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.