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Pre- Conditioning course
Prepared by DP Venter from pre-conditioning booklet as well as AEL presentation slides.
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Facebursts
What is a “faceburst”?A “faceburst” is defined as the ejection of rock from mining faces at a very high velocity into the mine workings due to the violent release of
accumulated strain energy.
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Rock types and strengths
VCR Reef 280 MPa
Lava 320 MPa
Quartzites 240-260 MPa
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High stress on face -facebursts
High vertical stress
Face deforming resulting in face burstswhen energy is released violently
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PreconditioningWhat is preconditioning?Preconditioning, also called “destress blasting”, is a rockburst control technique that regularly involves setting off designed blasts ahead of the stope face. The blasts dissipate energy by promoting slip on fractures, which transfers the stresses further away from the stope face (Figure 1). This is achieved by remobilising the existing fractures in the rockmass so that strain energy is not accumulated immediately ahead of the working face. When the preconditioning blasts redistribute stress away from the working face, a cushion of stress-relieved rock ahead of the stope face is provided, and this is able to absorb energy from distant events.
stress profilebefore preconditioning
preconditioned
after
stress profilebefore preconditioningstress profilebefore preconditioning
preconditioned
after
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Pre-conditioned face
Pre-conditioned hole
Face fractured and cannot burst
High vertical stress
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Face conditions –without preconditioning
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Face condition with preconditioning
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Benefits of pre-conditioning
What are the benefits of pre-conditioning?The benefits of preconditioning are as follows:minimised/eliminated faceburst” hazard (no faceburst incident has been reported from panels where preconditioning was regularly and properly implemented);reduced damage by distant seismic events;reduced accident/injury rate (seismicity, f.o.g.);improved H/W conditions better stoping-width control;improved face conditions (clean and straight break, no sockets);improved hangingwall fracturing (less shallow dipping fractures); increased face advance per blast, therefore greater productivity and production bonuses;increased drilling rate;reduced fragment size of blasted reef, therefore easier cleaning;reduced dilution;reduced stoping cost; and
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STRIKE GULLY
PRECONDITIONING CYCLE
DAY 1
DAY 2
DAY 3
DAY 4
3 m
Unmined
Methods of preconditioingFace-Perpendicular Preconditioning
The face-perpendicular preconditioning technique involves the drilling and blasting of face-preconditioning holes as an integral part of the actual
production blast cycle. Face perpendicular preconditioning is suited to normal production faces as it integrates well into the mining cycle.
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GULLY 1
GULLY 2P
AN
EL
1
PA
NE
L 2
PA
NE
L 3
PercussionDrill Rig
Pre
co
nd
itio
nin
g H
ole
PreviousPreconditioningHole
3.5 to 5.5m
UNMINED
Methods of preconditioning cont.Face-parallel preconditioning layout in an overhand mining layout (not to scale) As shown in this figure, Panel 2 has been mined to the limit of effective preconditioning, i.e. the position of the previous preconditioning hole. The next hole must be drilled between 3.5 m and 5.5 m from the current face. The hole must be kept parallel to the face to ensure a consistent preconditioning effect along the entire length of the face
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Beast panels
MINING DIRECTION
DIP
UNMINED
(D1)
(D2)
(D4)
(D5)
(D3)
APPROVED BY:
SECT HEAD R/E
ASST. MINE MNGR.
MINE MANAGER
DATE
Layout of Preconditioned holesBreast Panel
DAY 1 (D1) DAY 2 (D2) DAY 3 (D3) DAY 4 (D4) DAY5 (D5)
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Updip hole positions
Updip Panel
APPROVED BY:
SECT HEAD R/E
ASST. MINE MNGR.
MINE MANAGER
DATE
Laout of Preconditioned HolesUpdip Panel
DAY 1 (D1) DAY 2 (D2) DAY 3 (D3) DAY 4 (D4)
DIP
MINING DIRECTION
UNMINED
LAYOUT OF PRE-CONDITIONED
UP-DIP PANEL
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Face drill holes
Pre condition and blast holes
2.0 - 3,5 MM4,5m 3m(M
ax)
Blasting holes
Preconditioning holes
2.0 – 3.5 Metres
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Section showing Blast & preconditioning holes
Tamping must be used
Face Blast Hole Fuse inserted in first capsule
Preconditioning Hole Fuse inserted in last capsule
Face Blast Hole Fuse inserted in first capsule
Fuses in the blast & preconditioning holes must almost be lined up vertically.
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Priming and tamping
SIDE VIEW SHOWING TOP PRIMING AND TAMPING
TAMPING CLAYPRODUCTION
HOLESEXPLOSIVES
PRIMER (TOP)
HANGING WALL
FOOTWALL
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Blasting of pre conditioned holes
FRE
E
FACE
DIRECTION OF BLAST
Assume to be three meters Assume to be three meters
Green Line - Extenders with HandiMastersYellow Line - HandiMastersOrange Blocks - HandiMaster Blocks 200ms delay
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Reverse timing - block pattern with pre-conditioning in slow motion (From AEL)
Free
face
Direction of blast
Assume to be three meters Assume to be three meters
Blue line - Extenders with REEFMASTERSYellow line - REEFMASTEROrange blocks - REEFMASTER blocks 200m/s delay
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Reverse timing - block pattern with Pre-conditioning in fast mode (From AEL)
Free
face
Direction of blast
Assume to be three meters Assume to be three meters
Blue line - Extenders with REEFMASTERSYellow line – REEFMASTERSOrange blocks – REEFMASTER blocks 200m/s delay
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Free
face
Blue line - ExtendersYellow line - REEFMASTERSOrange blocks – REEFMASTER blocks 200m/s delayRed holes - Production holesBlue holes – Pre-conditioning holes
Reverse timing - block pattern with pre-conditioning in 3D ( From AEL)
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Free
face
Blue line - ExtendersYellow line - REEFMASTERSOrange Blocks – REEFMASTER blocks 200m/s delayRed holes - Production holesBlue holes – Pre-conditioning holes
Reverse timing - block pattern with pre-conditioning in 3D fast mode (From AEL)
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Free
face
Blue line - ExtendersYellow line – REEFMASTERSOrange blocks – REEFMASTER blocks 200m/s delayRed holes - Production holesBlue holes – Pre-conditioning holes
Reverse timing - block pattern with pre-conditioning in 3D slow mode (From AEL)