Full Scale Near Field Flow Behaviour at the Ridgeway Deeps Block Cave Mine

7/25/2019 Full Scale Near Field Flow Behaviour at the Ridgeway Deeps Block Cave Mine

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Full Scale Near Field Flow Behaviour at the Ridgeway Deeps Block Cave Mine

Ian Brunton

Senior Geotechnical Engineer, Newcrest Mining Limited

Senior Research Fellow, W. H. Bryan Research Centre, University of QueenslandGlenn Sharrock

Principle Geotechnical Engineer, Newcrest Mining Limited

Adjunct Associate Professor, W. H. Bryan Research Centre, University of Queensland

James Lett

Senior Geotechnical Engineer, Newcrest Mining Limited

Abstract: Full scale marker experiments were designed and implemented at the Ridgeway Deeps

block cave mine from March 2008 to October 2010. The experiments aimed at quantifying and

assessing the geometry of the extraction zone, development of the extraction zone over time,

variability of flow behaviour, and factors affecting flow behaviour in both the near and far field of thecave column. Over 3,000 markers were installed over a two year period, making these experiments

the most extensive undertaken by a block caving operation to date. Results in the near field (markers

recovered within 30 m above the undercut level) provide insight into the development of the

extraction zone during undercutting and subsequent draw. These results highlight early material

recovery in the vicinity of the major apex, which expands towards the centre of the drawbell as more

tonnes are drawn. Furthermore, marker recovery is not spatially uniform during material extraction,

indicating disturbed flow behaviour. This type of behaviour significantly deviates from conventional

flow theory based on numerical models and scaled physical models using narrow distributions of

idealised particles or crushed aggregates.

1.

Introduction

An understanding of gravity flow mechanisms in caving operations is critical for the ongoing success

of caving mines. Gravity flow impacts on both design and operational aspects of the cave, including

extraction/undercut level layout and design, drawbell geometry, cave propagation, air gap formation,

and ore reserve recovery. Most existing knowledge of the mechanisms controlling gravity flow come

from idealised numerical and small scale experiments on gravels and sands with narrow particle sizes

and shape distributions (Sharrock and Hashim, 2009). This knowledge, or conventional flow theory, is

based on isolated flow ellipsoids (and derivations of such) and interactive/interaction flow behaviour.

Generally, these theories are applied to extraction level design (drawpoint spacing) and used to

calibrate cave material flow codes (e.g. PCBC, REBOP, NCA, PGCA, FlowSim, and DEM codes;

refer Sharrock et al, 2012 for further detail) for the modelling of ore recovery, waste ingress, air gap

formation, and coupling to cave propagation finite element models.

In contrast to small scale experiments, actual caves are known to have wide fragmentation size

distributions with large discrete blocks, or interlocking groups of particles, which result in hang-ups

and other disturbances to the flow displacement field (Sharrock and Hashim, 2009). Such behaviour is

termed disturbed flow, and can result in significant deviation from conventional flow behaviour

(Sharrock and Hashim, 2009). To date, limited data exists in the literature to describe the development

and geometry of disturbed flow behaviour in operational caves. Although existing numerical and

scaled physical experiments indicate the likely differences between disturbed flow behaviour and

conventional flow theory, this has not been confirmed in detailed full scale experiments.

To improve the understanding of full scale material flow behaviour, a marker experimental program

was adopted at the Ridgeway Deeps block cave mine. The major objectives of this program were toquantify extraction zone development and shape (the shape that defines the original location of the


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excavated material, defined by recovered markers), and indirectly interpret general material

movement mechanisms. The experiment was divided into two broad components related to the

quantification of 1) near field flow (within 30 m of the undercut level), and 2) far field flow

(extending from 30 m to 240 m above the undercut level). Due to the early draw tonnes extracted

from the cave to date (approximately 17 % of final design tonnes), the results from the near field

markers are discussed. These results detail material recovery and the transient growth of the extraction

zone in the vicinity of the undercut, stagnant regions in the vicinity of the undercut and potential flow

mechanisms driving this behaviour.

2. Ridgeway Deeps Block Cave Mine

The Ridgeway Deeps mine is located approximately 250 km west of Sydney, Australia. The operation

is located 3 km to the north west of the Cadia Hill open cut gold mine, and approximately 25 km

south of Orange, New South Wales (Figure 1). The Ridgeway Deeps, Ridgeway, Cadia East, and

Cadia Hill gold mines form Cadia Valley Operations, which is owned and operated by Newcrest

Mining Limited. The Ridgeway gold-copper orebody was discovered in November 1996, with initial

mining commencing in March 2002 using the sublevel caving method (Ridgeway SLC). A production

transition from the Ridgeway SLC to Ridgeway Deeps block cave occurred in 2010. The Ridgeway

Deeps block cave is located approximately 210 m below the existing Ridgeway SLC and 1100 m

below ground surface. The expected mine life is eight years (2017) based on current reserves.

