................................................................................ 12

2.2.2 Full-scale experiments ........................................................................................................... 16

2.3 Conceptual flow models ........................................................................................................ 25

3 CONCLUDING REMARKS ....................................................................................33

4 REFERENCES .......................................................................................................34

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FIGURES

Figure 1. Sublevel caving at the Kiruna LKAB iron ore mine. ............................................................... 1

Figure 2. Fragmentation in the context of sublevel caving. .................................................................... 3

Figure 3. Mechanisms of gravity flow (Kvapil, 1998). ........................................................................... 5

Figure 4. Shape and eccentricity as a function of material mobility (Kvapil, 1998). .............................. 6

Figure 5. Successive phases of extraction (Kvapil, 1982). ...................................................................... 6

Figure 6. Velocity distribution in the ellipsoid of loosening (Kvapil, 1998). ......................................... 7

Figure 7. Ellipsoid where all particles along the contour of the ellipsoid would move with the same

velocity (Kvapil, 1998). ...................................................................................................... 7

Figure 8. Gravitational (F1) and resisting forces (F2) acting on a rock particle (Kuchta, 2002). ........... 8

Figure 9. Draw body constructed using the Berg-mark-Roos equation with s1 = 10 m and αG = 70°

(Kuchta, 2002). ................................................................................................................... 8

Figure 10. Results of three-dimensional SLC models using the numerical modeling code PFC3D

(DeGagné & McKinnon, 2006). ....................................................................................... 11

Figure 11. Single boulder blockage in model-scale experiment (Stazhevskii, 1996). .......................... 13

Figure 12. Influence of a blast created slot on waste rock ingress (Stazhevskii 1996). ........................ 14

Figure 13. Geometry of the SLC blast model and order of initiation (Rustan, 1970). .......................... 15

Figure 14. Model after blasting (Rustan, 1970). ................................................................................... 15

Figure 15. Full-scale versus model-scale results (Janelid, 1973). ......................................................... 18

Figure 16. Draw bodies at Longtan iron ore mine (Rustan, 2000). ....................................................... 19

Figure 17. Marker recovery from different zones, total of 24 rings (Larsson, 1998). ........................... 20

Figure 18. North-South section showing different ring layouts at levels 849, 878 and 907 m. ............ 21

Figure 19. Typical result from marker trials (left ║ and right ┴ ring planes), Ridgeway mine, double

ring interactive draw in 5 m drifts, cross-cuts X0 & X2 with ring 51 & 52 at the 5250

mine level (Power, 2004a). ............................................................................................... 23

Figure 20. Extraction zone shapes noted by Power (2005) on the basis of marker trials at Ridgeway

mine. ................................................................................................................................. 23

Figure 21. Typical result from marker trials, Perseverance mine, cross section looking west and long

section looking north (Hollins & Tucker, 2004). .............................................................. 24

Figure 22. Development of waste inflow with percentage extraction (modified after Quinteiro et al.,

2004) ................................................................................................................................. 25

Figure 23. Waste rock inflow from backbreak of previous ring (Gustafsson, 1998). ........................... 28

Figure 24. Draw bodies of ring no 5 and 6, drift 9, level 335 m (Janelid, 1973). ................................. 29

Figure 25. Palm-and-finger draw body shape (Gustafsson, 1998). ....................................................... 29

Figure 26. Explanation of the pulsation seen in large scale sublevel caving (Larsson, 1996 cited by

Hustrulid, 2000). ............................................................................................................... 30

Figure 27. Sequences of cavity formation and failure (after Gustafsson, 1998). .................................. 30

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Figure 28. Shallow draw phenomenon. ................................................................................................. 31

Figure 29. Formation of a compacted interface due to blasting (Kvapil, 2008). ................................... 31

Figure 30. Location of observation drift at Ridge-way mine (Power, 2004b). ..................................... 32

Figure 31. Photographs taken from observation drift, width of opening about 1.5 m (Power, 2004b). 32

Figure 32. Vertical cross section showing section along drift axis and incompletely blasted rings. Long

arrow indicates camera viewing direction (Selldén & Pierce, 2004). ............................... 32

Figure 33. Open gap between blasthole plane and a combination of confined ore and compacted waste

in previous gaps. The damaged brow is the brighter material in the far left of the picture.

(Selldén & Pierce, 2004). .................................................................................................. 32

TABLES

Table 1. Conceptual models of gravity flow mechanisms in sublevel caving. ..................................... 26

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1 SUBLEVEL CAVING (SLC)

1.1 Basic considerations

Sublevel caving is a mass mining method based upon the utilization of gravity flow of blasted ore

and caved waste rock (Kvapil, 1998). It relies on the principle that ore is fragmented by blasting

while the overlying host rock fractures and caves under the action of mine induced stresses and

gravity. Thereby the caved waste originating from the overlying rock mass fills the temporary void

created by ore extraction. The method itself has been initially applied in the early 1900s to extract

soft iron ores found in Minnesota and Michigan (Cokayne, 1982). At that time heavily timbered

drift support was sequentially removed at the end of a drift initiating the ore to cave and then was

being slushed out. As dilution became excessive the next set of timbers was removed and so on.

Today many uncertainties of fragmentation and ore cavability are eliminated since each tonne of

ore is drilled and blasted from the sublevels. Breaking the ore by blasting removes the dependency

on “natural” fragmentation as the mechanism for ore breakage shared by most other methods of

caving. For this reason SLC is strictly speaking not considered a caving method any longer as far as

the ore is concerned, but SLC does rely on the walls caving and thus the name is retained. As

practiced today the method should probably be given another name, such as sublevel retreat

stoping, continuous underhand sublevel stoping or something similar that better reflects the process

(Hustrulid, 2000). SLC is nowadays usually applied in hard, strong ore materials in which the

hanging wall progressively caves, keeping pace with the retreating rings.

Key layout and design considerations are to achieve high recovery with an acceptable amount of

dilution. Current SLC geometries (Figure 1) consist of a series of sublevels created at intervals of

between 20 and 30 m beginning at the top and working downward.

Figure 1. Sublevel caving at the Kiruna LKAB iron ore mine.

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A number of parallel drifts are excavated on each sublevel with drifts being offset between the

individual sublevels. From each sublevel vertical or near vertical fans of holes are drilled upward to

the overlying sublevels. The burden between the blast rings is about 2 - 3 m. Beginning typically at

the hanging wall the rings are blasted one by one against the material lying in front consisting of

ore from overlying slices and caved waste. The extraction of ore from the blasted slice continues

until a total dilution or some other determining measure reaches a prescribed level. Thereupon the

next slice is blasted and the process continued. Depending on the thickness of the orebody the

technique may be applied using traverse or longitudinal retreat.

1.2 Factors influencing flow behavior

The major disadvantage of SLC is the relatively high dilution of the ore by waste which is based on

the flow characteristics of both materials. Fragmentation of the ore slice itself can be regarded as a

core element for successful SLC (see Figure 2). The general tendency is that more finely

fragmented ore has greater mobility in the stope area. Thorough fragmentation allows drawing of

the ore from over the entire width of the extraction drift and from deep in the muck pile. Both of

these factors allow for a uniform gravity flow and this promotes a higher recovery of ore and hence

overall effective use of the SLC method. In this respect blasting has been throughout the literature

identified as the initial, but also the major impact upon primary fragmentation and later material

flow characteristics (Janelid, 1968; Cullum, 1974; Marklund, 1976; Kvapil, 1982; Stazhevskii,

1996; Bull & Page, 2000; Hustrulid, 2000; Rustan, 2000; Power, 2004a-b; Selldén & Pierce, 2004;

Minchinton & Dare-Bryan, 2005; Zhang, 2005 & 2008; DeGagné & McKinnon, 2005).

