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ROCK MECHANICS – THE BASIC MINING SCIENCE: CHALLENGES IN UNDERGROUND

MASS MINING

E T Brown

2007 Müller Lecture

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Brian FlowersJames FootsCharles FairhurstEvert HoekJohn JaegerManuel RochaDoug StewartHugh Trollope

Elders and betters, mentors and teachers

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Theme 7 Panel, First ISRM Congress, Lisbon, 1966

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Barry BradyJohn BrayGideon ChitomboCharles FairhurstGermán FloresNeal HarriesEvert HoekJohn HudsonAntonio KarzulovicTao Li

Colleagues and former students

José LobatoRob MorphetDon McKeeStephen PriestKevin RosengrenErnesto VillaescusaJohn WatsonChris WindsorJian Zhao

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Sten BjurströmJohn FranklinMaria de Lurdes EusébioNuno GrossmanArnaldo SilvérioKalman KovariShun SakuraiHorst WagnerWalter Wittke

ISRM colleagues

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First ISRM Congress, Lisbon, 1966

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ROCK MECHANICS – THE BASIC MINING SCIENCE: CHALLENGES IN UNDERGROUND

MASS MINING

E T Brown

2007 Müller Lecture

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Orebodies at the Neves Corvo Mine, Portugal

ZAMBUJALLOWERCORVO

NEVES SOUTH

NEVES NORTH

UPPERCORVO

CORVO SEZINC

GRAÇA SWZINC

GRAÇA SE

ShaftPillar

Backfill (Nov.04)

Copper Ore

Zinc Ore

Copper Ore (Pillar)

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Underground ore loading at the Neves Corvo Mine, Portugal

700 HAULAGE LEVELTORO 40 LOADING

AT ORE PASS10

Overview of modern mass miningIntroduction to block and panel caving (BPC)Industry trends in BPC, including the next generation of “super caves”Rock mechanics challengesImplications for the discipline of rock mechanics

Outline

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Mining by block and panel caving, sublevel caving, and open and sublevel stoping, and their variants; and

Production rates of at least 30,000 tonnes of ore per day, or about 10 million tonnes per year.

Underground mass mining involves

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Evolution of daily production rates at selected large underground mines

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1880 1900 1920 1940 1960 1980 2000 2020 2040

YEARYEAR

TON

NES

PER

DA

YTO

NN

ES P

ER D

AY

Bingham Canyon

Climax

Salvador

Kiruna

Mount Isa

San Manuel

El Teniente

MiamiRidgeway & RWD

Olympic DamAndina

Freeport IOZ/DOZ

Henderson

MalmbergetPalabora

PremierKidd Creek

CEU

El Teniente

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40000

60000

80000

100000

120000

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1880 1900 1920 1940 1960 1980 2000 2020 2040

YEARYEAR

TON

NES

PER

DA

YTO

NN

ES P

ER D

AY

Bingham Canyon

Climax

Salvador

Kiruna

Mount Isa

San Manuel

El Teniente

MiamiRidgeway & RWD

Olympic DamAndina

Freeport IOZ/DOZ

Henderson

MalmbergetPalabora

PremierKidd Creek

CEU

El Teniente

Grasberg UG/ Resolution

Chuquicamata UG

Bingham Canyon UG

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Andina Sur Sur underground project, ChileChuquicamata transition project, ChileEl Teniente New Mine Level, ChileGrasberg, IndonesiaOyu Tolgoi, MongoliaResolution, USA

Some planned “super caves”

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Known operating and planned block andpanel caving mines

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Open pit and block cave, Palabora Mine,South Africa, April 2004

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Unfavourable economics of further open pit cut-backs

Inability to ensure the continuing stability and safety of high open pit slopes

Adverse environmental impact of further open pit development

Cost and productivity advantages of BPC over other underground mass mining methods

Reasons for a transition from open pit tounderground mining by block and panel caving

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Outline

John O’ReillyRio Tinto, 2004

“Projections based on current reserves show that in about 15 years’ time, Rio Tinto could be producing

more copper from underground than from open pits”

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Classification of underground mining methods (after Brady & Brown 2004)

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Schematic showing the essential features of block caving

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Mechanised panel caving(Flores 2005)