Production is expected to be 1.6 Moz of gold and 0.21 Mt copper, with an annual mining rate of 5.6

Mt (Newcrest, 2011).

Figure 1 Location of Cadia Valley Operations and Ridgeway Deeps block cave mine

The Ridgeway deposit is a structurally controlled gold-copper porphyry orebody characterised by

stockwork and sheeted quartz veins containing copper sulphides (Smart and OSullivan, 2006). The

deposit is centred on a subvertical monzonite stock of the Late Ordovician to Early Silurian. The

orebody is contained within the Forest Reef Volcanics and sediments of the Weemalla Formation, and

has a maximum dimension of approximately 400 m east-west, 250 m north-south, and in excess of

1000 m vertically (Smart and OSullivan, 2006). Mineralisation extends over 1000 m in vertical

extent, from 500 m below the surface and is open at depth.

The design of the Ridgeway Deeps block cave consisted of an extraction level (4786 level) and an

undercut level (4804 level) 18 m above the extraction level. The general design of these levels andassociated drawbells is shown in Figure 2. The block cave was established using an advanced

undercut crinkle cut design. Total undercutting area was approximately 85,000 m2 (approximate

Cadia Valley Operations

Ridgeway SLC and Ridgeway Deeps

Cadia Hill

Cadia East

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maximum dimensions 490 m x 180 m), and was commenced in the north east footprint corner and

retreated to the south west. A total of 248 drawpoints were developed, with production from all

drawpoints occurring by November 2010. Material handling to surface is achieved through two

underground jaw crushers, and conveyor belt system to surface.

Figure 2 Ridgeway Deeps extraction and undercut level geometry (after Newcrest, 2007)

3. Full Scale Experimental Program

Methodologies for cave scale monitoring of flow behaviour can be divided into two broad categories

indirect and direct measures. Indirect measures rely on flow behaviour being inferred from indirect

measurements such as grade recovery, visual identification of geological markers at the drawpoint,

timing and location of waste ingress such as clay, and recovery of infrastructure from overlying

abandon levels. Although such information can be valuable, the interpretation of such data can be

misleading or inconclusive.

In contrast, direct measures rely on the direct measurement of either the extraction or movement zone.

Measurement of the extraction zone is achievable through the installation of markers (generally

plastic, metal, or electronic) in the cave. The development and shape of the extraction zone is simply

defined by the markers recovered. The major limitation with such monitoring programs is achieving

the required density of markers to satisfactorily define the extraction zone. Historically, such marker

experiments have been confined to sublevel caving operations where sufficient marker density can be

installed in selected blast rings (Janelid, 1972; Gustafsson, 1998; Power 2004; Brunton, 2009). To the

authors knowledge, in block/panel caving operations, no rigorous direct marker experiments have

been attempted or documented. To date, the technology has not been developed to measure material

movement characteristics (displacement, velocity, extent) in the full scale.

Full scale marker experiments were designed and implemented at the Ridgeway Deeps block caveoperation from March 2008 to October 2010. The experiments aimed at quantifying and assessing the


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development and shape of the extraction zone, potential flow mechanisms controlling flow behaviour,

identify possible sources of waste ingress, and ascertain the degree of flow behaviour variability. The

design of the experimental program was based upon a number of criteria related to the experimental

objectives, marker density both laterally and vertically, budget constraints, drill availability and

capability, underground development and access, existing location of open holes (exploration and

hydrofracture), undercutting schedule, and expected cave geometry. Based upon these design criteria

the installation of markers was focused in specific areas of the block cave column (Figure 3). Markers

were installed in up holes drilled from the 4804 undercut level (maximum hole length 30 m), deep

down holes drilled from SLC development (maximum hole length 250 m, drilled from 5040 and 5070

levels), SLC production crosscut pillars (5010 level), and down cave back monitoring open holes

during cave propagation.

Figure 3 Ridgeway Deeps experimental marker layout (looking south west)

Markers were designed to mimic flow behaviour of rock in the mine within the limitations of the

installation techniques available. They had to be individually identifiable, robust enough to survive

any initial blasting process and subsequent cave flow, and be recovered in a relatively easy and

reliable fashion to ensure sufficient data for further analysis. Based upon these requirements, markers

were constructed from 42 mm diameter hollow steel pipe (inside diameter 38 mm) cut to 250 mm

lengths (Figure 4a). A four letter code was welded on the pipe to uniquely identify each marker. An

electronic marker system (Elexon, 2011) was trialled in selected areas of the cave (undercut and open

holes). The system was found to be robust and reliable; however the technology was notcommercially available at the time of marker installation (therefore resulting in the large proportion of

metal markers being used in the experimental program).