In SLC, blasting takes place in a semi-confined situation, where the blasted material is allowed to

swell due to the compaction of the caved material and to a minor extent swell into the void volume

of the production drift. Even though several analytical and empirical models have been developed

in the past the interaction of semi-confined blasting conditions, SLC blast design and rock mass

characteristics on blast performance are not well understood. Layout criteria for ring blasting

concerning overall geometry (ring inclination, shoulder hole angles, design powder factor),

burden/spacing ratios, explosive properties and timing are commonly based on site experience.

With the general trend towards larger blast layouts over the past years and considering the

fundamental importance of blasting to the success of a SLC mining operation it is remarkable that

only a limited number of well documented experiments have been undertaken to quantify the

impact of altered blast design parameters on the resulting material flow characteristics (Rustan,

1970; Kosowan, 1999; Quinteiro et al., 2001; Zhang, 2004; Power, 2004a-b; Clout, 2004;

Quinteiro, 2004; Zhang, 2005; Brunton et al., 2010). The impact of measured blast performance

such as vibration records, VOD measurements, and backbreak studies should be of particular

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interest when gravity flow is studied (Rustan, 1993; Hedström, 2000; Fjellborg, 2002; Zhang,

2005; Brunton, 2009; Wimmer et al., 2009).

Figure 2. Fragmentation in the context of sublevel caving.

In practice the quantification of the physical and mechanical properties of blasted or caved rock is

difficult. Rustan (2000) stated that the most important parameters influencing flow width are

fragment size distribution, shape factor of particles, surface friction of fragments, attrition, density,

shear strength, cohesion of the bulk material and moisture content. The rock material properties

internal angle of friction, limit border angle and angle of repose at dumping or loading have been

assessed. The properties swelling, packing and porosity vary though in space and time and it has

not been possible to assess them or assign a value to them.

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2 GRAVITY FLOW OF BROKEN ROCK

Flow behavior of broken rock has been investigated through small-scale experimental studies

aimed at understanding fundamental mechanisms and factors influencing material flow behavior in

storage bins. Experiments of this type are well suited to construct general applicable mathematical

models.

Mine-based experimental models have been set up to model specific situations. The literature

outlines that the development of efficient design and the operation of SLC mines relied upon

results obtained from experiments that directly modeled specific situations in small- or full-scale

tests.

Generally, the latter focuses on quantifying the impact of various mine design parameters based

upon a specific geometry of the mine and its orebody on material flow behavior and subsequent ore

recovery as well as dilution. They also serve to validate and further improve numerical models. The

following summarizes the current understanding of SLC material flow behavior.

2.1 Modeling gravity flow

2.1.1 Ellipsoid Theories

The theory proposed by Kvapil (1965) was one of the first attempts to fit general mathematical

models using physical models to the flow of granular material. Although it has been developed

using small-scale 2D models aiming at modeling flow in storage bunkers is has become highly

significant for caving methods and was extensively used as a design tool for these methods before

other modern modeling approaches became accepted.

The results obtained by Kvapil are based on studies of free discharge of granulated material

through an outlet at the bottom of a hopper. Central to this theory is the progressive expansion of a

flow ellipsoid which progresses upwards as material is discharged. Meanwhile the geometry of

granular flow is described as the concept of “ellipsoids of motion”. It also outlines dependencies of

the ellipsoid of motion on particle sizes and how the design of a hopper could be determined given

this knowledge.

In subsequent years the flow ellipsoid has been divided into two ellipsoids, each with distinct

boundaries (see Figure 3). These are named the ellipsoid of extraction and the ellipsoid of

loosening (Janelid & Kvapil, 1966). The ellipsoid of extraction is stated as the limiting boundary,

which defines the original location of material that has been extracted from the outlet whereas the

ellipsoid of loosening defines the boundary between stationary material and material that has

moved from its original location at any given point in time material is discharged.

5


Shapes defining the ellipsoid of extraction and loosening have been referred to in a number of

different ways in the literature (Trueman, 2004). Beyond the loosening ellipsoid all particles

remain stationary in a region known as the passive zone. As drawing proceeds, the material within

the extraction ellipsoid is removed and replaced by surrounding particles. However, it is only the

material within the loosening ellipsoid that has the opportunity to enter the extraction ellipsoid. The

size and eccentricity of both ellipsoids gradually develop as material is removed.

Figure 3. Mechanisms of gravity flow (Kvapil, 1998).

By placing markers in a certain pattern within the granular material in a 3D model the validity of

the existence of both zones has been demonstrated. Markers extracted defined the ellipsoid of

extraction and those that just moved to the draw point the ellipsoid of loosening.

The shape of a given ellipsoid is described by its eccentricity related to the major and minor semi-

axes of the ellipsoid. As a rule, the volume of loosening ellipsoid is about 15 times larger than the

volume discharged: expressed in terms of heights this yields a 2.5 times larger loosening ellipsoid.

It is well known that particle size directly influences eccentricity as, for instance, smaller particles

will generate thinner ellipsoids with a proportionally higher eccentricity. Also the eccentricity of

the ellipsoid of extraction and loosening increases with the height of the ellipsoid. This effect,

which is relatively small in SLC, has a much greater importance in block caving due to the very

large block heights. Moreover, eccentricity depends on a number of other factors (Kvapil, 1998),

such as shape (spherical, irregular), surface roughness of the particles (smooth, rough) material

properties (density, strength, moisture content), extraction rate (high, low and continuous versus

interrupted).

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Consideration of all these factors, results in a certain flow behavior which might be expressed in

terms of the mobility of granular or coarse material. A greater mobility results in easier flow and a

higher eccentricity of the ellipsoids as shown qualitatively in Figure 4.

Figure 4. Shape and eccentricity as a function of material mobility (Kvapil, 1998).

Considering models with horizontally layered white and black granulated material and studying

deflections of the layers indicated the active zone and also that the drawdown of the material itself

occurs in the form of an inverted cone (see Figure 5). This indicates that the vector velocity in the

center of the draw is highest and is reduced proportionally on either side of the draw cone axis until

a particle velocity of zero is achieved at the boundary.

Figure 5. Successive phases of extraction (Kvapil, 1982).

The velocity distribution is shown in Figure 6, which represents the velocity distribution through

sections E-E` to A-A`. The boundaries of the loosening ellipsoid have an instantaneous velocity of

zero and the central flow axis vectors indicate the progression of relative values such that v4 > v3 >

v2 > v1. For better visualization the velocity vectors are constructed perpendicular to the axial

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section of the ellipsoid. From the previous figure one can derive zones of the same particle velocity

v1 defined on the boundary shown in Figure 7. A line connecting the particles of the same velocity

forms an elliptic looking figure in 2D and an ellipsoid of same velocity in 3D. Evidently, the shape

of the gravity zones is controlled by a specific distribution of the velocity of motion, resulting in

ellipsoids of the same velocity. Therefore, not only does the zone of loosening have the shape of an

ellipsoid, but so also does the zone from which the discharged material was extracted (ellipsoid of

extraction).

Figure 6. Velocity distribution in the ellipsoid of loosening (Kvapil, 1998).

Figure 7. Ellipsoid where all particles along the contour of the ellipsoid would move with the

same velocity (Kvapil, 1998).

