Techniquescontrolledblasting 091217003405-phpapp01
3 likes2,892 views
Controlled blasting techniques can be used to mitigate adverse impacts of blasting in mining and construction. These include line drilling, trim blasting, smooth blasting, pre-splitting, optimizing blast design parameters, accurate timing delays, and muffle blasting. Signature hole analysis uses monitoring of a pilot blast to model blast vibration and optimize delay timing to reduce vibration energy at structural resonance frequencies. Adopting controlled blasting techniques can help restrict ground vibrations and overbreak while improving safety, environmental, and economic outcomes.
Techniquescontrolledblasting 091217003405-phpapp01
- 1. TECHNIQUES OF CONTROLLED BLASTING TO IMPROVE OVERALL ENVIRONMENTAL & SAFETY STANDARD *** MEANS TO MITIGATE ADVERSE IMPACT OF BLASTING IN OPEN PITS, QUARRIES, TUNNELS, UG METAL MINES AND CONSTRUCTION WORKINGS Partha Das Sharma (sharmapd1@gmail.com), website: http://miningandblasting.wordpress.com/
- 2. In both the mining and construction industries, blasting is the predominant method for fragmentation of consolidated mineral deposits and rocks. The public relation problems of users of explosives have increased greatly in the past few years as explosives are being consumed in increasing quantities. Increased population and spread of urbanization near to the construction & mining sites have affected more people by blasting. 2
- 3. Controlled blasting methods are used to control adverse impacts such as: Over-break Reduce ground vibrations Reduce fractures within remaining rock walls Reduce noise Reduce dilution / waste of ore etc. 3
- 4. Following are the techniques of controlled blasting: Line drilling, Trim (Cushion) blasting, Smooth (contour or perimeter) blasting, Pre-splitting, Selecting and employing various parameters of blast design, using modern technology, Precise and accurate timing delays, Muffle blasting at critical and congested areas. 4
- 5. Line drilling: This system involves a single row of closely spaced uncharged holes along the neat excavation line. This provides a plane of weakness to which the primary blast can break. It also causes some of the shock waves generated by the blast to be reflected, which reduces shattering and stressing in the finished wall of the host rock. Line drill holes are generally percussive hammer holes having spaced two to four times the hole diameter, drilled along the excavation line. The blast holes directly adjacent to the line drill holes (buffer holes) are generally loaded lighter (about 50% of primary holes) and are closely spaced (about 50 to 75 %) than primary holes. This technique gives maximum protection to the host rock to preserve its original strength. The disadvantage of this system is high drilling cost due to closed spacing and results are often unsatisfactory because of poor hole alignment. 5
- 6. Trim (Cushion) blasting : Like line drilling trim or cushion blasting involves a single row of holes along the specified final excavation line. This technique generally uses 2 to 4 inch diameter holes. Holes are loaded with light charge, well-distributed, completely stemmed and fired after the main excavation is removed. By firing the trim holes with minimum or no delay between holes, the detonation tend to shear the rock web between holes and give a smooth wall with minimum over-break. It is better to put trim holes just before removing the final berm. As compare to line drilling technique trim or cushion blasting is simpler and economical as increased hole spacing are used. 6
- 7. Smooth (contour or perimeter) blasting: A technique used mostly in underground blasting, closely spaced drill holes are loaded with decoupled charges and fired simultaneously to produce an excavation contour without fracturing or damaging the rock behind or adjacent to the blasted face. For promoting safety and economy in underground workings, performance of blasting in headings, drivages, tunnels and stopes becomes very important factor. An ideal blast results in a minimum of damage to the host rock with minimum of over-break. Perimeter or contour holes are drilled along specified final excavation limits and are lightly loaded than that of buffer holes and production holes. The spacing is kept closer than buffer holes and production holes. As a thumb rule 10 to 12 times hole diameter in medium to tough rock and 5 to 6 times hole diameter in poor, fragmented rock are kept as spacing. 7
- 8. Example: Blast damage and overbreak in Tunneling Results of Controlled Blast: Smooth walls, Minimal fractures, Minimal overbreak, 8 Less support required, Better economy & safety.