Installation of the marker in up holes required the use of a redcap and spider at the base and top of

the marker respectively (Figure 4a). The redcap was designed to hold the weight of the marker in the

hole, while the spider was used to centralise the marker during installation. Markers were loaded into

holes using an explosive truck, which allowed the distance of the marker up the hole to be accurately

measured (Figure 4b). Markers in downholes were installed with the aid of PVC tube segments and a

wire line attached to a winching system (Figure 4c). Markers were taped to the PVC tube before being

lowered into the hole to ensure their original position was maintained. The spacing of markers in both

up and down holes was 2 m. Once installation was completed, markers were grouted in place to

ensure no premature movement. Based upon previous experiments at the Ridgeway SLC (Power,

2004), recovery of markers was undertaken using magnetic separation in the material handling system

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(after the primary crusher). This method has been found to provide high levels of marker recovery

compared to other methods such as visual collection at the drawpoint or conveyor belt system (Power,

2004). The main disadvantage of magnetic separation is that it does not provide information

concerning the drawpoint location, time, or tonnes excavated from the drawpoint at recovery. This is

an important limitation in the use of metal markers, as the degree of lateral movement across the cave

cannot be determined.

Figure 4 Installation of metal markers in up and down holes

4. Near Field Experimental Results

Due to the relatively low tonnes extracted from the cave to date (approximately 17 % of final design

tonnes December 2011), the results from markers installed in up holes drilled from the 4804

undercut level are only considered for analysis. Markers were installed in three separate areas of theundercut level (Figure 3): 1) eastern flank to monitor flow behaviour in proximity of a cave boundary

(4804 XN22 524 markers), 2) central region to monitor general flow behaviour over three drawbells

(4804 XN3, 5, 7, and 9 1291 markers), and 3) western region to assess the performance of an

electronic marker system (4804 XN27 112 markers). For the purposes of this paper, the marker

recovery in the central region of the cave is discussed in further detail. This region is considered

representative of the expected material flow behaviour within the cave as it is located in the dominant

rock type (Volcanics), spatially removed from the cave boundary, and located above drawbells with

typical design draw tonnes.

The installation layout consisted of five separate rings drilled from the 4804 XN3, 5, 7, and 9

crosscuts (Figure 5). Each ring was dumped forwards 10 (to match the blast ring dump angle) and

contained 24 holes (diameter 102 mm) ranging in length from 12 m to 30 m. Spacing between eachring was designed at 10 m to provide sufficient marker density across the drawbell (located mid

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burden for the inclined and flat blast rings). The experimental area was located within the Volcanics

rock type (average UCS = 116 MPa, average RMR(L90)

= 59) approximately 60 m south of the

northern cave boundary. Based on digital photographic analysis, the average measured fragmentation

distribution P80and block top size volume for the area was 1.1 m3and 18 m

3respectively (for a drawn

tonnage range of 9,000 t to 11,000 t), while production statistics indicate an average drawpoint hang-

up frequency per 1000 tonnes of 2.1 (drawn tonnage range from 15,000 t to 20,000 t). Drawpoint

hang-ups were removed through a combination of techniques including water cannon, explosives

(bombing and ballistic mortar), and mechanical methods.

Figure 5 Isometric views of 4804 level central region up hole marker rings and associated development

To delineate the extraction zones, five vertical sections perpendicular to the major apex were

constructed through the three drawbells (Figure 6). The sections were spaced at nine metres,beginning at the centre of the southern drawbell (Section 1) and progressing to the north. Sections 1,

3, and 5 were located in the centre of each drawbell, while Sections 2 and 4 were constructed through

the minor apex. A nine metre sample clip (4.5 m to the south and north of the section line) was used to

determine which markers reported to each section. For recovered markers, a drawpoint tonnage was

assigned based on: 1) the closest drawpoint in which the marker was initially located to, and 2) the

drawpoint tonnes on the day in which the marker was recovered from the material handling system.

This is an important assumption, as the flow displacement path is assumed to be the shortest distance

to the drawpoint (in reality this is not always likely to be the case).

