Kamis, 11 Desember 2014


The choice of demolition method depends on the project conditions, site constraints, sensitivity of the neighbourhood and availability of equipment. Implosion is one of the demolition methods. Implosion is the preferred method for efficiently demolishing large structures.

>>> Pre-blast Considerations

If it is intended to blast a building structure, the Registered Specialist Contractor (Demolition) shall carry out a comprehensive Risk Assessment Report and an Environmental Assessment Report on the effect of implosion on the affected neighbourhood. With positive results on both the risk assessment and environmental impact assessment and agreed by the relevant approval Authority, the Registered Specialist Contractor (Demolition) may begin studying the structure of the building and develop a blasting design. The design may include pre-weakening of the structure, the strategy in placement of the explosives and time delay so that the building will collapse in a safe manner. Pre-weakening of the structure may include cutting out a portion of the shear walls and other structural elements. A test blast may be conducted to verify  the strength of the structural member and to fine tune the explosive design. Protection of the adjacent properties and habitats is also an important consideration.

>>> General Concerns

General concerns and good practices in controlled demolition by blasting are discussed in the following:
  1. Pre-weakening of the structure shall be designed to ensure the structural stability before the implosion;
  2. To minimize the dispersion of building debris into adjoining land after blasting, a trench or bund wall shall be installed outside the building to contain the debris, unless a basement exists;
  3. A good design will provide adequate and sufficient time delay to allow only one or two floors of the building debris to fall on ground level at a time in order to limit the magnitude of the impact on the ground;
  4. A good design will cause the structure to fall towards the center of the building and/or within the protected area;
  5. The design must also identify an exclusion zone to evacuate all residents or inhabitants during the blasting. The impacts of noise and dust generated during the blasting shall be considered. Radius of the typical exclusion zone shall not be less than 2.5 times the building height;
  6. If there are slopes and earth retaining walls or features, a geotechnical assessment shall be conducted to ensure that the blasting will not affect the stability of these features;
  7. The entire site shall be under 24-hour security from the installation of explosive until final blasting. Handling and storage of explosives shall be in conformance with the Dangerous Goods Ordinance, any requirements of the Commissioner of Mines and other relevant regulations. The implosion expert shall have proven experience and track records in design and supervision of blasting similar building structures to the satisfaction of the Commissioner of Mines. The blasting expert shall have acquired the relevant training and practical experience in using the proposed explosives. The blasting expert shall obtain from the Commissioner of Mines an authorization to carry out blasting. All personnel must be evacuated from the site before and during blasting;
  8. The Registered Specialist Contractor (Demolition) must coordinate with the government and local community to determine the best procedures in notification, schedules for the events, traffic routing, design for the sequence of events, evacuating residents, clear out personnel from the building and assigning responsibilities during blasting. For the purpose of crowd control, blasting should be carried out in the early morning of a Sunday or public holiday;
  9. An emergency plan shall be prepared to handle emergency situation such as premature explosion, misfire or interruption due to bad weather including thunder and lightning;
  10. After the explosion, the blasting expert must check to make sure that there is no unfired explosive left on site. The entire area must remain clear and under security control until the unfired explosives have been detonated or safely dealt with by the blasting expert;
  11. As far as practicable, non-electrical initiation system should be used to avoid the risk of pre-mature detonation by stray currents, external electro-magnetic waves or radio frequencies. The installation shall include a redundant system to ensure successful detonation. Nitroglycerine based explosives are not permitted to be used.
  12. The Registered Specialist  Contractor (Demolition) must provide evidence of his capability to safely perform the demolition and shall illustrate to the approving authorities that the procedures are safe;
  13. The mode of collapse shall be demonstrated to ascertain that: (1) no part of building beyond the protected area; (2) the impact of the structural collapse will not cause significant vibration affecting any underground tunnels and any underground utilities and any adjoining properties;
  14. The structural safety of the building to be imploded shall be checked and certified to be sound and safe at all stages prior to implosion.
Demolishing steel columns is a bit more difficult, as the dense material is much stronger. For buildings with a steel support structure, blasters typically use the specialized explosive material cyclotrimethylenetrinitramine, called RDX for short. RDX-based explosive compounds expand at a very high rate of speed, up to 27,000 ft/s (8,230 m/s).

Minggu, 07 Desember 2014


Key Performance Indicators, also known as Key Success Indicators (KSI), help a department define and measure progress toward organizational goals. Based on my experience, there are seven indicators in Drill & Blast Department at open-pit mining.