Cox (1969) made model experiments and also underground studies at Mufulira mine in the

Zambian copper belt, initially without the knowledge of theory of ellipsoid flow (Kvapil & Janelid,

1966), but his findings were in close agreement with the theory. Further validation of the theory

and a relatively close fit have been shown by full-scale tests from the Grängesberg SLC operation

(Janelid, 1973), see chapter 2.2.2. A reflection of the general level of acceptance of this theory is

that even today, many general mining textbooks with sections on granular flow, use the ellipsoid

model as their basic flow theory (Kvapil, 1998; Hustrulid, 2000; Brady & Brown, 2004).

During the period in which ellipsoid theory was gaining acceptance other workers have added

valuable contributions to gravity flow theory. Worth mentioning is experimental work by Gardner

(1966) on flow in bins and hoppers in a 2D model. There a mathematical model was presented that

8


predicted the shape of the dead zones at the bottom of a bin as a function of the internal angle of

friction of the model media.

The ellipsoid theory presented has since then been further refined, taking into consideration a near

elliptical form of the extraction draw body but with a maximum width occurring above the upper

half of the draw body; the so-called “drop hypothesis”. The model assumes that a particle moves in

a straight line from its resting point to the opening and that rock is removed continuously.

Consideration of a decomposition of forces acting on a rock particle in opposite directions (see

Figure 1), namely one component of the gravitational force and a resisting force from surrounding

particles, yields the so-called Bergmark-Roos equation (Bergmark, 1975; Hedén, 1976):

s(α) = s1 * [(sinα – sin αG) / (1 – sin αG)]

s…travel distance

α...travel angle

αG..maximum angle at which the broken rock does not flow

s1…height of the extraction draw body

By use of the Bergmark-Roos equation the shape of the extraction draw body can be constructed

(see Figure 2).

Figure 8. Gravitational (F1) and resisting forces (F2) acting on a rock particle (Kuchta, 2002).

Figure 9. Draw body constructed using the Berg-mark-Roos equation with s1 = 10 m and αG = 70°

(Kuchta, 2002).

One of the shortcomings of this equation is that the width of the draw body continues to increase

with increasing extraction heights due to the assumption that rock particles travel in a straight-line

path towards the opening. On the other hand research has shown that in rock, the width of the draw

9


body will reach a maximum value at very great extraction heights and does not continue to increase

with increasing extraction heights (Rustan, 2000). Indeed, at low or moderate extraction heights, an

approximate shape of the draw body may be derived.

Kuchta (2002) presented a revised version of the Bergmark-Roos equation accounting for a non-

zero opening width. This version includes equations derived for the area, volume and maximum

width of the draw body for a given extraction height.

2.1.2 Newer modeling approaches

The problem of analyzing the progressive flow of rock through an enclosed stope could not be seen

as wholly analogous to theories developed for the flow of granular materials (Yenge, 1980). This is

due to the discrepancies in particle size, the relative sizes between the containers and particles and

boundary conditions. Three distinct differences exist between the flow of materials in bins and in

an SLC environment and they can be summarized as follows:

The friction between broken rock and the solid face of the unbroken ring affects the flow

pattern of the blasted rock, i.e. in a bin the material is surrounded by four solid walls

whereas the rock in SLC is surrounded on three sides by broken rock and on the fourth side

by the in-situ rock of the next slice to be blasted,

Blasting the ore column creates density variations within the ore and between ore and

waste,

SLC exists under substantially higher overburden pressures than are usually found in bin

flow.

There are also appreciable differences between the slower discharge rates, the large increases in

void volume and the delayed ground caving response in SLC as compared to granular solid models.

A number of problems that appear when describing SLC flow by idealized ellipsoid theory have

long been recognized, both when doing small and full-scale experiments (Fröström, 1970; Just &

Free, 1971; Cullum, 1974; Janelid, 1975; Just, 1981; Yenge, 1981; Kvapil, 1982; Peters, 1984;

Gustafsson, 1998; Rustan, 2000; Clout, 2004; Hollins & Tucker, 2004; Power, 2004a-b).

Consequently more advanced model and full-scale experiments have been set up. Subsequent

research has been directed towards mathematical methods to address phenomena observed in the

experiments and to improve model performance such as:

Stochastic methods (Chen, 1997; Gustafsson 1998) assume that gravity flow is a stochastic process,

i.e. they include the probabilities of downwards propagation of a particle or upwards propagation of

voids (void diffusion). Another conceivable way of creating voids would be a differential flow of

10


particles but is actually not considered in these models. It is important to understand that the

physics of granular flow are almost completely ignored with these methods.

Plasticity theory (Pariseau & Pfleider, 1968; Nedderman 1992) is the commonest coarse grained

approach for the prediction of velocity distribution in granular materials. In these models, the stress

distribution in the static material is first calculated and from this the velocity distribution is

obtained. Although little progress has been achieved for the prediction of velocity fields in specific

cases, plasticity theory provides more realistic predictions for granular flows in hoppers since it

only applies when particles are small and the material may be considered to act as an equivalent

continuum. There is, however, difficulty in determining a range of the required material properties.

Cellular automata, CA (Sharrock et al., 2004; Alfaro & Saavedra, 2004; Castro et al., 2009) divide

the volume of material into a large number of cells that interact according to differential equations,

describing the physics of the system. In this approach, cells contain discrete objects that are

categorized by individual state parameters that evolve dynamically according to the partial

differential equations.

An example of cellular automata model is, for example, e.g. FlowSim (Castro et al., 2009) which

uses local rules that simulate the gravity flow from discrete elements that change their state through

local rules. The movement of grains is simulated through two mechanisms, one incorporating the

increase in porosity and another rule incorporating the movement of particles driven by gravity.

Both local rules are summarized by two adjusting parameters which require calibration through

experimental data. A distinct difference with discrete element methods (Cundall & Strack, 1979) is

that in FlowSim particle shape or forces are not explicitly calculated. These simplifications were

intentionally made so that FlowSim could computationally be faster than other methods based on

discrete elements. This does not necessarily mean a loss in the rigorousness of the model, as the

aim of a mathematical flow model for block caving applications is to help in defining the design or

draw control practices for a large number of draw points and elements.

Another example of cellular automata, CA, is CAVE-SIM. However, both of these examples aim to

develop an engineering tool to determine the dilution entry point, mixing of grades and recovery in

a large, actual, production block cave scenario.

Discrete element method, DEM (Hustrulid, 1997; Selldén & Pierce, 2004; Minchinton & Dare-

Bryan, 2005; DeGagné & McKinnon, 2005 & 2006) involves computing the contact forces and

resulting Newtonian dynamics of individual particles in an assembly. As a result, the values of

shear and normal forces, rotation, velocity and displacement are determined for each particle.

Figure 10 demonstrates a possible modeling outcome using PFC3D.

11


Figure 10. Results of three-dimensional SLC models using the numerical modeling code PFC3D (DeGagné & McKinnon, 2006).

A further development to this is the modeling software REBOP (Rapid Emulator Based on PFC3D;

Lorig & Cundall, 2000) which incorporates rules based on mechanisms observed and determined in

PFC3D simulations, confirmed by physical model tests performed at the JKMRC (Power, 2004a)

and calibrated at various mine sites such as Henderson, Northparkes, Palabora and Cullinan

(Pierce, 2004). It embodies the incremental evolution of IMZ (Isolated Movement Zone) and IEZ

(Isolated Extraction Zone) for each draw point, those volumes being equivalent to the ellipsoid of

loosening and ellipsoid of extraction in the conceptual model derived by Kvapil (1998). In contrast

to gravity flow simulation packages using fixed draw cone shapes, REBOP does not make

assumptions about the geometry of the IMZs and IEZs. The shape of these three dimensional

volumes evolves continuously as an iteration of quite simple micro rules applied in a time step

fashion that mimics production from the draw points on a daily basis. The equations of these micro

rules govern the material flow from one horizontal layer to the underlying layer by mechanisms

such as collapsing arches at the top of an IMZ and erosion of material in the vertical walls between

adjacent IMZs. The overall objective of the REBOP simulation modeling is to be a practical tool

for engineers for mining design and production control mainly in a larger context e.g. in block

caving operations. REBOP allows predictions of extracted ore grade or other caved rock properties

in caving operations and offers visualization of the movement and distribution of material above

the draw points in three dimensions.