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Late 19th century: Precursor to modern block caving developed in the iron ore mines, northern Michigan, USAEarly 20th century: The block caving method developed in the USA for iron ore and then copper mining in the western states1920s: Block caving started in Canada and Chile Late 1950s: Block caving introduced into southern African diamond mines and then chrysotile asbestos mines

Historical development of block and panel caving

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Late 1960s: LHD vehicles developed for underground mining1970: LHDs used with block caving at El Salvador mine, Chile1981: Mechanised panel caving introduced in the primary ore at El Teniente mine, Chile1990s: Planning of the new generation of block caves with larger block heights in stronger orebodies (e.g. Northparkes, Palabora)2005 on: Planning the next generation of “super caves”

Historical development of block and panel caving (continued)

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Requires an orebody of large horizontal and vertical extent with well disseminated mineralisationMay be used in thick flat-lying deposits, steeply dipping veins of sufficient width and diamondiferous pipesThe lateral extent of the orebody must be large enough to allow caving to be initiated This width will depend on the strength of the orebody– traditionally weak, but now may be strongThe height of the ore column should allow adequate production lives of individual drawpoints and an acceptable rate of return on development and production costsBecause production costs are low, BPC may be used for lower grade orebodies than other underground mass mining methods

Conditions under which BPC may be used

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Application to stronger, less easily cavableand more coarsely fragmenting orebodiesApplication to deeper orebodies (including “blind” deposits) under higher stressesApplication to lower grade orebodies than those mined by other underground methodsLarger block heightsIncreased automation towards the “underground rock factory” concept and miner-less mining

Trends in block and panel cave mining

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New ore handling philosophies and underground layoutsTransitioning from large open pits to underground mining by BPC methodsGreater risks and an increased emphasis on “getting it right” in early decision makingChanging demands on, and skill sets of, managers, engineers and operators

Trends in block and panel cave mining (continued)

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Their massive scale and the associated project management challengesVery long lead timesHigh capital costsLarge amounts of pre-production development required with long design livesHigh in situ rock temperatures at depth and the associated ventilation and refrigeration requirements

Some engineering challenges in the next generation of super caves

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The desirability of no entry mining and the automation of excavation and productionThe development of new and improved methods of working and productionWater and environmental managementUnderground communications and control systems

Some engineering challenges in the next generation of super caves (continued)

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Key block cave design activities

GEOLOGICAL BLOCK MODEL

GEOTECHNICAL CHARACTERISATION

Cavability Assessment

Fragmentation Assessment

Caving Performance Assessment Including Undercutting

CONCEPTUAL MINE DESIGN

Operational Hazard Assessment

Initial and/or Trial Excavation

FINAL MINE DESIGN

Implementation of Final Design

Monitoring During Construction and Mine Operations

Draw Trials

Draw Strategy

Monitoring

Excavation Stability Analysis

Ore HandlingSystem Selection

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Caving mechanics and cavabilityassessmentFragmentation assessmentCaving performance, including caving initiation by undercutting and continued propagationExtraction level stability under the range of stress conditions applying throughout the life of the caveAssessing the risk and ameliorating the effects of major operational hazardsSurface subsidence prediction

Key rock mechanics challenges

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Vertical section through a rock mass that is well-suited to gravity caving

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Conceptual model of stress caving(Duplancic & Brady 1999)

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Progress of caving, Northparkes E26 Lift 1

(van As & Jeffrey 2000)

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Schematic section showing the path of the Northparkes E26 Lift 1 air blast

(Ross & van As 2005)

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Evolution of fracturing in a SRM sample (Mas Ivars et al 2007)

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Advanced microseismic analysis in caving prediction

(ASC & Itasca 2007)

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Summary of preferential structures observed in SRM samples and microseismic clusters, Northparkes E26 Lift 2, Australia

(Reyes-Montes et al 2007)

SRM sample In-situ seismicity

Dom

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Professor Hugh Trollope AO FTSE

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Fragmentation at Salvador Mine, Chile

Coarse fragmentation at Salvador Mine, Chile

Good fragmentation at Salvador Mine, Chile

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FracMan model of a 50 m x 50 m x 70 m block, and identification of blocks connected to the undercut

(Rogers et al 2007)

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Extension of pre-existing discontinuities

Opening of filled or healed discontinuities

Opening of bedding or schistosity planes

Crushing of blocks under superimposed weight

Failure of blocks in compression arising from arching

Failure of blocks in induced tension arising from point

or line loading

Bending failure of elongated blocks

Abrasion of block corners and edges

Mechanisms of secondary fragmentation

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Drill and blast pre-conditioning trial, Panel III, Andina, Chile