1) Volume Blasting

At beginning of the year Department Engineering always gives its planning concerning volume blast that shall be blasted by Department Drill & Blast. If there are any changes at planning, they will give updating-blasted volume at the outset of the month.

2) Powder Factor

Powder factor (PF) is use of explosives divided by volume blasting. For the new company which mine at open pit mining firstly has to trial calculating PF to get optimum PF. Remember that every rock has different characteristic.

3) Utilization of Unit Drilling

Unit Utilization is effective working hours of unit  divided by total working hour. Because unit drilling is enough expensive and costly fuel consuming, Department Drill & Blast has to know the need of total units and distributing them well.

4) Productivity of Unit Drilling

Productivity of unit drilling is total depth per hour. The supervisor has to check weekly how operator do traveling unit in drill area. Moreover, the supervisor has to monitor the pressure, flushing and rotary of bottom bit from inside of cabin operator. All of these are done to acquire optimum minute of every drilled hole.

5) Bit Life

Based on my experience for bottom bit like New RB 30 J, minimum bit life is 9,000 meter. You can search the appearance of New RB 30 J on Google. Generally rock condition defines types of bottom bit.

6) Safety Stock at Magazine

It is no funny if you wanna blast proposed area, but you don't have stock explosives and blasting agents in the magazine. Department head usually will account the stock weekly in order the needs of explosives and blasting agents to be fulfilled. The capacity of the magazine is restricted because of either local or government regulation.

7) Bit Consumption
Every manufacturer has trailed and gathered data of either maximum bit life or minimum bit life. Usually the manufacturer of bottom bit has proposed the bit life. It is no funny if the proposed of bit life is not suitable in the field.



Seismic waves are waves that travel through the earth. These waves represent the transmission of energy through the solid earth. Other types of wave transmission of energy are sound waves. light waves, and radio waves. Earthquakes generate seismic waves. The science that studies earthquake is seismology, the name being derived from the Greek word seismos meaning to shake. Some of man's activities such as blasting and pile driving produce seismic waves which people can feel. Seismic waves are divided into two large classes, body waves and surface waves.
  • Body Waves
Body waves travel through the mass of the rock, penetrating down into the interior of the rock mass. There are two kinds of body waves: compressional waves and shear waves.The compressional wave is a push-pull type wave that produces alternating compression and dilatation in the direction of wave travel, such as occurs in a stretched spring. The shear wave is a transverse wave that vibrates at right angles to the direction of wave travel. The motion of a shear wave can be seen in a rope which is strongly flexed at one end. The rope moves up and down, but the wave travels outward towards the other end. Liquids do not transmit shear waves.

A schematic representation of the motion for compressional wave and shear wave is shown in Figure 1 and Figure 2.

Figure 1: Compressional Wave

Figure 2: Shear Wave

  • Surface Waves
Surface waves travel over the surface of rock mass but do not travel through it. The depth to which the rock is affected by the wave motion is approximately one wave length. Surface waves are generated by body waves which are restrained by physical and geometrical conditions from traveling into the interior of the rock mass. Surface waves produce the largest ground motions and are the large energy carriers.


Seismic waves are elastic waves. Elasticity is a property of matter which causes a material to regain its original shape or size if it is deformed. A very familiar example of elastic behavior is that of a stretched rubber band which springs back to its original length when released. Rock materials are highly elastic and thus produce strong elastic or seismic waves when deformed. Deformation occurs in two ways, a change in volume which is a compression or in shape which is a shear.

Material resist deformation and this resistance is called an elastic modulus. If the deformation is a compression, the resistance is measured by modulus of incompessibility or the bulk modulus. If the deformation is a shear, the resistance is measured by the modulus of rigidity or the shear modulus. Thus, there are the two types of seismic waves, compressional and shear.

Operations such as blasting will always produce vibration or seismic waves. The reason for this is a quite simple. The purpose of blasting and other such operations is to fracture rock. This requires an amount of energy sufficient to exceed the elastic limit. When this occurs, the rock fractures. As fracturing continues, the energy is used up and eventually falls to a level less than the strength of the rock and fracturing stops. The remaining energy will pass through the rock, deforming it but not fracturing it because it is within the elastic limit. This will result in the generation of seismic waves. A simple schematic representation of compression and shear is shown in Figure 3 and Figure 4.

 Figure 3: Deformation by Compression

 Figure 4: Deformation of Shear


The fundamental properties that describe wave motion are called wave parameters. These are measured and quantified when discussing wave motion or vibration. Consider the simple harmonic wave motion illustrated in Figure 5 and represented by the equation:

 Figure 5: Wave Motion and Parameters

Period and frequency are reciprocals so that:

Wave length L is the distance from crest to crest or trough to trough measured in feet and is equal to the wave period multiplied by the propagation velocity V.