Instead of a general ellipsoid shape being fitted for each section, the extraction zone was defined by a

series of polygons based upon marker recovery tonnage increments. Delineation of these polygons

was based upon a number of criterion or rules consisting of: 1) polygon boundary defined by the

half way point between two markers (in both the x and y direction on the section), 2) markers not

recovered in the material handling process are assumed to represent material not extracted from the

cave to date, 3) areas within the experimental area that do not contain markers are treated as not being

monitored, with extraction polygons terminating at these regions (i.e. polygons do not extend into

areas with no marker coverage). These criteria are considered important as they provide a systematic

and consistent approach in defining extraction zones. Although the polygon assembly represents only

an approximate model of the extraction zone and not the exact shape, they do provide an insight into

the asymmetric, disturbed and non-uniform nature of full scale gravity flow which has not been

measured before.

Extraction zone polygons were constructed for five separate draw tonnage increments 1) markersrecovered during the undercutting process, 2) 1 t to 10,000 t draw, 3) 10,000 t to 20,000 t draw, 4)

20,000 t to 30,000 t draw, and 5) 30,000 t to 40,000 t draw (Figure 6). Drawpoint extraction tonnages

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in the experimental region ranged from 28,500 t to 43,000 t (tonnages varied significantly due to

hang-ups limiting drawpoint availability). To date, approximately 50 percent of markers have been

recovered from the experimental region. Further marker recovery is expected with additional tonnes

drawn from the region. It should be noted that the extraction zone polygons represent material

recovery in both the undercutting and subsequent draw process. During the undercutting process,

potential exists for both lateral and vertical movement of markers in the lower section of the marker

rings (due to blast related overbreak and commencement of caving). It would therefore be expected

that before excavation from the drawpoints commences, a percentage of markers would not be in their

original position, but would travel a path defining the near field flow displacement field.

Figure 6 4804 level marker recovery sections (looking north east) for 0 to 40,000 tonnes extracted

The marker recovery results summarised in Figure 6 shows a general trend towards initial recovery

above the major apex region, which expands towards the centre of the drawbell with increased

excavated tonnes. Markers located in close proximity to the inclined blast rings were recovered during

the initial undercutting process. Development of recovery through the centre (Figure 6 Sections 1, 3,and 5) and across the minor apex (Figure 6 Sections 2 and 4) of the three drawbells was similar. It is

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concluded that the geometry of the extraction zone defined by the polygons are irregular in nature and

cannot be described by general ellipsoid flow theory.

5. Conceptual Model of Near Field Flow Behaviour

A four stage conceptual model based on the marker experiments, fragmentation data, and anecdotalinformation such as drawpoint observations, stress analysis and micro-seismic data is presented in

Figure 7. In all stages, disturbed gravity flow is evident. Disturbed flow occurs when a large particle,

or group of interlocking particles acting as a large particle, disrupts the displacement field of an

assembly of smaller particles (or mixture of small and large particles).

Figure 7 Conceptual model of near field disturbed gravity flow at Ridgeway Deeps block cave mine

Stage 1 (Figure 7a) represents the initial state in proximity to the undercut advance face immediately

after firing of the crinkle cut, but before mucking of the undercut. Three different zones representing

particle size distributions exist 1) the blasted material, 2) destressed caved material, and 3) stress

damaged zones above the major apex. The packing (porosity) and size ratio between the particles in

each zone is thought to control the movement and segregation mechanisms observed in the next three

stages.

In Stage 2 (Figure 7b - 10,000 t) slow moving and arched rock in the destressed zone acts somewhat

like an inclined SLC ring, promoting the movement of the fine particles, which carry coarser rock

fragments (containing markers) from the lower edge of the destressed zone, and hence from above the

major apex. As a result coarse particles in contact or embedded in the matrix of fine particles movevery quickly and are recovered earlier than the core material. The fine particles act to rapidly transport

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particles from the damaged and destressed zones to the drawpoints. This is somewhat like a diffusion

or concentration driven segregation mechanism observed in other disciplines of granular science

(Mosby et al, 1996, de Silva et al, 2000). By contrast the coarser particles in the centre of the drawbell

move very slowly and a core of coarse particles accumulates in this region. This type of disturbed

flow has been noted in scaled physical modelling experiments and has been termed kinematic

disturbance. Kinematic disturbance results from differential movements between the fine and coarse

or larger fractions (Sharrock and Hashim, 2009). Large particles move slower, and effectively create a

moving boundary, which disturbs the flow of other particles.

