TECHNIQUES OF CONTROLLED BLASTING TO TECHNIQUES OF CONTROLLED BLASTING TO TECHNIQUES OF CONTROLLED BLASTING TO TECHNIQUES OF CONTROLLED BLASTING TO

IMPROVE OVERALL ENVIRONMENTAL & IMPROVE OVERALL ENVIRONMENTAL & IMPROVE OVERALL ENVIRONMENTAL & IMPROVE OVERALL ENVIRONMENTAL &

SAFETY STANDARDSAFETY STANDARDSAFETY STANDARDSAFETY STANDARD

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MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF MEANS TO MITIGATE ADVERSE IMPACT OF 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, QUARRIES, TUNNELS, BLASTING IN OPEN PITS, QUARRIES, TUNNELS, BLASTING IN OPEN PITS, QUARRIES, TUNNELS, BLASTING IN OPEN PITS, QUARRIES, TUNNELS,

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

WORKINGS WORKINGS WORKINGS WORKINGS

Partha Das Sharma (sharmapd1@gmail.com), website: http://miningandblasting.wordpress.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 past

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explosives 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 vibrations

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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 controlledblasting:

q Line drilling,

q Trim (Cushion) blasting,

q Smooth (contour or perimeter) blasting,

q Pre-splitting,

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q Pre-splitting,

q Selecting and employing various parametersof blast design, using modern technology,

q Precise and accurate timing delays,

q Muffle blasting at critical and congested areas.

Line drilling:

q This system involves a single row of closely spaced uncharged holesalong the neat excavation line.

q This provides a plane of weakness to which the primary blast canbreak.

q It also causes some of the shock waves generated by the blast to bereflected, which reduces shattering and stressing in the finished wall ofthe host rock.

q Line drill holes are generally percussive hammer holes having spacedtwo to four times the hole diameter, drilled along the excavation line.

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two to four times the hole diameter, drilled along the excavation line.

q The blast holes directly adjacent to the line drill holes (buffer holes)are generally loaded lighter (about 50% of primary holes) and are closelyspaced (about 50 to 75 %) than primary holes.

q This technique gives maximum protection to the host rock topreserve its original strength.

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

Trim (Cushion) blasting :

q Like line drilling trim or cushion blasting involves a singlerow of holes along the specified final excavation line.

q This technique generally uses 2 to 4 inch diameter holes.

q Holes are loaded with light charge, well-distributed,completely stemmed and fired after the main excavation isremoved.

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q By firing the trim holes with minimum or no delay betweenholes, the detonation tend to shear the rock web betweenholes and give a smooth wall with minimum over-break.

q It is better to put trim holes just before removing the finalberm.

q As compare to line drilling technique trim or cushionblasting is simpler and economical as increased hole spacingare used.

Smooth (contour or perimeter) blasting:

q A technique used mostly in underground blasting, closely spaceddrill holes are loaded with decoupled charges and firedsimultaneously to produce an excavation contour without fracturingor damaging the rock behind or adjacent to the blasted face.

q For promoting safety and economy in underground workings,performance of blasting in headings, drivages, tunnels and stopesbecomes very important factor.

q An ideal blast results in a minimum of damage to the host rock

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q An ideal blast results in a minimum of damage to the host rockwith minimum of over-break.

q Perimeter or contour holes are drilled along specified finalexcavation limits and are lightly loaded than that of buffer holes andproduction holes.

q The spacing is kept closer than buffer holes and production holes.

q As a thumb rule 10 to 12 times hole diameter in medium to toughrock and 5 to 6 times hole diameter in poor, fragmented rock are keptas spacing.

Example: Blast damage and overbreak in Tunneling

88Results of Controlled Blast: Smooth walls, Minimal fractures, Minimal overbreak, Less support required, Better economy & safety.

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

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Pre-splitting:

q Pre-splitting is the smooth blasting method in which cracks for thefinal contour are created by blasting prior to the drilling of the rest ofthe holes for the blast pattern.

q This is an effective way of restricting back-break and groundvibration in large open pit, quarry blasting.

q Pre-splitting helps in isolating blasting area from remaining rockmass by creating an artificial discontinuity along the final designedexcavation line / plane against which subsequent main blast breaks.

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excavation line / plane against which subsequent main blast breaks.

q A row of holes are drilled at the periphery (three sides) of the mainblasting block at a closer spacing, charged with lesser quantity ofexplosives than the production blast and blasted prior to the mainblast in an effort to create a fractured line and a reflective plane atthe excavation limit or plane.

q Some of the shock waves from subsequent main blast arereflected at the pre-split plane which results in arresting aconsiderable portion of blast induced ground vibration generated inthe main blast to propagate.