 4.1 Seismic Sensor

The function of vibration instrumentation is to measure and record the motion of the vibration earth. In basic scientific terms, this is a seismograph comprised of a sensor and recorder.

The sensor is in fact three independent sensor unit placed at right angles to each other. One unit is set in the vertical plane, while the remaining two units lie in the horizontal plane at right angles to each other. Each sensor will respond to motion along its axis. Three are necessary to completely determine the ground motion. The three units are enclosed in a case as shown in Figure 6.

 Figure 6: Seismograph Sensor

The configuration of the sensor case varies with the manufacturer, and may be round, square, rectangular, or triangular.

The sensor is usually an electromagnetic transducer which converts ground motion into electrical voltage. Inside the sensor is a coil of wire suspended in a permanent magnet field. The magnet is attached to the sensor case and cannot move, but the coil suspended in the magnetic field by springs or hinges is free to move. Any movement of the coil relative to the magnetic field will generate an electrical voltage proportional to the speed of the coil movement. If the coil moves slowly, a small voltage is generated. If the coil move fast, a large voltage is generated. When the ground vibrate, but the suspended coil inside will tend to remain motionless due to its inertia, thus producing relative motion between the coil and the magnetic field, resulting in the generation of an electrical voltage.

A schematic diagram of the sensor transducer is shown in Figure 7.

 Figure 7: Sensor Mechanism

The recorder takes the voltage output of the sensor, converts it back into motion, and produces a visual record of the ground motion. Since the sensor consists of three mutually perpendicular independent units, there will be three traces on the record, one for each sensor unit. This record is then ready for analysis and interpretation.

The recorder changes the output voltage of the sensor into motion by use of galvanometer. When a voltage is generated at the sensor, a current will flow through the circuit causing the galvanometer coil to move. Thus the electrical energy has been changed back into motion and may be amplified in the process. The recorder also puts timing lines and calibration signals on the record. Finally, the recording of the motion may be done photographically or by heat stylus.

The ground motion may also be recorded on magnetic tape. To obtain a record from magnetic tape, it is necessary to have a playback system and a chart recorder. This system is somewhat more involved, but adds increased flexibility. The tape can be played back at different amplifications or for varied analysis techniques. In addition, many events (i.e., blast) can be recorded on a single magnetic tape. The cassettes are inexpensive and easily available.

4.2 Seismograph System

There are many seismograph system, or simply seismographs, available today, each of which performs the basic function of measuring ground motion. The many variations are a response to need, constraints, and advancing technology. A brief description of the main types of seismograph will be helpful.
  • Analog seismograph is a three component system that produces a record of the ground motion. It is called analog because the record is an exact reproduction of the ground motion only changed in size, amplified, or de-amplified.
  • Tape seismograph is the same as the analog seismograph, except that it records on a magnetic tape cassette instead of producing a graphic record. A record of thee ground motion is obtained by use of a playback system and a chart recorder.
  • Vector sum seismograph is the standard seismograph system consists of three mutually perpendicular components. The vector sum seismograph performs the mathematical calculation electronically; that is, it squares the value of each of the components for each instant of time, adds them, and takes the square root of the sum. It then produces a record of the vector sum. 
  • Bar graph seismograph is a three component system that differs in its recording system. Instead of recording the wave from the ground motion at each instant of time, only the maximum ground motion of three components is recorded as a single deflection or bar whose magnitude can be read from the record graph. This is a very slow speed recording system which can be put in place and left to record for periods up to thirty or sixty days.
  • Triggered seismograph is an analog or tape seismograph which automatically starts to record when the ground vibration level reaches a predetermined set value, which triggers the system.

Most seismographs are equipped with meters that register and hold the maximum value of the vibration components and the sound level. Other seismographs are equipped to produce a printout which gives a variety of information such as maximum values for each vibration component, frequency of vibration for the maximum value, vector sum, and sound level. Blast information such as date, blaster number, time, location, job designation, and other pertinent information can also be added to the printout.

