1. BLASTING FOR REDUCED ROCK DAMAGE AND CONTROLLING STABILITY

2. DAMAGE TO REMAINING ROCK

3. Damage Resulting From Conventional Blasting

4. SIDES BACK BREAK UNDER BREAK & BACK SHATTER
Overbreak at
SIDES BACK BREAK UNDER BREAK & BACK SHATTER

5. MECHANISM OF BLAST DAMAGE
BLAST DAMAGES
The 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.
MECHANISM OF BLAST DAMAGE
Crushing Around Borehole
Radial Fracturing
Gas Pressure
Internal Spalling
Induced Strain
Release of Load Fracturing

6. DAMAGE 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

7. DAMAGE RESULTING FROM VIBRATION

8. VIBRATION LEVELS
FOR ROCK DAMAGE
Blast measurements adjacent to explosive charge are important.
Semi-empirical methods of field measurements of peak particle velocity and rock parameters correlation.

9. FRAGMENTATION Ease of excavation Transport of muck
Requirement of customer
With use of appropriate techniques fragmentation can be improved
Use of appropriate technique
Conventional Blasting

10. FACTORS AFFECTING BLAST 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 configuration
Strength of explosive as well as concentration & distribution of explosive within hole
HOLE SHAPE
Conventional hole
Notching of blast hole
PROTECTIVE DEVICES
Damages can be reduced by use of liner
DECOUPLING OF CHARGE
To diminish excessive peak pressure

11. BLASTING TECHNIQUES FOR DAMAGE REDUCTION
Ensuring Adequate
Burden Relief
Reducing Explosives
Energy Concentration
Controlled Blasting
Techniques

12. PARAMETERS IN CONTROLLED BLASTING
Precision In Drilling
Explosives
Interval Timing
Rock Characteristics

13. PRECISION IN DRILLING Hole deviation and collaring error
Spacing, burden and their ratio
Shape of opening
Actual blast geometry may differ from design

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

15. INTERVAL TIMING Number of delays in perimeter holes
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

16. INITIATING DEVICES

17. ROCK CHARACTERISTICS
Rock properties, structure and groundwater normally dominant in wall stability are not controllable.
ROCK STRESS
ROCK STRENGTH
ROCK STRUCTURE

18. REDUCING ENERGY CONCENTRATION
Initiation Sequence
Charge Distribution
Hole Diameter
Effective Sub Drilling

19. CHARGE DISTRIBUTION

20. CHARGE DISTRIBUTION

21. STEMMING 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

22. CONTROLLED BLASTING TECHNIQUES
Line Drilling
Presplitting
Smooth Blasting
Cushion Blasting
Air Decking
Controlled Fracture Growth

23. Line drilling holes along the final excavation

24. BLAST HOLE LOADING SYSTEM FOR PRESPLITTING
Example of presplitting with and without presplitting

25. Presplitting in a blast

26. Preslitting 1

27. presplitting2

32. CUSHION BLASTING
Closely spaced lightly loaded holes at the perimeter.

33. CONTROLLED FRACTURE GROWTH
Drill Hole Liners
Metal Tube
Plastic Pipe
Card Board Tube
NOTCHED HOLES

34. BLASTHOLE LINERS GI PIPE LINE PVC PIPE LINER PLASTIC LINER
CARDBOARD LINER
CARDBOARD LINER
PAPER TUBE LINER

35. AIR DECKING

36. Longer stemming in front and at the back

37. BLAST DESIGN AND IMPLEMENTATION
Fragmentation Process
Rock Characteristics
Explosives
Initiation
Measurements Before Blasting And Design Implementation
Computer Aided Blast Design

38. MEASUREMENTS BEFORE BLASTING
Actual Blast Geometry May Differ From Design
Boretrak Blasthole Logger
Laser Profiler

39. ROCKFACE LASER PROFILER
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

40. MEASUREMENTS DURING BLASTING
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. 

41. MEASUREMENTS AFTER BLASTING
FRAGMENTATION Size Distribution, Photographic Techniques, Wipfrag
MUCK PILE DISPLACEMENT Maximum Throw, Overall Displacement, Muck Pile Swell
BLAST DAMAGE BEYOND THE BLAST LIMITS Cautious Blasting
DIGGING PRODUCTIVITY Bucket Fill Factor, Overall Productivity, Time Lost In Handling Oversize, Downtime For Cleanup

42. WALL STABILITY BLASTING FOR
Any reduction in explosive consumption will lead to a reduction in damage to the rock.
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.
Effective burden on perimeter holes should not be greater than about 25 times the blast hole diameter, preferably about 20 times.
Limit the width of the blasts to no more than 1.5 times the bench height

43. 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.
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.
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.

44. 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.
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.
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).
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.

45. Choke blasting into excessive burden or broken muck piles should be avoided.
The front row charge should be adequately designed to move the front row burden.
The main charge and blast hole patterns should be optimized to give the best possible fragmentation and digging conditions for the minimum powder factor.
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

46. BLAST DESIGN case study
DRILLING THE PRE SPLIT LINE -
Blasthole Diameter: 165mm, 15–25o to the vertical Effective Burden: 3.0 m
Spacing: 4.0 m
Pattern: Staggered
LOADING THE SPLIT LINE -
ANFO/ Polystyrene blends with low pour
densities of 0.4 – 0.53 gm/cc
Stemming Length: Subgrade: 0.6m
Delay pattern is hooked up for shooting in the direction of the dip

47. BLAST DESIGN DRILLING THE BUFFER LINE - LOADING 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.