CAVE RISKCAVE RISK

CONTENTSCONTENTS

• Types of risk• Major operational risks• Black hat action strategies

Types of riskTypes of risk

• Heslop 2000• Design risks• Operational hazards• Draw risks• Technology risks

Design risksDesign risks• Economic effect• Based on incorrect

geotechnical data• Above collected years before• Poor assessment of ground

conditions• Common example

- partially logged core (eg UCS & RQD)- Forced to use because of lack of funds- Results other parameters have to be guessed- uncertainty in design parameters- Under support of drifts by 25%

Common geotechnical failuresCommon geotechnical failures• No study done• Study missing• Lack of detail for study level• Detail default• Scoping study detail accepted for

DEFS• Classification design used only• Not addressing the key issues

mentioned at earlier stages• Conceptual stage not correct• Not understanding implications of

recommendations• Not understanding design philosophy• Not reading study reports

- 80% do not read report- 15 % executive summary- 5 % report

Practical measuresPractical measures• Focuss study/ reasonableness

test- Review and compile other studies- Read extensively ( take 1 week)- Literature study- Bench mark - develop feel for the project- Findings

• Find out ( Summers 2000) :- What we know- What we don’t know- What we know we don’t know

• Study standards (AusIMM)• Unresolved issues

Unresolved issuesUnresolved issues• Aspects that can not be resolved

during the study• Special bridging studies (i.e.

caving )• Rule for special studies

- Impact on project greater than contingency- Infrastructure stability- Cavability- dilution and dilution blankets

• If no cash or time for special study- Run project on blue sky- Sensitivity on worst case- opinion required

Operational risksOperational risks

• Heslop 2000• Loss of life• Close mine or section of the mine• Rock bursts• Air blasts• Mud rushes/floods

Rock/ strain burstingRock/ strain bursting

• Stress increases as undercut is developed.

• Stresses increases further before propagation.

• Perimeter stresses increase with area undercut and height of undercut

• Rock/ strain burst damage greatest before propagation

• Rock bursting related to caving• Rock bursting related to

management of caving process

Rock burst cave prone cavesRock burst cave prone caves

• > 300- 400m• High tectonic stresses• Mountains• MRMR >35• Difficulty in propagated the cave• Rapid rates of advances of pre-break/UC• El- Tiente 1976

Rock burstingRock bursting• Fractured zone exists around the

cave• 50m from the cave face• As the cave progates this zone

increases• Fracture zone increase due to

stress relaxation• During this stages areas up to

500m from cave effected• Strain bursts can occur in places

like crusher, plats, cribb rooms• Strain bursting, ejection

Rock burstingRock bursting

• Bursts occur with cave propagation• High levels of seismisity before propagation.• Mag of up to Richter 3.6• Large caves have up to 6 large bursts if not managed• Seismic events during propagation are shear related

Common areas for burstingCommon areas for bursting• 20 to 40m from cave face/front• Strain bursting and rock ejection at

breakaways/ intersections up to 500m from cave

• 100m below the cave• Large excavations with 50m from

ore body• Pillars between conferencing cave

fronts• Before propagation.

Rock burst prevention strategiesRock burst prevention strategies

• Mesh walls/ backs of intersections/ large excavations 500m from cave

• Cone bolt intersections/ breakaways

• Install yielding support within 40m from propagation face position

• Reduce block cave draw rates <2000 /tons/day before propagation

• Reduce block cave/ SLC pre-break advance rates below 3000m2 /month.

• Propagate the cave first.• SLC < 45m vertical/m/yr• Mine towards the solid

Major air blastsMajor air blasts

• 4 conditions• A gap between the muck pile

and cave back• Caving has slowed or stopped• A large volume of cave back

must be unstable• Air escape routes

GeotechnicalGeotechnical

• Hard intrusive through the ore body• Hard ore zones above uc level• Necking or tapering of ore body• No pit subsidence• History of contact ore wedge formation• History of stall• Hard caves

Muck pile Cave gapMuck pile Cave gap• No real hard rules• Experiential• Criticized• Old mining rule 200mm/ day draw

rate• Realistic cave back gaps are

between 2m to 15m• 2m to 5m good for normal

production• 15-30m draw rate greater caving

rate• 30m> problem of draw correction/

air blasts• 30m-50m of muck pile• All above wrong if tunnels into

cave

Cave stallCave stall• Caving stops or slows• Vertical MRMR > than UC HR• Caving hoop stresses strength

rock mass• Block height 2x footprint width • Draw rate greater than caving rate• Hard caves < 44mm/ day• Soft caves < 75mm/ day