Other, DEM codes are FASTDISC and FLOW3D. Because of the accuracy of DEM, it is well

suited for detailed gravity flow analysis on factors affecting complex flow phenomena in sublevel

or block caving mines. However, its application has several limitations associated with numerical

stability and computation time.

12


The key motivation for development of these models was to simulate the effects of different

geometries and draw strategies on the economic performance of SLC operations. Furthermore, the

development of theories and models is important for the ongoing success and economic viability of

the mining method. A much more detailed summary of numerical models used to emulate the

gravity flow of fragmented rock is given by Castro et al. (2009).

Attempts to simulate sublevel caving are hindered by the physical scale of the operation. In

addition, blasting takes place rapidly while the draw of material may last several days for an

individual SLC ring. Incorporating flow from other higher or adjacent levels extends the process to

months. Incorporating the period of draw of rock form other higher or adjacent levels extends the

process to months or even years.

Validation of small and full-scale experimental results is critical for all numerical models. It is

therefore of great value that a number of experiments have been conducted which allowed this

validation to be undertaken (Brown, 2003; Power, 2004a-b; Selldén & Pierce, 2004; Brunton &

Chitombo, 2009).

2.2 Experimental work

2.2.1 Small-scale experiments

Physical modeling was carried out for the optimization of block caving mines in the US from 1916

onwards, and later was carried out in Africa (Lehman, 1916; Bucky et al., 1943; McNicholas et al.,

1946; Airey, 1965). The objectives of these models have been the identification of the parameters

of the process, the extent to which these parameters influence material flow, and whether the

results could be applicable to full-scale production. Model complexity evolved continually from

early bin models to models incorporating mine geometry as well as material flow properties of

blasted material.

In the late 1950s SLC became increasingly used. This called for more efficient and rational design

techniques, and small-scale testing considering different SLC layouts commenced. In these

experiments the interaction of parameters such as sublevel height, drift spacing and shape, ring

burden and inclination, fragment size and excavation techniques on material flow behavior have

been extensively studied (Sjöstrand, 1957; Koppanyi, 1960; Finkel & Skalare, 1963; Redaelli,

1963; Airey, 1965; Janelid & Kavpil, 1966; Free, 1970; Janelid, 1972; Tessem & Wennberg, 1981).

Most of these experiments have been of two-dimensional character as extraction and movement

zones have been studied through plexiglass side windows. They confirmed the ellipsoid theories as

discussed in detail in chapter 2.1.1.

13


By contrast McCormick (1968) observed in laboratory tests with sand that the flow above a draw

point expands vertically upwards in a funnel shape during which a flow channel with parallel walls

develops. Furthermore he recognized two main mechanisms, namely collapses of hang-formations,

and normal draw-down flow. He suggests that flow cones develop because sand breaks down

continuously from the sides of arches. Once the cone becomes so wide that temporarily stable

arches could no longer develop and finally collapse, normal flow arises and a channel with parallel

wall develops.

A major limitation of all these early works was the difference between material properties produced

by SLC blasting and those selected for the model experiments. These material properties include

fragmentation distribution, bulk density or degree of compaction of the ore and waste material,

friction, cohesion properties and stress distribution within the material. A number of small-scale

experiments were conducted which took account of previous uncertainties (Panczakiewicz, 1977;

Yenge, 1981; Stazhevskii, 1996; Kosowan 1999). Of particular importance is the work by

Panczakiewicz (1977) and Stazhevskii (1996) which attempted to incorporate complex material

properties encountered in full-scale SLC rings in small-scale experiments.

Panczakiewicz (1977) constructed both 2D and 3D models to investigate various SLC geometries

for the Mount Isa Mine in Australia and the impact of fragmentation distribution and material bulk

density on flow behavior. In these studies fragmentation was divided into uniform and well-graded

distributions. Bulk density contrasts between ore and waste rock were realized by three different

categories of compaction: uncompacted, light and heavy. Arching was observed for compacted

materials which led to significant changes in flow behavior, or, in extreme cases, to a complete

blockage. Stazhevskii (1996) summarizes another attempt to model the inhomogeneous nature of

blasted rock material in model-scale by investigating the influence of material bulk density,

fragmentation distribution and oversize, by which is meant boulders, on flow behavior, see Figure

11.

Figure 11. Single boulder blockage in model-scale experiment (Stazhevskii, 1996).

14


Figure 12. Influence of a blast created slot on waste rock ingress (Stazhevskii 1996).

However, a major drawback in the Stazhevskii (1996) study is that a discussion of model geometry

or testing procedure is lacking. From this study it is shown that both the material bulk density and

the presence of boulders have a significant influence on material flow behavior and hence on the

ingress of waste material into the extraction zone. It was concluded that a strictly symmetric flow

pattern in mining conditions would be rather exceptional.

The modeling of a slot or void created during the blasting process is also of note. The theory in the

study which originates from Markenzon (1967) suggests that a slot is formed as the burden moves

forward and compresses the caved material during the blasting process (see Figure 12). Based upon

modeling (Stazhevskii, 1996) concludes that the material above the blasted burden could then

access the newly formed slot resulting in a layer of broken material that is inhomogeneous in

composition and density with indeterminate thickness and boundaries. Such a layer would be likely

to cause waste dilution at an early stage of excavation, but also pulsating phenomena of alternating

ore and waste rock inflow observed at SLC operations.

Model-scale experiments incorporating confined blasting in the study of gravity flow have seldom

been carried out. Rustan (1970) simulated blasting and loading of SLC rounds and benches in

models on a scale of 1:75, see Figure 13 and Figure 14. The blasting was carried in a container

filled with limestone. The development of a material for the model that would give a scaled

fragmentation of full-scale fragmentation was important. To achieve this, it was necessary to insert

artificial weakness planes in the model material, because, without weakness planes, the middle size

fractions were lacking (Rustan, 1990). With a mixture of two-thirds of coarse magnetite and one-

third of fine magnetite an artificial orebody with high density could be created. Finally, the

magnetite was mixed with cement, water, and crushed microscope glass plates that would act as

natural weakness planes.

15


Figure 13. Geometry of the SLC blast model and order of initiation (Rustan, 1970).

Figure 14. Model after blasting (Rustan, 1970).

After blasting the loaded material has been separated in terms of magnetite and waste rock

(limestone) by means of a magnet. Many of the phenomena observed at full-scale also could be

recognized in the model such as overbreak effects at the breakage surface, and variations in waste

rock content during the extraction process.

The influence of specific charge, timing, compressive strength as well as joint frequency of the

model material and confining pressure on the fragment size distribution has been studied in great

detail.

With the aim of studying flow in more detail, additional, holes have been drilled within the burden

to position markers inside some of the blasts. Several markers could be found on the muck pile

directly after blasting. The extraction showed that more markers were found to originate from the

mid-part and closest to the blasted ring plane of the entire ring. Additionally, some waste rock

fragments were found on the muck pile directly after blasting. It was explained that the dynamic

movement of the burden towards the waste rock likely opens up a slot between the burden and the

front in which waste rock can fall down from the top of the blasted ore (Stazhevskii, 1996).

