Pre- Conditioning course

Prepared by DP Venter from pre-conditioning booklet as well as AEL presentation slides.

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.

Rock types and strengths

VCR Reef 280 MPa

Lava 320 MPa

Quartzites 240-260 MPa

High stress on face -facebursts

High vertical stress

Face deforming resulting in face burstswhen energy is released violently

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

Pre-conditioned face

Pre-conditioned hole

Face fractured and cannot burst

High vertical stress

Face conditions –without preconditioning

Face condition with preconditioning

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

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.

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

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)

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

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

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.

Priming and tamping

SIDE VIEW SHOWING TOP PRIMING AND TAMPING

TAMPING CLAYPRODUCTION

HOLESEXPLOSIVES

PRIMER (TOP)

HANGING WALL

FOOTWALL

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

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

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

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)

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)

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)