Upload
panxopaula-zepeda-santos
View
215
Download
0
Embed Size (px)
Citation preview
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 action
3. Ground material lies in muck pile voids
4. Water mixes with material to form mud
5. Mud is drawn out or flows out
6. Mud can be forced out by collapsing muck pile voids
shaleRain water
Draw point
Rock
Mud rush
Shale weathers and slakes in slope, and
sloughs
Waste capMudpocket
Shale weathers further in pit bottomGroundwater flows towards waste cap
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
Tailings saturated muck pile
Drawing of dam base material causes plugging of muck pile voids and prevents tailings drainage
Water
Mud plug discharged ahead of tailings causing air blast
Dam base material
Surface
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 ?