Blast damage control Blast damage control measuresmeasures

In mining, damage to a rock mass is unavoidable. The objective is to minimise its extent, severity and its impact

The Paradox:We want to totally destroy the competence of one area without damaging the neighbouring area !

Methods to control underground blast damage

• Compliance with smooth wall blasting techniques

• Develop design parameters to suit prevailing conditions

• Accurate drilling of holes• Implementation of Quality Control

and Quality Assurance practices

Development blasting - terminology

Perimeter/Contour holes

Lifter holes

Cut holes(Relief and charged holes)

Easer holes(stoping holes)

Driv

e he

ight

Centre line

Grade line

Drive outline (toe)Drilling offset

Knee Holes

Perimeter easer holes

Smooth wall blasting

• A method where the row of holes adjacent to the planned contour is fired at the end of the round, with a light charge, with a small spacing in comparison to the burden

• Care must also be taken to the potential damage caused by adjacent rows to the contour (inner easer holes)

Smooth wall blasting - general rules

• Explosive energy per meter of blasthole reduced• Blasthole spacing is approximately 75% of the

spacing for stripping holes within the round• Blasthole burden is approximately 1.1 to 1.4 times

the spacing around the perimeter (pre-splitting effect)

• Smooth blasting charges should ideally be initiated simultaneously, to create a clean break between blastholes

Design guidelines for perimeter blasting

Persson et al 1994

In smooth blasting S/B ratios of 0.8 are generally used

Blasthole deviationVariability in the toe position for a

round with a total drill deviation of 2 degrees

Variability in the toe position for a round with a total drill deviation of 4

degrees

Dyno Nobel (2005)

Inaccurate drillingCumulative drilling errors

Rock excavation handbook - Tamrock

Sandvik drilling control system

• KEY FEATURES – Rock detect

• Drifter moved forward with controlled speed and low power until solid rock contact

• Accurate alignment of holes and less stress on drill steel

– Collaring automatics• After rock detect, collaring proceeds with

collaring pressures to set collaring depth• Then power ramped to normal drilling

pressures– Adjustable feed controlled percussion

• Optimized feed force to meet all percussion levels

– Sensitive anti-jamming automatics• Safe and fast drilling even in poorest

rock conditions

Atlas copco ABC system

Decoupled charging of perimeter holes

Decoupled charging of perimeter holes

QA/QC

• A formal set of procedures that allows the systematic collection of data that supports the implementation of recommended designs for different environments

• The main components are– Site inspections and data logging– Data storage and management – Data analysis and reporting

(feedback to miners)

As drilled round – collar positions

Photo courtesy of De Beers mines

As drilled round – hole geometry

Wetherelt and Williams, 2006 – Fragblast 8

Initiation & Sequencing

Electronic Detonators Vs Pyrotechnic

Experimental Work by Ichijo et al ,1994

Cross-Sectional Area 8m2Drill hole spacing 450mmLength of line of least resistance 450mmDrill hole diameter 42mmNumber of holes per round 59Explosive type DinamaitoVOD 6000-7000 m/sCharge length 2 - 2.5m

SG UCS (MPa) TensileStrength(MPa)

E (GPa) Poisson'sratio

Vp (m/s)

2.66 300 12 73 0.24 5800 100

Rock conditions

Initiation & Sequencing

Electronic Detonators Vs Pyrotechnic

Open pit control measures

• Several different techniques are used to reduce blast induced slope damage. They include:

– trim blasting;– buffer blasting;– pre or mid split blasting;– post split blasting; and– line drilling.

(Chitombo, Onederra and Scott, 2006)

Trim blasting

• In trim blast designs, production blast designs are modified to reduce wall damage. The common modifications are:

– a free face is created for horizontal relief;– the pattern width is reduced to three to six rows deep;– the delay sequence is modified to control vibration levels

and displacement; and– sub-drilling is reduced or eliminated above the catch

berm.– The last row of holes is placed in front of the designed

batter face. This is known as the standoff distance.

(Chitombo, Onederra and Scott, 2006)

Trim blast design featuresBickers et al , 2001

Buffer blasting

• In buffer blasting the relationship between explosive energy distribution, confinement and level can be enhanced with the use of airdecks, pattern modifications and/or reduced hole diameters

• Buffer blasts are typically three to five rows wide and shot to free face that has a consistent burden. In some adverse geology, additional rows may have to be added to the blast to protect the slope from damage caused by the production blast

• Batter face angles between 60º and 75º degrees are fairly typical for cushion blast designs

(Chitombo, Onederra and Scott, 2006)

Buffer and production blast

6 m

7 m

4 m

5 m

1,5 m

(Enaex, 2001)

165 mm

Buffer and production blast

J = 1.5 m

Stemming - 4 m

Anfo- 68 Kg4 m

Blendex 950 - 98 Kg3.5 m

10 m

6 m

1.5 m 5 m

Buffer

B940 - 51 Kg2 m

Air deck3 - 5 m

Air bagat 4 m

(Enaex, 2001)

Buffer blast design guidelines

Blasthole Diameter (mm)

Charge (kg/m)

Burden (m) Spacing (m) * Offest From Toe (m)

76 0.6 1.7 1.4 0.3089 0.75 1.9 1.5 0.36102 0.9 2 1.7 0.41114 1.2 2.4 1.9 0.46127 1.4 2.6 2 0.51153 2 3.1 2.4 0.61165 2.3 3.3 2.6 0.66200 3.4 4.1 3.1 0.80229 4.6 5 3.5 0.92270 6 5.5 4.1 1.08311 7.8 6.1 4.8 1.24

* spacing may need to be half the inner buffer row spacing to aid with pattern tie-in.