Type 1 collapse (Bell 1999)Type 1 collapse (Bell 1999)

MUCKPILE

AIR GAP

Surface

Back Fails

Type 2 collapse (Bell 1999)Type 2 collapse (Bell 1999)

MUCKPILE

AIR GAP

Surface

Block falls from back

Type 3 (Flores 1993)Type 3 (Flores 1993)

wedge

STRUCTURE

Air sources & escape routesAir sources & escape routes

• 2 main air sources routes- air in muck/cave back gap- air in muck pile voids

• Air escape routes- Via undercut draw points- Muck pile to draw points- Old workings holed in cave

effectseffects

• Loss of life• Sudden subsidence• Collapse of block contact area• Point loading on drifts• Hang-ups• Destruction and damage to services• Loss of production• Drift rehab

PreventionPrevention• Define rock mass condition 100m vertical intervals• HR designed hard geotechnical unit• Major wedge structure assessment• Correct definition of draw rate• Keep draw points full• Good draw control• CSM cave if possible / 3d draw profiles• Plug all tunnels into cave• Site plugs outside break back zone• Tele-remote draw points• Draw points full• Monitor cave back• Evacuate mine in failure/ danger times

MUD RUSHESMUD RUSHES

• Butcher (1999-2000)• Effects

- Loss of life- Loss of mine- Production loss- Air blasts- major rehab

Mud rush conditionMud rush conditionWater

Mud materials

Discharge point

Disturbance

Mud forming materialsMud forming materials• Cave wastes with shale• Tailing/ slimes or mud deposited in glory hole• Weathered overburden/ soil• Suspected TKB Kimberlite material• Gypsum/ clay minerals• Fine cave waste

- 20% sand size material- Moisture > 8.5%

MUD

Mud forming mechanicsMud forming mechanics

1. Shale/ mud forming material drawn into cave.2. Above material is ground down muck pile action3. Ground material lies in muck pile voids4. Water mixes with material to form mud5. Mud is drawn out or flows out6. Mud can be forced out by collapsing muck pile voids

Discharge pointsDischarge points

Micro design: Extraction level layout TYPE 1 DRAWPOINTS

Mud transport excavations

MTE’sMTE’s

• Mud transport excavations• Mufairia- more people killed from MTE cut-offs• Cut-off on lower levels and behind• MTE’s

- Ore passes- RAP’s- Declines- Accesses

FactsFacts• Tailings inrushes greater

danger• Wide draw points greater

discharge• SLC poor control• 2 mining blocks lost with SLC• Poor muck pile drainage

greater danger• Air blasts occur after mud

rushes• Flooding occurs • Destruction of services

Mud rush preventionMud rush prevention

• 3D’S PRINCIPAL• Distance- keep mud forming materials out of

muck pile• Drain- keep muck piles dry• Draw –good draw control standards

MeaningMeaning• Good draw control

- Uniform draw down- draw discipline

• Drainage• - Mine pumping

- Surface pumping- keep underground workings dry

- Prevent inflows in muck pile from underground and surface- Hydro-geological study

• Distance - bottom-up caving methods ( reduce SLC’s )

- Site tailing/slimes dams away from glory hole- Do not dispose of mining waste in glory hole- remove mud forming material if possible- correct open pit slope design

MeaningMeaning• Use small draw point size (3.8m x3.8m)• Seal old passes and workings• Install more pumps• Install water prevention stalls• Mud rush alarms• Monitoring pumping levels• Lower in pumping means water in muck pile• Tele-remotes• Closed bogger cabs with oxygen• Re-inforced bogger cabs• Flood prevention if main pumps destroyed

BLACK HAT ACTION STRATEGIESBLACK HAT ACTION STRATEGIES

• OPERATION GEOTECHNICAL RISK ASSESMENT

• MOST NEGATIVE ASPECTS• AMERIORATION STRATEGIES• NOW GET OUT OF THIS• NORMAL RISK ASSSESMENT

PROCESS- Hazard- Likelihood- Consequences- mitigated by- Residual risk

SLC BLACK HATSSLC BLACK HATS

• Drift instability• Isolated draw• Rock/ strain bursting around

footprint• Infrastructure threaten by glory

hole break backs• Mass dilution ingress• Drift access collapse• Contact collapse

Block cave black hatsBlock cave black hats• Undercut crushing/ bursting• Undercut stress induced

damage• Point load destruction of drifts• Footprint perimeter rock strain

bursting• Excessive drift repair• Cave/ muck pile stall• Cave infrastructure break back

DISCUSSIONDISCUSSION

• What Are the causes ?• Answers to problems ?• Answers realistic ?• Answers tactical ?