Carefully excavating the model stepwise also allowed an investigation of swelling and bulking of

the blasted round by means of the markers. Correlations with timing, compressive strength and

confinement stresses have been made.

Moreover, burden kinematics in a confined state, such as swelling, velocity, acceleration and

retardation have been studied on bench blasts by using a high-speed camera (4100 fps).

16


Zhang (2004) conducted gravity flow experiments based on SLC blasting models made of concrete

at a scale of 1:50. After blasting, the covering material of the models was manually excavated and

separated from the blasted material, which was colored a priori representing different heights. The

experiments demonstrated that the draw body is of complex shape with a volume much smaller

than the actual blasted volume. With respect to fragment sizes a tendency for coarser particles with

increasing height and remnants at the toe region has been observed. Further it is pointed out that in

order to solve the ore dilution problem associated with the discharge process, the blast and ore

discharge processes need to be treated together.

In summary, small-scale experiments have evolved from simplistic bin models to relatively

complex models incorporating SLC geometry and inhomogeneous material parameters. Numerous

limitations associated with small-scale experiments have been discussed in the literature relating to

issues of similitude (Free, 1970; Sandström, 1972; Alford, 1978; Gustafsson, 1998; Power, 2004a),

model design and properties of ore and waste (Sandström, 1972; Cullum, 1974; Janelid, 1975;

Panczakiewicz, 1977; Alford, 1978; Yenge, 1981; Stazhevskii, 1996; Gustafsson, 1998; Kosowan,

1999; Hustrulid, 2000). Despite these limitations it has at the same time been concluded that small-

scale experimental work has provided quantitative and qualitative results usable for the design and

operation of SLC mines. Further, due to the importance of material properties on flow behavior

future direction of SLC small-scale flow modeling needs to incorporate blasting (Rustan, 2000).

2.2.2 Full-scale experiments

Results from full-scale experiments investigating material flow behavior are crucial for further

development, assessment and validation of numerical and small-scale models.

Monitoring SLC material flow is generally done by means of markers (metal or plastic objects with

unique identification numbers stamped upon them) installed and grouted in drill holes located

inside the burden to be blasted. Recovery is usually done visually at the draw-point or by magnetic

separation during the later material handling process, for example at the primary crusher. A number

of limitations are associated with both types of markers. This has initiated an effort to develop an

electronic marker system based on existing RFID technique. This would allow for real time

detection of markers at the draw-point (Brunton, 2009). Details about the development and final

shape of the extraction zone are obtainable based upon the recovered markers. On the other hand,

studying the progression of the movement zone is a much more complex matter, but the following

developments are noteworthy. There is ongoing research (Baiden et al., 2008) in which synthetic

instrumented rock pieces are deployed to sense rock flow and transmit their actual positions via

VLF communication methods in real time. There are, however, no actual test results from a mine.

There is also an earlier parallel development of instrumented boulders (Hanisch et al., 2003) to

study the dynamics of debris flow on mountain slopes. The authors claim that the configuration and

17


the redundant layout of the built-in 3D accelerometers and the differentiation and combination of

translational and rotational movements could be computed with reasonably stable long-time

behavior. There are no field data to support these claims though.

Geophysical methods could also be used to study gravity flow in-situ. Passive seismic tomography,

for example, applied in block caving to obtain information around and within the cave; for example

cave propagation, stress conditions, fragmentation and compaction zones (Glazer & Lurka, 2007;

Lynch & Lötter, 2007). Furthermore gravity measurements are carried out to estimate cave back

positions in block caving (Gaete et al., 2007). However, the study of gravity flow of individual ring

blasts would require methods that have much finer resolution. A recent attempt to pinpoint

magnetite ore residues within caving debris by means of various geophysical borehole probes

(GPR, magnetometer and susceptibility measurement) has given some promising results, but also

shown clear limitations related to the inhomogeneous nature of the material studied (Wimmer &

Ouchterlony, 2008).

Visual observations, which allow a sporadic insight into the caving flow (Selldén & Pierce, 2004;

Power, 2004a-b) are very important. The acquisition of geo-referenced 3D images from inside

cavities and behind rings in the LKAB Kiruna mine in the case of openings of a new draw point or

in hang-up situations is a promising direct approach (Wimmer et al., 2009).

It is worth noting that only a few full-scale SLC draw marker trials have been carried out in the

past due to the complexity and costs involved in such tests. The following surveys the most

important ones.

a) Grängesberg mine (Sweden)

Between 1969 and 1970 full-scale marker trials were carried out at different sublevels in the

Timmergruvan mine (Janelid, 1973). At this location the magnetite orebody has a dip of 60 - 70

degrees and varying width of 20 - 25 m. The sublevel height was either 13 or 7 m with drifts 3.3 m

wide and 3.2 m in height. The SLC ring inclination was vertical in all cases. Most blasts were

either fired individually (23 tests, each 500 tonnes), but sometimes two rings were shot

simultaneously (8 tests, each 1000 tonnes). A total of 12628 plastic markers have been inserted in

holes drilled downwards from higher sublevels and about 70 % of these have been recovered

visually at the draw points. Marker density can be regarded as very high (five rings with installed

markers inside a 1.5 m burden) but were restricted to the upper part of the expected draw body.

Besides the recording of marker identities, the additional parameters measured included the

fragment size distribution (Fröström & Lamperud, 1973), the hang-up frequency, visual estimates

of the percentage of waste rock, and wagon weights.

18


Figure 15. Full-scale versus model-scale results (Janelid, 1973).

In these tests clear correlations were observed between the shape of the body of extraction and

disturbances, which were observed in the field such as hang-ups, boulders and unsymmetrical

draw. The apex of motion was found to be situated at a distance of around 0.5 m from the mining

front, both with single and double ring blasts. Maximum width and depth of the draw body was

found to be at the same height above the sublevel. A comparison of extraction zones from full-scale

tests and those of previous small-scale tests yielded similar results, but the small-scale results were

more repeatable, see Figure 15. The small-scale tests gave a slightly higher and narrower extraction

zone, which was attributed to a lower compaction degree of the caving debris. It has also been

observed that the draw body in full-scale tests could take the shape of an inverted drop at 60 - 70

percent of the extraction.

b) Longtan mine (China)

In China a series of full-scale experiments have been undertaken at various mine sites from 1975 to

1985 (Gustafsson, 1998). Examples are the trials undertaken at the Longtan SLC iron ore mine

from 1976 to 1977 (Chen & Boshkow, 1981; Gustafsson, 1998; Rustan, 2000). The ore was a low

grade magnetite (3800 kg/m3) with an orebody 30 – 50 m thick dipping at 80 - 90 degrees. Both the

geometry and blasting conditions for this operation were unique insofar as the blasted ring height

amounted to 50 m at a burden of approx. 1 m. Eight vertically drilled rings with a total burden of

8.4 m were blasted simultaneously yielding 32000 tonnes of ore. A total of 3520 markers (wood

filled plastic tubes or ventilation pipes) were placed in 177 marker holes. Marker rings consisted of

9 to 12 holes drilled from the lower level and 18 - 19 holes from the higher sublevel. Recovery of

markers was done by visual means; no waste rock percentage during loading was reported. The raw

19


data itself indicated that the extraction zone was not ellipsoid but rather had the shape of a drop, see

Figure 16.

Referring to vertical sections drawn, the final depths, widths and heights of the draw body were

about 7 m, 12 m and 54 m respectively.