Principle of Pre-splitting

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1212CHARGING OF PRE-SPLIT AND PRODUCTION HOLES

Muffle blasting:

q Fly-rock is another important adverse impact ofblasting operations, specially, when conducted in thevicinity of dense human habitation / congested areas.

q Muffling or covering of blast holes properly beforeblasting, is the common solution to prevent fly-rock fromdamaging human habitants and structures.

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damaging human habitants and structures.

q Generally, mat or mesh (40 mm x 40 mm size) made ofpreferably of locally available steel ropes (5 to 6 mm) areused for muffling purpose. Sand bags weighing 40 to 50kg are kept over the mesh at an interval of 3 m.

q Efficiency of arresting of fly-rock depends mainly onthe quality of muffling system implemented.

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

q Delay blasting (with millisecond delays) permits to divide theshot into smaller charges, which are detonated in apredetermined millisecond sequence at specific time intervals.

q Millisecond delay initiation of the explosive charge is atechnique used in most open pit, quarry, tunnel and undergroundrock blasting operations.

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q It serves to enhance fragmentation and direct rock movementfor increasing productivity.

q The major advantages of delay blasting are: (i) Improvedfragmentation, (ii) Reduction of ground vibrations and air blast,(iii) Reduction of over-break and fly-rock, (iv) Improvedproductivity and lower cost.

q Charge weight per delay is the most important parameter forcontrolling blast induced ground vibration and air-blast.

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

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

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

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‘Signature-Hole’ Blast Analysis for Vibration Control by usingAccurate Delay Timing Electronic Detonator System:

q Structural response to blast-induced ground vibration is aphenomenon that has been analyzed for many years.

q Residential structure’s level of response to blast induced groundvibration is dependent on both the peak particle velocity and thefrequency of the waveform.

q Researchers have shown that, above ground structures resonatewhenever they are excited by a vibration containing adequate energy

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whenever they are excited by a vibration containing adequate energymatching the fundamental frequency of the structure.

q The value of this frequency is mainly dependent upon the mass,height and stiffness of the structure.

q The maximum response of a house to blast induced groundvibration occurs whenever the frequency of the ground vibrationmatches the natural resonant frequency of the house.

q When little or no energy at the resonant frequency of thestructure, the structural response to the vibration will be negligible.

q A method of controlling blast vibrations other than by modifyingthe scaled distance came into use some time ago.

q The crucial point of the methodology is the use of a pilot-blastsignal which takes account of the seismic properties of all complexgeology between the blast and the target locations.

q Therefore, it does not require any geological model orassumption.

q The delay interval between blast-holes can be chosen to controland minimize the vibration energy within the structural response

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and minimize the vibration energy within the structural responseband of most houses.

q Studies had indicated that blast vibration could be simulated bydetonating a “Signature Hole” with the vibration monitored at criticallocations, and then using a computer to superpose the waveformswith varying delays.

q 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 andannoys neighbors could be reduced.

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q In this method, accurate delay times are crucial toeffective vibration control, scatter in the firing timeslimited the method severely.

q Electronic detonators have scatter less than amillisecond.

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millisecond.

q In light of all these, researchers have started findingboth limitations and new potential of this new techniqueof controlling blast vibration.

q In other words, “Signature Hole Analysis” is a modelingtechnique, to help control adverse effects of blast inducedvibrations.

q The process involves controlling the frequency content byadjusting delay times within a blast containing severalexplosive charges.

q The risk to adjacent structures is thereby mitigated.

q Thus, with the growing adoption rate of electronic initiation

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q Thus, with the growing adoption rate of electronic initiationsystems as a tool to control nuisance of vibrations, themodeling techniques are becoming more popular.

q M/s Instantel has recently developed and launched a‘Signature Hole Analysis’ software tool, which allows users tosimulate a large number of charge delay times very quickly.

q The software, in fact, is a modeling technique used to helppredict and control blast induced vibrations.

Advantages of the technique, ‘Signature Hole Blast Analysis’, forVibration Control:

q This technique provides optimum electronic timing whilemaintaining high level of production with efficiency by raisingquantity of explosives per delay (kg/delay) and provide overallstructural safety of blast surroundings.

q Blast with shorted duration results in mitigating effects of blastinduced vibration.

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q Therefore, as post-blast vibrations are reduced by raisingfrequencies, much larger blasts can be undertaken with betteroperational performance, without compromising stringent safetystandards of environment.

q Thus, Signature Hole Analysis software tool available can beused to help optimize and improve overall operational efficiency.

q It has also been observed that this vibration control method isfeasible for underground mining ring blasts as well.

By adopting these precautions

q The ground vibration is restricted to ease the public relation problem,

q The mines’ / construction’s techno-

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q The mines’ / construction’s techno-economics are improved,

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

q As far as possible moderntechniques and equipments are alsoto be used in order to mitigate theadverse blast effects

q Regular monitoring is needed

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q Regular monitoring is needed

q Training to the blastingsupervisors on controlled blastingtechniques is very much essential.

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