In Stage 3 (Figure 7c 20,000 t) most particles from the blasted zone have been extracted. As draw

continues more arching and interlocking of the coarse particles occurs, leading to percolation of fine

particles through the interstitial voids. In this stage there is ongoing recovery of markers from above

the major apex. It is noteworthy that a high number of hang-ups were experienced in the marker

experimental area (due to oversize), and as a result substantial disturbed flow and skewing of the

extraction zones are evident in the marker recovery data. Hang-ups result in the formation of stagnant

zones, forming zones of rapid material movement elsewhere to satisfy requirements for mass balance.

In Stage 4 (Figure 7d 30,000 t) the majority of the caved rock from the destressed zone has beenrecovered, while marker recovery shows very high levels of disturbed flow. Large rock fragments and

arches disturb the displacement field, resulting in significant disorder and asymmetric growth of the

extraction zone, but higher recovery above the drawbell. This type of disturbed flow has been termed

static disturbance which occurs when a stationary zone of particles forms, either from a fixed

boundary or a large static particle. Such a situation is shown in Figure 7d, where the particle is

stationary due to arching, while the smaller particles with higher mobility can pass. In this case, the

static particle (or group of particles) effectively forms a new boundary, which disturbs the flow of

material. This zone changes as flow continues, if for example the arch is broken, or stress

redistributions result in movement or elimination of the stationary zone.

6.

Conclusions

This paper documents what is believed to be the first recorded full scale marker experiment in block

caving. The experimental marker recovery results for the near field provides the first insights into the

development of the extraction zone during undercutting and subsequent draw. The results highlight

early material recovery in the vicinity of the major apex, which expands towards the centre of the

monitored drawbells as more tonnes are drawn. In addition, marker recovery is not spatially uniform

during material extraction, indicating disturbed flow behaviour. This type of behaviour significantly

deviates from conventional flow theory based on numerical models and scaled physical models using

narrow distributions of idealised particles or crushed aggregates. A conceptual model is proposed to

describe the basic flow mechanisms associated with the observed near field flow behaviour. This

model incorporates concepts associated with concentration driven segregation and disturbed flow

kinematic and static disturbances.

7. Acknowledgements

The authors wish to thank Newcrest Mining Limited (NML) for the support and permission to publish

this paper. The initiation and implementation of this project would not have been possible without the

assistance of David Finn, Geoff Dunstan, Stephen Duffield, Geoff Capes, Michelle Morgan, Robert

Lowther, Luca Popa, Tim Thornhill, and Joseph Emmi.

8. References

Brunton, I. D., 2009. The impact of blasting on sublevel caving material flow behavior and recovery.PhD thesis, University of Queensland, 2009, p 562.


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de Silva, S., Dyroy, A., and Enstad, G. G., 2000. Segregation mechanisms and their quantification

using segregation testers. In AD Rosato and DL Blackmore (eds), IUTAM Symposium on

Segregation in Granular Flows, Kluwer Academic Publishers, Boston, pp 11-29.

Elexon, 2011. http://www.elexonelectronics.com/

Gustafsson, P., 1998. Waste rock content variations during gravity flow in sublevel caving: analysis

of full scale experiments and numerical simulation. PhD thesis, Lulea University of Technology,

Sweden, p 228.

Janelid, I., 1972. Study of the gravity flow process in sublevel caving. In International sublevel caving

symposium, Atlas Copco, Stockholm, p 23.

Mosby, J, de Silva, S.R., and Enstad, G. G., 1996. Segregation of sugar during flow. Transactions of

ASAE, vol. 41, no. 5, pp 1469-1476.

Newcrest, 2007. Ridgeway Deeps geotechnical feasibility report. NML internal report, p 94.

Newcrest, 2011. http://newcrest.com.au/projects.asp?category=2

Power, G. R., 2004. Modelling granular flow in caving mines: large scale physical modelling and full

scale experiments. PhD thesis, University of Queensland, Brisbane, p 303.

Sharrock, G. B., and Hashim, H., 2009. Disturbed Flow in Block Caving, Proc. American Rock

Mechanics Symposium, Asheville, USA, July 2009, p12.

Sharrock, G.B., Beck, D.A., Capes, G.W., Brunton, I.D. , 2012. Applying coupled Newtonian Cellular

Automata-Discontinuum Finite Element models to simulate propagation of Ridgeway Deeps Block

Cave, Proc. Massmin 2012 Conference, Sudbury.

Smart, G., and OSullivan, T., 2006. Local scale estimation of sublevel cave stocks is it possible? A

case study in recon-ciliation of metal production Ridgeway Mine, New South Wales. In Proceedings

6th International Mining Geology Conference. Melbourne: Australian Institute of Mining and

Metallurgy, pp 323-33.

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Full Scale Near Field Flow Behaviour at the Ridgeway Deeps Block Cave Mine