- 9. Charging for Smooth (contour or perimeter) blasting in Tunneling: 9
- 10. Pre-splitting: Pre-splitting is the smooth blasting method in which cracks for the final contour are created by blasting prior to the drilling of the rest of the holes for the blast pattern. This is an effective way of restricting back-break and ground vibration in large open pit, quarry blasting. Pre-splitting helps in isolating blasting area from remaining rock mass by creating an artificial discontinuity along the final designed excavation line / plane against which subsequent main blast breaks. A row of holes are drilled at the periphery (three sides) of the main blasting block at a closer spacing, charged with lesser quantity of explosives than the production blast and blasted prior to the main blast in an effort to create a fractured line and a reflective plane at the excavation limit or plane. Some of the shock waves from subsequent main blast are reflected at the pre-split plane which results in arresting a considerable portion of blast induced ground vibration generated in the main blast to propagate. 10
- 11. Principle of Pre-splitting 11
- 12. CHARGING OF PRE-SPLIT AND PRODUCTION HOLES12
- 13. Muffle blasting: Fly-rock is another important adverse impact of blasting operations, specially, when conducted in the vicinity of dense human habitation / congested areas. Muffling or covering of blast holes properly before blasting, is the common solution to prevent fly-rock from damaging human habitants and structures. Generally, mat or mesh (40 mm x 40 mm size) made of preferably of locally available steel ropes (5 to 6 mm) are used for muffling purpose. Sand bags weighing 40 to 50 kg are kept over the mesh at an interval of 3 m. Efficiency of arresting of fly-rock depends mainly on the quality of muffling system implemented. 13
- 14. Using millisecond delay sequence and use of in-hole delays in decks: Delay blasting (with millisecond delays) permits to divide the shot into smaller charges, which are detonated in a predetermined millisecond sequence at specific time intervals. Millisecond delay initiation of the explosive charge is a technique used in most open pit, quarry, tunnel and underground rock blasting operations. It serves to enhance fragmentation and direct rock movement for increasing productivity. The major advantages of delay blasting are: (i) Improved fragmentation, (ii) Reduction of ground vibrations and air blast, (iii) Reduction of over-break and fly-rock, (iv) Improved productivity and lower cost. Charge weight per delay is the most important parameter for controlling blast induced ground vibration and air-blast. 14
- 15. In-hole Delay blasting in Open-pits for reduction of Ground Vibration: 15
- 16. Surface connection of In-hole Delay blasting in Open-pits: 16
- 17. Blasting Ring with deck & In-hole delays in UG metal mines: 17
- 18. ‘Signature-Hole’ Blast Analysis for Vibration Control by using Accurate Delay Timing Electronic Detonator System: Structural response to blast-induced ground vibration is a phenomenon that has been analyzed for many years. Residential structure’s level of response to blast induced ground vibration is dependent on both the peak particle velocity and the frequency of the waveform. Researchers have shown that, above ground structures resonate whenever they are excited by a vibration containing adequate energy matching the fundamental frequency of the structure. The value of this frequency is mainly dependent upon the mass, height and stiffness of the structure. The maximum response of a house to blast induced ground vibration occurs whenever the frequency of the ground vibration matches the natural resonant frequency of the house. When little or no energy at the resonant frequency of the structure, the structural response to the vibration will be negligible. 18
- 19. A method of controlling blast vibrations other than by modifying the scaled distance came into use some time ago. The crucial point of the methodology is the use of a pilot-blast signal which takes account of the seismic properties of all complex geology between the blast and the target locations. Therefore, it does not require any geological model or assumption. The delay interval between blast-holes can be chosen to control and minimize the vibration energy within the structural response band of most houses. Studies had indicated that blast vibration could be simulated by detonating a “Signature Hole” with the vibration monitored at critical locations, and then using a computer to superpose the waveforms with varying delays. By choosing delay times (∆t) that create ‘destructive interference’ at frequencies that are favored by the local geology, the “ringing” vibration that excites structural elements in structures, houses and annoys neighbors could be reduced. 19
- 20. 20
- 21. In this method, accurate delay times are crucial to effective vibration control, scatter in the firing times limited the method severely. Electronic detonators have scatter less than a millisecond. In light of all these, researchers have started finding both limitations and new potential of this new technique of controlling blast vibration. 21
- 22. In other words, “Signature Hole Analysis” is a modeling technique, to help control adverse effects of blast induced vibrations. The process involves controlling the frequency content by adjusting delay times within a blast containing several explosive charges. The risk to adjacent structures is thereby mitigated. Thus, with the growing adoption rate of electronic initiation systems as a tool to control nuisance of vibrations, the modeling techniques are becoming more popular. M/s Instantel has recently developed and launched a ‘Signature Hole Analysis’ software tool, which allows users to simulate a large number of charge delay times very quickly. The software, in fact, is a modeling technique used to help predict and control blast induced vibrations. 22
- 23. Advantages of the technique, ‘Signature Hole Blast Analysis’, for Vibration Control: This technique provides optimum electronic timing while maintaining high level of production with efficiency by raising quantity of explosives per delay (kg/delay) and provide overall structural safety of blast surroundings. Blast with shorted duration results in mitigating effects of blast induced vibration. Therefore, as post-blast vibrations are reduced by raising frequencies, much larger blasts can be undertaken with better operational performance, without compromising stringent safety standards of environment. Thus, Signature Hole Analysis software tool available can be used to help optimize and improve overall operational efficiency. It has also been observed that this vibration control method is feasible for underground mining ring blasts as well. 23
- 24. By adopting these precautions The ground vibration is restricted to ease the public relation problem, The mines’ / construction’s techno- economics are improved, Preservation of host rock strength and safety standard are improved to a considerable level 24
- 25. As far as possible modern techniques and equipments are also to be used in order to mitigate the adverse blast effects Regular monitoring is needed Training to the blasting supervisors on controlled blasting techniques is very much essential. 25
- 26. THANK YOU 26