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BLASTING FOR REDUCED ROCK DAMAGE AND CONTROLLING STABILITY

BLASTING FOR REDUCED ROCK DAMAGE AND CONTROLLING STABILITY. DAMAGE TO REMAINING ROCK. Damage Resulting From Conventional Blasting. Overbreak at SIDES BACK BREAK UNDER BREAK & BACK SHATTER. BLAST DAMAGES.

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BLASTING FOR REDUCED ROCK DAMAGE AND CONTROLLING STABILITY

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  1. BLASTINGFOR REDUCED ROCK DAMAGE AND CONTROLLING STABILITY

  2. DAMAGE TO REMAINING ROCK

  3. Damage Resulting From Conventional Blasting

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

  5. 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 RESULTINGFROM 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 appropriatetechniques fragmentation can be improved Use of appropriate technique Conventional Blasting

  10. FACTORS AFFECTING BLAST DAMAGES • 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 • 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

  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. 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 • 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 SYSTEMFORPRESPLITTING Example of presplitting with and without presplitting

  25. Presplitting in a blast

  26. Preslitting 1

  27. presplitting2

  28. CUSHION BLASTING • Closely spaced lightly loaded holes at the perimeter.

  29. Drill Hole Liners Metal Tube Plastic Pipe Card Board Tube CONTROLLEDFRACTURE GROWTH NOTCHED HOLES

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

  31. AIR DECKING

  32. Longer stemming in front and at the back

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

  34. MEASUREMENTSBEFORE BLASTING • Actual Blast Geometry May Differ From Design • Boretrak Blasthole Logger • Laser Profiler

  35. 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

  36. MEASUREMENTSDURING 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.

  37. MEASUREMENTSAFTER 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

  38. BLASTING FOR WALL STABILITY • 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

  39. 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.

  40. 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.

  41. 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

  42. 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: 2.4 Subgrade: 0.6m Delay pattern is hooked up for shooting in the direction of the dip

  43. 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.

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