BLASTINGBLASTINGFOR FOR REDUCED ROCK DAMAGE REDUCED ROCK DAMAGE AND AND CONTROLLING STABILITYCONTROLLING STABILITY

DAMAGE TO DAMAGE TO REMAINING ROCKREMAINING ROCK

Damage Resulting From Conventional Blasting

Overbreak atOverbreak atSIDES BACK BREAK UNDER BREAK & SIDES BACK BREAK UNDER BREAK &

BACK SHATTERBACK SHATTER

BLAST DAMAGESThe blast damage refer to any deterioration of the strength of the remaining rock/block due to the presence of blast induced cracks and extension of pre-existing or newly generated fractures.

Crushing Around Borehole Radial Fracturing Gas Pressure Internal Spalling Induced Strain Release of Load Fracturing

MECHANISM OF BLAST DAMAGE

DAMAGE FORMSDAMAGE FORMS Separation due to

breakage Increased fracture

frequency Degradation in

discontinuity surfaces Changes in the

aperture of discontinuity

Development of new cracks and their bifurcation

DAMAGE DAMAGE RESULTINGRESULTING

FROM FROM VIBRATIONVIBRATION

VIBRATION LEVELS

• Blast measurements adjacent to explosive charge are important.

• Semi-empirical methods of field measurements of peak particle velocity and rock parameters correlation.

FOR ROCK DAMAGE

FRAGMENTATIONFRAGMENTATION

• Ease of excavation• Transport of muck• Requirement of customer

With use of appropriatetechniques fragmentation can be improvedConventional Blasting

Use of appropriate technique

FACTORS AFFECTING FACTORS AFFECTING BLAST DAMAGESBLAST DAMAGES

ROCK PROPERTIES Dynamic breaking

strength: increase in DBS reduce over break

Structural properties: joint orientation, joint spacing

Rock Mass Rating (RMR) higher RMR are less prone to rock damage

CHARGE PROPERTIES Type of explosive, Charge configurationStrength of explosive as well as concentration & distribution of explosive within hole

HOLE SHAPEConventional holeNotching of blast hole

PROTECTIVE DEVICESDamages can be reduced by use of liner

DECOUPLING OF CHARGETo diminish excessive peak pressure

BLASTING TECHNIQUES FOR BLASTING TECHNIQUES FOR DAMAGE REDUCTION DAMAGE REDUCTION

Ensuring Adequate Burden Relief  Reducing Explosives Energy Concentration

Controlled Blasting Techniques

PARAMETERS IN PARAMETERS IN CONTROLLED BLASTINGCONTROLLED BLASTING

Precision In Drilling

Explosives Interval Timing Rock

Characteristics

PRECISION IN DRILLINGPRECISION IN DRILLING

Hole deviation and

collaring error

Spacing, burden

and their ratio

Shape of opening

Actual blast

geometry may

differ from design

ANFO, Slurry / Emulsion, Special Products for Controlling Damage.

Velocity of detonation

Decoupling ratio

Density of explosive

Charge concentration

per metre

Length of borehole

Shape of the charge

EXPLOSIVES

INTERVAL TIMINGINTERVAL TIMING

Overbreak is reduced if the burden is easily pushed forward

Gas confinement for less time

Each charge should have progressive relief of burden during the blast

Number of delays in perimeter holes

Delay scattering in detonators

Misfired holes

Number of delays in perimeter holes

Delay scattering in detonators

Misfired holes

INITIATING DEVICES

ROCK CHARACTERISTICSROCK CHARACTERISTICS

• ROCK STRESS• ROCK STRENGTH• ROCK STRUCTURE

Rock properties, structure and groundwater normally dominant in wall stability are not controllable.

REDUCING ENERGY REDUCING ENERGY CONCENTRATIONCONCENTRATION

Initiation Sequence Charge Distribution Hole Diameter Effective Sub Drilling

CHARGE DISTRIBUTIONCHARGE DISTRIBUTION

CHARGE DISTRIBUTIONCHARGE DISTRIBUTION

STEMMINGSTEMMING

With 115 to 152 mm holes,

2.5 to 4.5 m stemming columns employed

With 76 to 102 mm stemming

Stemming can be reduced to 1.5 m-2.5 m

CONTROLLED BLASTING CONTROLLED BLASTING TECHNIQUESTECHNIQUES

Line Drilling

Presplitting

Smooth Blasting

Cushion Blasting

Air Decking

Controlled Fracture Growth

Line drilling holes along the final excavation

BLAST HOLE LOADING SYSTEMBLAST HOLE LOADING SYSTEM FORFOR PRESPLITTING PRESPLITTING

Example of presplitting with and without presplitting

Presplitting in a blast

Preslitting 1

presplitting2

CUSHION BLASTINGCUSHION BLASTING

Closely spaced lightly loaded holes at the perimeter.