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Caving mechanics and cavabilityassessmentFragmentation assessmentCaving performance, including caving initiation by undercutting and continued propagationExtraction level stability under the range of stress conditions applying throughout the life of the caveAssessing the risk and ameliorating the effects of major operational hazardsSurface subsidence prediction

Key rock mechanics challenges

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Cave initiation by undercutting: post-undercutting

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Remnant pillars on the undercut level withpre-undercutting

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Cave initiation by undercutting:advanced undercutting

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High extraction ratios and stressesVarying stress paths and stress levels throughout their service livesSeveral possible rock mass responses and failure modesStringent service requirements and long service livesDrawpoint and drawpoint brow stabilityDemanding service requirements of floors

Extraction level excavation design and stability issues

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Extraction level at the end of construction using an advanced undercut, DOZ mine, Freeport Indonesia

(Moss 2005)

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Subsidence resulting from block and panel caving, Salvador Mine, Chile

(Flores & Karzulovic 2002)

Crater perimeter

Caved rock

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Comparison of continuous and discontinuous subsidence(Kvapil et al 1989)

Continuous subsidenceDiscontinuous

subsidence

L1

HC

Original surface

Extracted block

L2

Steps

Exploitation of thin layer Exploitation of high block

HC

Crater with caved rock

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Block and panel caving subsidence terminology(Flores & Karzulovic 2004b)

ψb

δiz

Angle of breakInfluence zone

ψb ψb

Orebody

Stablezone

Influence zone δiz(continuous

deformations)Crater perimeter

(maximum extension of discontinuous deformations)

Cave material

Pre-existing open pit slopes

Undercut levelUnderground cave mining

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North-south vertical section, Palabora open pit and block cave, South Africa

(Moss 2005)

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The dip of the orebody

The plan shape of the orebody

The depth of mining and the associated in situ stress field

The strengths of the caving rock mass and of the rocks and soils closer to the surface

The slope of the ground surface

Factors influencing the angle of break

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Major geological features such as faults and dykes intersecting the orebodyand/or cap rock

Prior surface mining

The accumulation of caved or failed rock, or the placement of fill, in a pre-existing or a newly formed crater

Nearby underground excavations

Factors influencing the angle of break (continued)

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Indicative surface projection of the Resolution orebody

(Hehnke 2005)

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Surface features, Apache Leap, near Superior, Arizona

(Photographs: M Scoble)

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Resolution interpretive geology, East-West section (Hehnke 2005)

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Subsidence resulting from block caving operations, San Manuel Mine, USA

(Flores & Karzulovic 2002)

Fractured zone

Caved rock

Open pit

Crater perimeter

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0’ 600’300’

Depth (ft)

San Manuel subsidence reconstruction, July 1959 (Brummer 2006)

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San Manuel subsidence reconstruction, 1967 (Brummer 2006)

0’ 600’300’Depth (ft)

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San Manuel subsidence reconstruction, 1973 (Brummer 2006)

0’ 600’300’Depth (ft)

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3D subsidence reconstruction, 1955-73,San Manuel Mine, Arizona

(Brummer 2006)Buildings No. 1 and No. 4 Shafts

South Ore Body Crater

North Ore Body Crater

Northeast Ore Body Crater

Surface Cracking

2015 Grizzly & 2075 Haulage Levels

Mining Blocks

1415 (400 ft)

2015 (600 ft)

San Manuel, West & East Faults

1415 Grizzly & 1475 Haulage Levels

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Brown’s First Law of Caving Geomechanics

There are no easy answersThere are no easy answers

Corollary

There are no recipes or cook booksThere are no recipes or cook books

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The ISRM for honouring me with this award;Rob Morphet of Golder Associates, Brisbane, for support in the preparation of this lecture;Lorna Burns of Golder Associates, Brisbane, for preparing the PowerPoint presentation;several individuals and organisations for providing material used in the lecture;my Portuguese colleagues, particularly Professor Luis Ribero e Sousa, for their support and kind hospitality;my partner, Dale Spender, for her long-suffering tolerance of my interest in holes in the ground; andall of you for listening to me so patiently.

And finally, thanks to …..

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Glück Auf!