CONTROLLED BLASTING TECHNIQUESCONTROLLED BLASTING TECHNIQUESCONTROLLED BLASTING TECHNIQUESCONTROLLED BLASTING TECHNIQUES

************

MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF

BLASTING IN OPEN PITS, QUARRY, TUNNEL, BLASTING IN OPEN PITS, QUARRY, TUNNEL, BLASTING IN OPEN PITS, QUARRY, TUNNEL, BLASTING IN OPEN PITS, QUARRY, TUNNEL,

UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION UG METAL MINES AND CONSTRUCTION

WORKINGS, WORKINGS, WORKINGS, WORKINGS,

IN ORDER TO IMPROVE OVERALL IN ORDER TO IMPROVE OVERALL IN ORDER TO IMPROVE OVERALL IN ORDER TO IMPROVE OVERALL

ENVIRONMENTAL & SAFETY STANDARDENVIRONMENTAL & SAFETY STANDARDENVIRONMENTAL & SAFETY STANDARDENVIRONMENTAL & SAFETY STANDARD

By: Partha Das Sharma, B.Tech(Hons.), E.mail: sharmapd1@gmail.com

q In both the mining and constructionindustries, blasting is the predominant methodfor fragmentation of consolidated mineraldeposits and rocks.

q The public relation problems of users ofexplosives have increased greatly in the pastexplosives have increased greatly in the pastfew years as explosives are being consumed inincreasing quantities.

q Increased population and spread ofurbanization near to the construction & miningsites have affected more people by blasting.

Controlled blasting methods are used to control adverse impacts such as:

q Over-break

q Reduce ground vibrationsq 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 blastdesign, using modern technology,

q Precise timing delays,

q Varied density of explosives product by using bulkexplosives 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 holesalong 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 bereflected, which reduces shattering and stressing in the finished wall of thehost rock.

v Line drill holes are generally percussive hammer holes (2 to 3 inchdiameter) having spaced two to four times the hole diameter, drilled alongdiameter) having spaced two to four times the hole diameter, drilled alongthe excavation line.

v The blast holes directly adjacent to the line drill holes (buffer holes) aregenerally 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 itsoriginal strength.

vThe disadvantage of this system is high drilling cost due to closedspacing and results are often unsatisfactory because of poor holealignment.

Trim (Cushion) blasting :

v Like line drilling trim or cushion blasting involves a single row of holesalong 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 stemmedand fired after the main excavation is removed.and fired after the main excavation is removed.

v By firing the trim holes with minimum or no delay between holes, thedetonation tend to shear the rock web between holes and give a smooth wallwith 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 simplerand 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 decoupledcharges (charges with a smaller diameter than the drill hole) and firedsimultaneously to produce an excavation contour without fracturing ordamaging 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 becomesvery important factor.

v An ideal blast results in a minimum of damage to the host rock withv An ideal blast results in a minimum of damage to the host rock withminimum of over-break.

v In this technique perimeter or contour holes are drilled along specifiedfinal excavation limits and are lightly loaded than that of buffer holes andproduction 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 totough rock and 5 to 6 times hole diameter in poor, fragmented rock are keptas 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 whichcracks for the final contour are created by blasting prior to the drilling of therest of the holes for the blast pattern.

v Once the crack is made, it screens off the surroundings to some extentfrom ground vibrations in the main round.

v This is an effective way of restricting back-break and ground vibration inlarge open pit, quarry blasting.

v Pre-splitting helps in isolating blasting area from remaining rock mass byv Pre-splitting helps in isolating blasting area from remaining rock mass bycreating 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 mainblasting block at a closer spacing, charged preferably with lesser quantity ofexplosives than the production blast and blasted prior to the main blast inan effort to create a fractured line and a reflective plane at the excavationlimit or plane.

v Some of the shock waves from subsequent main blast are reflected at thepre-split plane which results in arresting a considerable portion of blastinduced 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 thecommon solution to prevent fly-rock from damaging human habitants andstructures.structures.

v Generally, mat or mesh (40 mm x 40 mm size) made of preferably oflocally available steel ropes (5 to 6 mm) are used for muffling purpose. Sandbags 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 ofmuffling system implemented.

Using millisecond delay sequence and use of in-hole delays in decks:

v Delay blasting (with millisecond delays) permits the explosive engineersto divide the shot into smaller charges, which are detonated in apredetermined millisecond sequence at specific time intervals.

v Millisecond delay initiation of the explosive charge is a technique used inmost open pit, quarry, tunnel and underground rock blasting operations.

v It serves to enhance fragmentation and direct rock movement forv It serves to enhance fragmentation and direct rock movement forincreasing productivity.

v The major advantages of delay blasting are: (i) Improved fragmentation,(ii) Reduction of ground vibrations and air blast, (iii) Reduction of over-breakand fly-rock, (iv) Improved productivity and lower cost.

v Charge weight per delay is the most important parameter for controllingblast 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, are improved,

q Preservation of host rock strength and safety standard are improved to a considerable level

As far as possible moderntechniques and equipments are alsoto be used

q In order to mitigate the adverseq In order to mitigate the adverseblast effects

q Regular monitoring

q Apart, training to the blastingsupervisors on controlled blastingtechniques is very much essential.

q Training also needed for maintainingq Training also needed for maintainingproper blast-log and procedures ofevaluation of blast from such blast-logs infuture.

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 micro-electronics have profoundly improved the design and use of drills, drill tools, blast-initiation products, explosives and drills, drill tools, blast-initiation products, explosives and seismographs.

q At the same time, development of safer, eco-friendly Non-Primary 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.

THANK THANK THANK THANK YOUYOUYOUYOUYOUYOUYOUYOU