Controlled Blasting

CONTROLLED BLASTING TECHNIQUES *** MEANS TO MITIGATE ADVERSE IMPACT OF BLASTING IN OPEN PITS, QUARRY, TUNNEL, UG METAL MINES AND CONSTRUCTION WORKINGS, IN ORDER TO IMPROVE OVERALL ENVIRONMENTAL & SAFETY STANDARD By: Partha Das Sharma, B.Tech(Hons.), E.mail: [email protected]

q In both the mining and construction industries, blasting is the predominant method for fragmentation of consolidated mineral deposits and rocks. q The public relation problems of users of explosives have increased greatly in the past few years as explosives are being consumed in increasing quantities. q Increased population and spread of urbanization near to the construction & mining sites have affected more people by blasting.

Controlled blasting methods are used to control adverse impacts such as: q Over-break q Reduce ground vibrations q Reduce fractures within remaining rock walls q Reduce noise q Reduce dilution / waste of ore etc.

Following are the techniques of controlled blasting: q Line drilling, q Trim (Cushion) blasting, q Smooth (contour or perimeter) blasting, q Pre-splitting, q Selecting and employing various parameters of blast design, using modern technology, q Precise timing delays, q Varied density of explosives product by using bulk explosives in both open pit quarries and UG workings, q Muffle blasting at a very critical and congested areas.

Line drilling: v This system involves a single row of closely spaced uncharged holes along the neat excavation line. v This provides a plane of weakness to which the primary blast can break. v 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. v Line drill holes are generally percussive hammer holes (2 to 3 inch diameter) having spaced two to four times the hole diameter, drilled along the excavation line. v 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. v This technique gives maximum protection to the host rock to preserve its original strength. vThe disadvantage of this system is high drilling cost due to closed spacing and results are often unsatisfactory because of poor hole alignment.

Trim (Cushion) blasting : v Like line drilling trim or cushion blasting involves a single row of holes along the specified final excavation line. v This technique generally uses 2 to 4 inch diameter holes. v Holes are loaded with light charge, well-distributed, completely stemmed and fired after the main excavation is removed. v 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. v It is better to put trim holes just before removing the final berm. v As compare to line drilling technique trim or cushion blasting is simpler and economical as increased hole spacing are used.

Smooth (contour or perimeter) blasting: v A technique used (rarely in surface and mostly in underground blasting) in which a row or closely spaced drill holes are loaded with decoupled charges (charges with a smaller diameter than the drill hole) and fired simultaneously to produce an excavation contour without fracturing or damaging the rock behind or adjacent to the blasted face. v For promoting safety and economy in underground workings, performance of blasting in headings, drivages, tunnels and stopes becomes very important factor. v An ideal blast results in a minimum of damage to the host rock with minimum of over-break. v In this technique perimeter or contour holes are drilled along specified final excavation limits and are lightly loaded than that of buffer holes and production holes. v The spacing is kept closer than buffer holes and production holes. v Generally, 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.

Example: Blast damage and overbreak in Tunneling

Results of Controlled Blast: Smooth walls, Minimal fractures, Minimal overbreak, Less support required, Better economy & safety.

Charging for Smooth (contour or perimeter) blasting in Tunneling:

Pre-splitting: v Pre-splitting or pre-shearing 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. v Once the crack is made, it screens off the surroundings to some extent from ground vibrations in the main round. v This is an effective way of restricting back-break and ground vibration in large open pit, quarry blasting. v 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. v A row of holes are drilled at the periphery (three sides) of the main blasting block at a closer spacing, charged preferably 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. v 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.

Principle of Pre-splitting

CHARGING OF PRE-SPLIT AND PRODUCTION HOLES

Muffle blasting: v Fly-rock is another important adverse impact of blasting operations, specially, when conducted in the vicinity of dense human habitation / congested areas. v Muffling or covering of blast holes properly before blasting, is the common solution to prevent fly-rock from damaging human habitants and structures. v 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. v Efficiency of arresting of fly-rock depends mainly on the quality of muffling system implemented.

Using millisecond delay sequence and use of in-hole delays in decks: v Delay blasting (with millisecond delays) permits the explosive engineers to divide the shot into smaller charges, which are detonated in a predetermined millisecond sequence at specific time intervals. v Millisecond delay initiation of the explosive charge is a technique used in most open pit, quarry, tunnel and underground rock blasting operations. v It serves to enhance fragmentation and direct rock movement for increasing productivity. v 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. v Charge weight per delay is the most important parameter for controlling blast induced ground vibration and air-blast.

In-hole Delay blasting in Open-pits for reduction of Ground Vibration:

Surface connection of In-hole Delay blasting in Open-pits:

Blasting Ring with deck & In-hole delays in UG metal mines:

By adopting these precautions q The ground vibration is restricted to ease the public relation problem, q The mines’ / construction’s techno-economics are improved, q Preservation of host rock strength and safety standard are improved to a considerable level

As far as possible modern techniques and equipments are also to be used q In order to mitigate the adverse blast effects q Regular monitoring

q Apart, training to the blasting supervisors on controlled blasting techniques is very much essential. q Training also needed for maintaining proper blast-log and procedures of evaluation of blast from such blast-logs in future.

Conclusion: q Fortunately, technology continues to move the process from an art to a science. q Computerized drills, Measure-While-Drilling (MWD) systems and Electronic Detonators, are gaining wider acceptance in quarries, opencast and underground mines. q Efficient blasting techniques with computers and microelectronics have profoundly improved the design and use of drills, drill tools, blast-initiation products, explosives and seismographs. q At the same time, development of safer, eco-friendly NonPrimary Explosives Detonators (NPED) has added the additional safety and efficiency in blasting system. q If the progress continues at this pace, days are not far, when we run our mines or excavation activities with no or very little nuisances of blasting and cleaner environment can be maintained.

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