Figure 16. Draw bodies at Longtan iron ore mine (Rustan, 2000).

c) Kiruna mine (Sweden)

The first full-scale field trials were undertaken in Kiruna between 1966 and 1967 (Haglund, 1968).

Totally 184 rings were monitored with a wide range of design parameters. Modified design

parameters included sublevel heights (9 - 13.5 m), ring inclination (70 - 90 degrees), ring burden

(1.2 - 2.4 m), and the number of blastholes (12 - 14). No details of the number or type of markers

are provided (if they were used at all). The results from these trials indicated best recovery values

for a ring inclination of 80 degrees, 1.8 m burden and 12 blastholes.

In the project “SLC 2000” the impact of SLC geometry, blast design, draw control procedures on

ore recovery, waste rock dilution and flow behavior has been studied in more detail (Quinteiro et

al., 2001). As part of this work, gravity flow behavior has been investigated during the years 1995-

97 by installing 908 markers halfway inside the burden in 24 rings (Larsson, 1998). Markers

consisted of 1 m long electric cable pieces, each with identification. A video system has been

installed on-site to facilitate locating markers at the front and to gather qualitative observations of

the outflow into the drift during the mucking operation. Totally 32 % of the markers have been

20


recovered and of these only a very small number originated from the sides of the ring, see Figure

17. On the other hand a large number of markers were recovered from the central part of the ring,

indicating predominant ore flow in the centre. It was concluded that this type of flow will result in

early waste rock dilution. Insufficient marker recovery made attempts to define extraction zones

difficult if not impossible (Gustafsson, 1998).

Figure 17. Marker recovery from different zones, total of 24 rings (Larsson, 1998).

Instead, the analysis was based upon ore recovery and waste rock content measured by weighing

the loaded bucket during mucking. The results indicated that an increase in ring burden (3 - 3.5 m)

resulted in a 32 % reduction in ore recovery and a 10 % increase in dilution. An increase in

production drift width (7 - 11 m) has shown both a 7 % improvement in ore recovery and dilution.

A significant reduction in dilution was observed by changing firing delays for blasting the 10 holes

of a ring, i.e. first blasting the four middle holes using short electronic delay intervals and after a

longer delay of 300 ms blasting the other holes in a ring. A delay of 100 ms was used between each

of the outer holes.

An important finding of the marker trials is that the layout designed with shorter and flatter side

holes to initiate interactive draw zones does not work. Flow rather occurs inside a relatively thin

vertical zone and this has caused some basic changes in the SLC ring layout, see Figure 18. The

present layout has, with a transition level in between, been changed to a silo-shaped layout.

21


Figure 18. North-South section showing different ring layouts at levels 849, 878 and 907 m.

Furthermore, a project was conducted at LKAB during 2003 - 2004 with the overall objective to

carry out research to achieve higher ore recovery and controlled dilution through improved

fragmentation of the blasted rings. The main outcome can be summarized as follows. Observations

made in the test area when measuring VOD is that top initiation can be an alternative layout since it

has shown promising results regarding ore recovery and waste content. Fragmentation analysis of

nine blasted rings using electronic detonators has shown no substantial differences in the average

fragmentation size when compared with a standard layout or a top initiation layout. Moreover,

about 18 % of the electronic detonators did not respond to logging just before blasting, indicating a

system malfunction probably induced probably by blasting nearby rings (shock problem,

communication problem and/or shear off problem of wires). Interestingly, image analysis

measurements indicate that fragmentation with a lower value of xc (characteristic size, 63.2 %

passing value) and higher value of n (Rosin Rammler uniformity index) gives better ore recovery

and lower dilution.

d) Stobie mine (Canada)

A series of full-scale experiments were conducted in 1996 (Kosowan, 1999) to assess ore recovery

for sublevel heights of 21 m (9 tests) and 31 m (8 tests). The percent of ore recovered for each trial

was estimated from visual grade control techniques (no markers used). A number of factors were

considered including drilling practices, blast design and implementation, excavation technique and

draw point width. Fragmentation for each trial ring was measured as well by means of image

analysis. In summary, fragmentation for the increased sublevel height was poor, which resulted in

an unacceptable level of ore dilution and recovery. The major problems identified were blast design

and initiation performance, with a high percentage of holes not detonating.

22


e) Ridgeway mine (Australia)

The most detailed full-scale experiments to date have been conducted at Ridgeway with over 70

individual trials conducted from 2002 -2006 (Power, 2004a-b; Brunton, 2009; Brunton et al.,

2010). The aim of the experimental program was to determine the geometry of the extraction zone,

investigate the hypothesis of interactive draw, gather information on flow mechanisms, determine

recovery and dilution factors and develop strategies for increasing metal recovery and/or reducing

costs. Markers adjusted to the approx. mean fragment size (steel pipes filled with concrete, Ø 42 x

250 mm) were grouted into place in either two or three marker ring planes inside a 2.6 m burden.

Initially marker recovery was undertaken with a combination of visual recovery and magnetic

separation during the handling process (Power, 2004a-b) and was changed due to low marker

recovery rates to magnetic separation exclusively (Brunton, 2009).

The results (Power, 2004a-b) indicated that the extraction zone was not of ellipsoid shape and that

the width and depth of the extraction zone was respectively narrower and shallower than the

blasted ring geometry, see Figure 19. At first the extraction zone develops along the mining front

(ring face) and then deepens. No evidence of interactive flow between adjacent extraction zones

could be seen. It turned out that the nature of the flow was episodic with flow proceeding in stages

from different parts of the ring. Furthermore dilution entry of waste could be identified to originate

from above the actual blasted ring at relatively low draw rates. Primary and combined recovery

(recovery at the current and next level) was determined to be on average 59 % and 75 %

respectively.

Based upon the marker trials a summary of the observed extraction zones was made (Power, 2005).

As a part of the experimental program a number of blast design parameters, i.e. number of

blastholes per ring, toe hole spacing, explosive density and initiation timing, were modified in

order to evaluate the impact on flow behavior.

Summarized, the draw point width at the brow and the depth of draw has an impact on the primary

recovery. Further, Clout (2004) concluded that blast powder factor, explosive sleep time and

number of blastholes had no major influence on primary recovery. However, Brunton (2009)

suggested, based on an additional independent analysis, a trend between blast powder factor and

primary ring recovery.

23


Figure 19. Typical result from marker trials (left ║ and right ┴ ring planes), Ridgeway mine, double ring interactive draw in 5 m drifts, cross-cuts X0 & X2 with ring 51 & 52 at the 5250 mine level (Power,

2004a).

Figure 20. Extraction zone shapes noted by Power (2005) on the basis of marker trials at Ridgeway

mine.

24


f) Perseverance mine (Australia)

Several full-scale trials were undertaken from 2002 - 2004 (Hollins & Tucker, 2004). The trials

were conducted in five production drifts on three different sublevels. The centre-to-centre crosscut

spacing was reduced from 17.5 - 14.5 m with the intention to cause interactive draw (Bull & Page,

2000). Essentially, this concept contains a uniform draw of the caved material, reducing early

dilution ingress and hence improved material recovery. A total of 1762 markers (steel pipes filled

with concrete, Ø 45 x 250 mm) were installed in the 3 m burden and at one meter intervals within

the marker ring. Markers have been recovered through a combination of visual identification (53

%) at the draw point and later magnetic separation (additional 20 %). The major conclusion of

these trials was that interactive draw between draw points does not occur, see Figure 21. The

maximum width of draw was on average 11.5 m which indicated that a zone of material located

between production drifts and the toes of the blastholes did not reach the draw points at all.