(Chitombo, Onederra and Scott, 2006)

Pre or mid split blasting

• Pre or mid split blasting involves drilling a row of closely spaced holes along the designed dig limit. These holes are loaded with decoupled charges to split the gap between holes in tension without causing compressional damage to the slope

• Pre-split differs from mid-split blasting only in the way the holes are initiated. Ideally, the pre-split will be fired before the holes in the adjacent blast are drilled.

• If the time between the detonation of the pre-split and adjacent holes is too great, the performance of the explosive in the adjacent holes can be adversely affected. As a result, a mid split (shot in the “middle” of the timing sequence) is timed to be fired a short time (around 100 ms) before the detonation of the adjacent holes.

(Chitombo, Onederra and Scott, 2006)

Mid split sequencing

point ofinitiation

(Floyd, 2006)

Combined pre-split, buffer and production blast patterns

2 - 3 m

6 m

7 m

4 m

5 m

3 m

(Enaex, 2001)

165 mm

115 – 165 mm

Combined pre-split, buffer and production blast patterns

J = 1.5 m

Stemming - 4 m

Anfo- 68 Kg4 m

Blendex 950 - 98 Kg3.5 m

10 m

6 m

3 m 5 m

Buffer

Anfo - 100 Kg6 m

2 m fromcollar

(Enaex, 2001)

Favourable pre-split conditions

• massive rock;

• tight joints;

• dominant joint orientation more than 30º off strike of designed face; and

• absence of weak structures that form wedges of daylight on the batter face and catch berm.

(Floyd, 2006)

Preliminary pre-split guidelines

Blasthole Diameter (mm)

Charge (kg/m)

Spacing (m)

Minimum Decoupled Charge Diameter (mm)

Maximum Decoupled Charge Diameter (mm)

76 0.5 1.1 22 2589 0.6 1.2 22 29102 0.7 1.4 25 32114 0.8 1.6 32 38127 0.9 1.8 32 44153 1.1 2.1 38 51165 1.2 2.3 44 51200 1.4 2.8 51 64229 1.6 3.2 64 76270 1.9 3.8 68 89311 2.2 4.4 78 103

(Chitombo, Onederra and Scott, 2006)

Pre-split loading options

Presplit Loading Options

singlecharge

bulkexplosive

multiplecharges

bulkexplosive

multiplecharges

decoupledcartridgeexplosive

continuousdecoupledcartridgeexplosive

nostem

airdeck

plugcharge

charge

nostem

charge

charge

charge

charge

charge

continuouscharge

increasing performance in unfavorable geology

(Floyd, 2006)

Pre-split fracture

Other options - post-split blasting

• Post-split blasting utilises a closely spaced, lightly charged row ofblastholes that is placed along the designed batter face. As opposed to pre-splitting, the row of holes is shot after the adjacent blast. In highly fractured rock, post-split holes have more relief and typically cause less damage to the slope

Blasthole Diameter (mm)

Charge (kg/m)

Spacing (m)

76 0.8 1.289 0.9 1.4102 1.0 1.6114 1.1 1.8127 1.3 2.0153 1.5 2.4165 1.6 2.5200 2.0 3.1229 2.3 3.5270 2.7 4.2311 3.1 4.8

(Chitombo, Onederra and Scott, 2006)

Other options - line drilling

• Line drilling consists of a line of unloaded holes drilled alongthe final limit.

• In weak material, the hole spacing is typically around 12 hole diameters. Hard massive rock requires the spacing to be reduced to three to six hole diameters.

• Initially, the buffer row should be placed 50% to 75% of the normal burden away from the line drill row. If the ground is saturated the burden will need to be increased to prevent overbreak.

• Line drilling is usually most cost effective in weakly cemented alluvium

(Chitombo, Onederra and Scott, 2006)

Line drilling weak alluvium

(Floyd, 2006)

Crest damage(Bye, 2006)

Structurally controlled Sub-drill / blast damage

Drill offsets to protect crest

standoffzone

0 1 2-1-2

0

Horizontal Offset From Crest (m)

blastholelocations

1

2

desiredcrest

-3

(Floyd, 2006)

GEOTECH ASSISTENT GEOTECH ASSISTENT

GEOTECH

SENIOR GEOTECH

ASSISTENT BLASTERSX 2

DRILL ASSIT. DRILL ASSIT.

DRILL OPERATOR

DRILL ASSIT. DRILL ASSIT.

DRILL OPERATOR

DRILL ASSIT. DRILL ASSIT.

DRILL OPERATOR

DRILL ASSIT. DRILL ASSIT.

DRILL OPERATOR

DRILLING FOREMANOR

DRILLING CONTRACTOR

SCALING OPERATORS4 SHIFT

BLASTING TECHNICIANWALL CONTROL SUPERVISOR

WALL CONTROL TEAM

PLANNING•Trims

•Presplits

•Schedules

SURVEY•Monitoring

•Staking

•Limits

(Bye, 2006)

Wall control management

Limit Blast Assessment

(Bye, 2006)

Hazard Plans(Bye, 2006)

Final wall scaling(Bye, 2006)

(Bye, 2006)

Limit blast design process and responsibilities (Mt Whaleback)

Bickers et al , 2001

Practical damage assessment (Mt Whaleback)

Bickers et al , 2001

Open discussion

• Underground– EDD versus Pyrotechnics – Blast damage from conventional undercutting– Blast damage from drawbell blasting– Single firing versus phased blasting of drawbells

• Open pit– Damage from EDD production blasting– Trim versus buffer blasts– Pre-splits

• Gas or shock effects• Angled versus vertical• Single bench versus double bench versus triple bench• Firing sequence (blasting with trim)• Influence of joint orientation

– Use of large diameter holes for contour blasting (> 165 mm)