CONTROLLEDCONTROLLEDFRACTURE GROWTHFRACTURE GROWTH

Drill Hole Liners Metal Tube Plastic Pipe Card Board Tube

NOTCHED HOLES

BLASTHOLE LINERSBLASTHOLE LINERS

GI PIPE LINE

PAPER TUBE LINER

PVC PIPE LINER

CARDBOARD LINER

PLASTIC LINER

CARDBOARD LINER

AIR DECKINGAIR DECKING

Longer stemming in front and at the back

BLAST DESIGN BLAST DESIGN ANDAND IMPLEMENTATION IMPLEMENTATION

Fragmentation Process Rock Characteristics Explosives Initiation Measurements Before Blasting And Design

Implementation

Computer Aided Blast Design

MEASUREMENTSMEASUREMENTSBEFORE BLASTINGBEFORE BLASTING

Actual Blast Geometry May Differ From Design

Boretrak Blasthole Logger

Laser Profiler

ROCKFACE LASER ROCKFACE LASER PROFILERPROFILER

• Laser ‘scans’ are made

• Operator points the laser at the face and measure: distance, horizontal and vertical angles

• Optimise design and drilling positions

• Process the data on site

Observation of the initiation sequence

Potential misfired blast holes 

Effectiveness of stemming material and length 

Face movement – degree and location 

Sources of fly rock, air blast 

Origin of oversize rock blocks 

Explosion gas products (fume), Indicating poor explosives performance-water contamination, etc. 

MEASUREMENTSMEASUREMENTSDURING BLASTINGDURING BLASTING

MEASUREMENTSMEASUREMENTS AFTER BLASTINGAFTER BLASTING

FRAGMENTATIONSize Distribution, Photographic Techniques, Wipfrag

MUCK PILE DISPLACEMENTMaximum Throw, Overall Displacement, Muck Pile Swell

BLAST DAMAGE BEYOND THE BLAST LIMITSCautious Blasting

DIGGING PRODUCTIVITYBucket Fill Factor, Overall Productivity, Time Lost In Handling Oversize, Downtime For Cleanup

i. Any reduction in explosive consumption will lead to a reduction in damage to the rock.

ii. Semi-rigid explosives cartridges should be used as decoupled charge. For example 55 mm diameter cartridges in 89 mm blast holes would be a suitably decoupled charge.

iii. Effective burden on perimeter holes should not be greater than about 25 times the blast hole diameter, preferably about 20 times.

iv. Limit the width of the blasts to no more than 1.5 times the bench height

BLASTING FOR

WALL STABILITY

v. The best spacing between back-row blast holes lie between 25 and 40 times the blast hole diameter. In multi-row shots, blast holes should be staggered.

vi. Drill angled rather than vertical blast holes at least for the last 3 to 4 rows in front of the final wall. Angled blast holes tend to cause less damage to the crest behind the back row. Angle of 20-30 to the vertical is recommended.

vii. For all blast holes except those in the back row, the length of the stemming column is commonly about 25 hole diameters. Because of the need to prevent surface over break, it is necessary to increase the stemming length in he back row.

viii. Subdrilling into the final crest or berm should be minimized because cracks generated by explosion gases will allow water into the berm, therefore increasing the rate of breakdown due to weathering.

ix. The initiation sequence should be selected so that there are minimum numbers of blast holes firing on the same delay, and preferably hole by hole.

x. Adequate delay should be used to ensure good movement towards free faces and the creation of new free faces for following rows. Utilize long delay intervals between rows of blastholes (around 20ms/m).

xi. Delays be used to control the maximum instantaneous charge to ensure that rock breakage does not occur in the rock mass, which is supposed to remain intact.

xii. Choke blasting into excessive burden or broken muck piles should be avoided.

xiii. The front row charge should be adequately designed to move the front row burden.

xiv. The main charge and blast hole patterns should be optimized to give the best possible fragmentation and digging conditions for the minimum powder factor.

xv. Back row holes should be drilled at an optimum distance from the final digline to permit free digging and yet minimize damage to the wall. Experience can be used to adjust the back row positions and charges to achieve this result

DRILLING THE PRE SPLIT LINE -Blasthole Diameter: 165mm, 15–25o to the vertical Effective Burden: 3.0 m Spacing: 4.0 mPattern: Staggered

LOADING THE SPLIT LINE -ANFO/ Polystyrene blends with low pourdensities of 0.4 – 0.53 gm/cc Stemming Length: 2.4 Subgrade: 0.6mDelay pattern is hooked up for shooting in the direction of the dip

BLAST DESIGN case study

BLAST DESIGN

DRILLING THE BUFFER LINE -Blasthole diameter: 165mm Effective Burden: 3.5m Spacing: 4.5m Holes are drilled vertical

LOADING THE BUFFER LINE -Hole toe is loaded with 12% Aluminized ANFO.Stemming Length: 2.2m Subgrade: 0.6m

BLASTING THE SHEAR LINE -Split line blasted 50 ms ahead of the buffer line with the production blast.