Figure 21. Typical result from marker trials, Perseverance mine, cross section looking west and long section looking north (Hollins & Tucker, 2004).

Concluding, all recent full-scale experiments conducted exhibited irregular and asymmetrical

shapes of the extraction zones of large-scale, modern SLC geometries. These results differ from

early full-scale tests: This discrepancy might be explained by the actual draw height and

consequently the stress regime having changed tremendously during these years. However, the fact

that the extraction rate was considered to be relatively uniform and shaped like a tear drop with an

effective ring height of 50 m, like in the Longtan mine, remains to be explained. An explanation of

this discrepancy might be the difference in blast design. It would be expected that multiple blast

rings with burdens of just 1 m instead of 2 - 3 m would provide a high explosive distribution and

well fragmented ore throughout the rings. This could have provided ideal flow conditions for a

uniform extraction zone to develop in the Longtan mine.

25


2.3 Conceptual flow models

Previous discussions (see chapter 2.2.2) have pointed towards irregular and asymmetrical shape of

the draw body for large-scale, modern SLC geometries. It follows that waste rock ingress, which is

measured by weighing buckets during the mucking procedure, exhibits a typically episodic

character, see Figure 22. To equalize fluctuations a simple moving average of 15 samples is

calculated.

For the ring in this figure, which was blasted using standard practice, waste first appeared when the

percentage of extraction was about 60 %. Afterwards the waste content increased with the

extraction percentage and the ring was abandoned when the extraction percentage reached 116 %.

At this point the average waste content of the last 25 % of the extraction exceeded 40 %. However,

the details of such curves usually vary considerably from ring to ring, and even between adjacent

rings.

Figure 22. Development of waste inflow with percentage extraction (modified after Quinteiro et al., 2004)

Simulations made so far have shown that quite different approaches could yield the same response,

such as with regard to pulsation effects in dilution entry curves. Thus response curves by

themselves are not uniquely related to the conditions imposed and hence are insufficient to validate

these conditions. Reduction of such ambiguities can only be made by in-situ observations which

aim to understand the relevant flow mechanisms. To summarize, the information acquired in the

research of gravity flow in SLC is still so limited that a generally accepted conceptual model

cannot yet be constructed.

26


Table 1 surveys the existing relevant conceptual models. This survey shows that characteristics

could greatly differ. Conceptual models, considering issues of disturbed flow (highlighted in Table

1) are examined in more detail in the following discussion.

Table 1. Conceptual models of gravity flow mechanisms in sublevel caving.

Conceptual model Dilution Ore losses Observation References

Extraction- and

loosening ellipsoid ideally none

zones outside

extraction

ellipsoid

shape & eccentricity of

both ellipsoids

Janelid & Kvapil, 1966;

Janelid, 1972

Drop hypothesis ideally none zones beyond

drop draw

body

shape of draw body

Fröström, 1970; Janelid,

1972; Chen & Boshkow,

1981. Bergmark-Roos

equation (Bergmark,

1975; Hedén, 1976;

Kuchta, 2002)

Geological variations in-situ in-situ variations of in-situ ore

grades

Gustafsson, 1998

Boulder blockage lateral above size, location of ore

boulders within caving

flow

Stazhevskii, 1996;

Gustafsson, 1998

Blast heave

explanation above above

waste rock from upper

level trapped within ore

during blasting process

Markenzon, 1967;

Stazhevskii, 1996;

Gustafsson, 1998

Backbreak lateral above backbreak between

boreholes within a caving

round

Janelid, 1972; Gustafsson,

1998; Hollins & Tucker,

2004; Brunton, 2009

Palm- and finger

draw body above/lateral

immobilized

material

within the

round

extended "mass flow

channels" from a main

draw body

Gustafsson, 1998;

Brunton, 2009

Cavity formation lateral above compacted ore in the upper

zones

Gustafsson, 1998;

Hustrulid, 2000

Shallow draw

phenomen above

frozen ore

band and

penetrated ore

within the

caved material

well-graded interfaces

(banding effect); possible

gap between the blasthole

plane at the brow and the

compacted material

Selldén & Pierce, 2004;

Power, 2004a-b; Brunton,

2009; Kvapil, 2008

27


a) Geological variations

In this model (Gustafsson, 1998), it is assumed that flow fields are time-independent and reflect

cyclic variations in the in-situ ore grade. By assuming that the ore grade varies spatially in a cyclic

way, a simple flow model would lead to varying waste rock content at the brow. Consideration of

the homogenous character of the ore bodies usually mined by SLC makes this assumption rather

unrealistic. With respect to the mine in Kiruna, waste rock inflow at the draw-point might occur

periodically close to the footwall as the lower part of a SLC ring is drilled within waste rock. Waste

lenses also exist in some rings and these lenses then will arrive at the draw point during loading,

but in general this is a rare phenomenon given the large size of the waste lens needed to distort the

waste rock curve.

b) Boulder blockage

Boulders and larger rock particles are here assumed to be the cause of waste rock content peaks.

Stazhevskii (1996) observed in model experiments that a single boulder could get stuck between

the mining front and the flowing rock, see chapter 2.2.1, Figure 11. This causes the ore above the

boulder to stop flowing and waste from previous blasted rings in front to flow in, causing a waste

rock peak in the extraction process. When flow under the boulder has gone on for a while, the

cavity which appears under it grows and the boulder finally becomes insufficiently supported. The

boulder then drops and ore flow from the ring could resume.

The proposed theory could also be extended to blockage by several large boulders. At the

beginning of the draw, finer size classes will flow faster, and flow between the larger particles. If

there are several larger boulders they may move until they come into contact with each other and

cause a blockage by interlocking with each other. When such a blockage is formed in the blasted

ring, coarser ore pieces from regions above with lower specific charge will be hindered in their

flow. Meanwhile, lateral waste fines will flow through the blockage and cause waste rock inflow to

the muck pile. When a significant amount of waste has flowed through the blockage the flow will

stop and the rock level below the blockage will sink. Stresses now occasioned solely by the

blockage will gradually increase until they are sufficient to break down the blockage, which ends

the waste rock peak.

The main flaw in this theory is that waste rock inflow in reality might also be present even when

very few boulders are actually observed during mucking. However, data show that there is a

correlation between many boulders in the beginning of a ring and an early beginning of waste rock

peaks (Gustafsson, 1998). This might also explain why several waste rock peaks usually are

observed. This explanation supposes that the excess waste rock in the waste rock peaks passes

through larger blocks which might be regarded as a “sieving effect”. It can be deduced that this

28


explanation predicts a size distribution of waste rock with a lower upper size limit than that before

or after the waste rock peak.

c) Blast heave explanation

According to this theory (Markenzon, 1967), the blast would cause the ore to penetrate into the

caved masses and thereby open a slot between the blasted rock and the remaining mining front

(ring face). Waste rock from above the ring would then have the chance to enter the draw body and

become trapped when the gas pressure decreases so that the blasted ring is pushed back into its

original position, see chapter 2.2.1, Figure 10.

There are several reasons why this explanation is physically incorrect. If it were true it is very

difficult to understand why in reality a series of waste rock inflows with intermittent ore inflows

reaches the draw point. Gustafsson (1998) has also employed a simplistic calculation that the

opening time is at a maximum in the range of a few tenths of a second. This would give the broken

rock from above far too little time to fall down far enough for it to appear early on the waste rock

curve. This explanation also implicitly supposes that the gas pressure during the blast is sufficient

to force about several thousand tonnes of ore into the caved masses, but that at the same time it

would not affect the free fall of broken rock into the slot from above.

d) Backbreak

Another possible explanation of waste rock inflow is backbreak from one ring to another. Evidence

that draw bodies could diverge to the location of backbreak, provided by markers, was given by

Gustafsson (1998) and Janelid (1973), see Figure 23 and Figure 24. However, both observed that

the hypothesis of waste inflow due to backbreak only was confirmed in a few individual ring blasts.

Further observations of backbreak influencing material flow were made by Hollins & Tucker

(2004) and Brunton (2009).

Figure 23. Waste rock inflow from backbreak of previous ring (Gustafsson, 1998).

29


Figure 24. Draw bodies of ring no 5 and 6, drift 9, level 335 m (Janelid, 1973).

e) Palm- and finger draw body

The general shape of the marker plots from Kiruna data (see chapter 2.2.2) indicate that the draw

bodies are much more complex than those of model experiments or conventional explanatory

hypotheses. Interpreting the results has indicated that the draw bodies of the experiments typically

consist of a lower compact part and several long structures above, termed as palm-and-finger draw

body shapes by Gustafsson (1998), see Figure 25. This complex shape of a draw body is thought to

be caused by spatial variations (e.g. related to blastholes) in the mobility of broken rock before

mucking. As a consequence fingers extending into the waste rock would cause the observed peaks

of waste rock.

Figure 25. Palm-and-finger draw body shape (Gustafsson, 1998).

30


f) Cavity formation

Another explanation of pulsation effects of waste rock inflow reflects the relative mobility of the

ore in the upper parts of the ring and the caved rock, see Figure 26 and Figure 27. This is one

interpretation of observations made at LKAB Kiruna mine.

Figure 26. Explanation of the pulsation seen in large scale sublevel caving (Larsson, 1996 cited by

Hustrulid, 2000).

Figure 27. Sequences of cavity formation and failure (after Gustafsson, 1998).

The assumption is that broken rock will flow much more easily in the lower ring parts than in the

upper parts. Consequently, the material of the lower part will start to flow at the start of mucking

whereas the upper part will remain in its post blasting position, gradually forming a gap. Regions of

31


material from the previous rings will ultimately start to bulge, their stability decrease and thereupon

they collapse and flow out into the gap until they reach their angle of repose. The sudden stress

change will also cause the upper part of the material to fill in the part of the gap which has not been

filled by waste rock so far. As mucking continues the granular material flows predominately along

the in-situ rock (ring face) rather than along the broken rock. This will create another air gap in the

ring and the process repeats is repeated.

Validating the cavity formation explanation is a difficult matter. The best chances actually would

be if one could observe the establishment and later breakdown of cavities, but this is impossible

with the observation techniques presently available, see chapter 2.2.2.

g) Shallow draw phenomenon

Field observations made at the Ridgeway mine (Power, 2004a-b) and at the Kiruna mine (Selldén

& Pierce, 2004) led to a model termed the “shallow draw phenomenon”, see Figure 28.

Figure 28. Shallow draw phenomenon. Figure 29. Formation of a compacted interface due to blasting (Kvapil, 2008).

Inadequate space for swelling of blasted ore means that only the material closest to the blast plane

is sufficiently broken to be mobilized, and the material further away is heavily confined and is not

mobile. Therefore, draw predominantly occurs closest to the blast front and progresses upwards,

which would make dilution entry from above possible once the top of the blasted ring is extracted.

The proposed theory has similarities with the hypothesis by Hustrulid (2000). The only difference

is that the shallow draw implies dilution entry from above. Interestingly, Kvapil (2008) has also

adopted a theory of draw bodies in SLC which says that ore fragments would penetrate into the

32


coarser cave rock, allowing for a preferential shallow, vertical and upwards orientated flow, see

Figure 29.

An observation drift above the actual draw level at the Ridgeway mine facilitated observations of

the mechanisms of a shallow draw which did not involve marker results, see Figure 30 and Figure

31. The width of the opening (= 1.5 m) is similar to the depth of many draw envelopes measured in

the marker trials. The compact and well-graded interface of the rock mass on the right side, and

also the arch, a remnant of the initial rock mass structure, are of note.

There is one documented observation of a hang-up formation at the Kiruna mine. This indicated

that there was an occurrence of shallow draw. A gap between the blast plane at the brow and the in-

situ ore can be identified in Figure 32 and Figure 33. The ore appears to be highly compacted if not

solid, but is believed to become rather well-fragmented rock if brought into motion again.

Figure 30. Location of observation drift at Ridge-way mine (Power, 2004b).

Figure 31. Photographs taken from observation drift, width of opening about 1.5 m (Power, 2004b).

Waste

Ore

Figure 32. Vertical cross section showing section along drift axis and incompletely

blasted rings. Long arrow indicates camera viewing direction (Selldén & Pierce, 2004).

Figure 33. Open gap between blasthole plane and a combination of confined ore and compacted waste in previous gaps. The damaged brow is the brighter

material in the far left of the picture. (Selldén & Pierce, 2004).

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3 CONCLUDING REMARKS

Attempts to simulate sublevel caving are hindered by the physical scale of the operation. In

addition, blasting takes place rapidly while the draw of material may last several days for an

individual SLC ring. Incorporating the period of draw of rock from other higher or adjacent levels

extends the process to months or even years.

Information on gravity flow behavior in sublevel caving (SLC) mines can be regarded as sparse.

This is a result of there being several unknowns and uncertainties with respect to the actual effects

of confined blasting in caving rounds. Subjects under continuing discussion are, for example:

Quantification of the physical and mechanical properties of blasted ore and caved rock

Remnant pillars, completed breakage and overbreak to subsequent rings

Mobilization of blasted ore with influence on the

o growth rate of the extraction and movement zone

o formation and failure of semi-stable arches, so-called hang-ups

Interaction effects between

o rings (interactive draw, i.e. material from adjacent rings enter the same draw point)

o blasted ore and pillar

o blasted ore and caving masses

Position variance within the

o blasted round (height, width, burden)

o deposit (longitudinal, transverse, depth)

Additional influences, e.g.

o drilling and charging procedure

o rock mass characteristics

o different confining pressure of caved masses, etc.

o temporal factor (sleep time of explosives, duration between blasting-loading, etc.).

Blasting exerts a great influence on the subsequent flow phase. Although blasting is of great

interest it is a neglected area of study. The probable reason for this neglect is that experimental

work in the SLC environment is a challenge. Various attempts have been made to increase

knowledge regarding gravity flow especially as the initial situation after blasting is rather obscure.

This is important because ore recovery, dilution as well as flow disturbances are the direct

consequences of flow behavior.

Because of this several conceptual flow models have been developed based upon small- and full-

scale experiments. Of these the phenomenon of “shallow draw” might well be paid special

attention since several studies of this recently have been made in large-scale, modern SLC

geometries.

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4 REFERENCES

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Alfaro, M. & Saavedra, J. (2004). Predictive models for gravitational flow. In A. Karzulovic &

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Santiago, Chile: Instituto de Ingenieros de Chile.

Alford, C.G. (1978). Computer simulation models for the gravity flow of ore in sublevel caving

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Netherlands: Kluwer Academic Publishers.

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Brunton, I. & Chitombo, G.P. (2009). Modeling the impact of SLC blast design & performance on

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Bucky, P.B, Stewart, J.W. & Boshkov, S. (1943). What is the proper drawpoint spacing for block

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gravity flow of broken rock in sublevel caving (slc ...990302/fulltext05.pdf · swebrec - swedish...

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