4.3 Vibration Parameters

Wave parameters were discussed earlier. Vibration parameters are the fundamental properties of motion used to describe the character of the ground motion. These are displacement, velocity, acceleration and frequency. As a seismic wave passes through rock, the rock particles vibrate, or are moved from the rest position. This is displacement, When the particle is displaced and moves, it then has velocity and can exert force that is proportional to the particle's acceleration. These fundamental vibration parameters are defined here:

  • Displacement is the distance that a rock particle moves from its rest position. It is measured in fractions of an inch, usually thousandths.
  • Velocity is the speed at which the rock particle moves when it leaves its rest position. It starts at zero, rises to a maximum, and returns to zero. Particle velocity is measured in inches per second.
  • Acceleration is the rate at which particle velocity changes. Force exerted by the vibrating particle is proportional to the particle acceleration. Acceleration is measured in fractions of “g”, the acceleration of gravity.
  • Frequency is the number of vibrations or oscillations occurring in one second, designated Hertz (Hz).

Vibration seismographs normally measure particle velocity since the standards of damage are based on particle velocity. There are, however, displacement seismographs and acceleration seismographs. Also, velocity seismographs can be equipped to electronically integrate or differentiate the velocity signals to produce a displacement or acceleration record.


5.1 Seismograph Record Content

Normally a seismograph record will show the following:

Four lines or traces running parallel to the length of the record. These traces are the vibration traces, while the fourth trace is the acoustic or sound trace. (There may not be an acoustic trace.)

Each of the four traces will have a calibration signal to show that the instrument is functioning properly. Timing lines will appear as vertical line crossing the entire record or at the top only, the bottom, or both top and bottom. An example of a typical seismogram, or vibration record, is shown in Figure 8.

 Figure 8: Vibration Record

One vibration trace or component is vertical, the other two horizontal. The components are usually specified as follows in Figure 9.
  • Vertical: motion up and down, designated V.
  • Longitudinal or Radial : motion along a line joining the source and the recording point, designated L or R.
  • Transverse : motion at right angles to a line joining the source and the recording point, designated T.

Figure 9: Vibration Components

The sensor normally has an arrow inscribed on the top. By pointing the arrow toward the vibration source, the vibration traces will always occur in the same sequence, with the arrow indicating the L component also the direction of motion will be consistent from shot to shot. The instrument manufacturer will indicate the proper sequences.

Each trace represents how the ground is vibrating in that component. If the seismograph is measuring velocity, then each trace shows how the particle velocity is changing from instant to instant in that component.

Similarly, if the seismograph is a displacement system or an acceleration system, the trace will show the instant to instant change in these parameters. The acoustic trace shows how the sound level changes with time.

5.2 Record Reading and Interpretation

The maximum vibration level is found by measuring the largest amplitude, either up or down from the zero line on any given trace. This value, in inches (usually fractions of an inch, e.g., 0.54), is then divided by the instrument gain setting. The result is the maximum ground motion. Figure 10 illustrates the operation.

Figure 10: Measure of Vibration Amplitude and Period

Assuming the seismograph is a velocity seismograph, the result in 0.108 in/s. If it is a displacement seismograph, the the result is 0.108 inches. If it is an acceleration seismograph, then the result is 0.108 g's.

To measure the frequency of vibration, it is necessary to examine the wave motion on the record. A sinusoidal wave would have equal amplitudes for the crests and through, and the distance between successive crests or troughs measures the period of the wave or the time for one complete oscillation. This is an ideal condition that does not normally occur on vibration records. That is, successive equal crests or trough do not usually occur. Hence, ti will be necessary to modify the procedure. The procedure will be illustrated for both the ideal case and for the normal occurring case.

Figure 10 shows several approximately equal successive crests. The distance between these successive peaks is measured by counting the number of timing lines from the first to the second peak. There are three,and since each timing space has a value of 0.01 seconds, then:

Now assume that the vibration record looks as following Figure 11.

 Figure 11: Half Period Measurement

Since there are not successive peaks or troughs of full equal amplitude, measurement will be made from the trough to the crest, this is a half period. There are 2.7 timing spaces from the trough to the crest in the first T/2 space going left to right. It is a trough to crest value since each timing line represents 0.02 seconds.

Notice the difference in T/2 spacing values. The second T/2 spacing represents the zero crossing value. The third spacing is a crest to trough value. Each of these spacing values will yield a different frequency. When records are read manually, this is a judgment call by the analyst and depends on this experience and expertise. The time values were read as multiple of the timing spaces. In practice it may be necessary to measure fractions of a timing space.

Frequency determined by reading from the record is highly susceptible to error. Frequency values read electronically by the seismograph system are much more precise and reliable. Early investigations and research show a wide scatter in data, and much of it no doubt is due to to the difficulties associated with reading frequency off a record.

Acoustic level are read in much same way, except that there is usually a base level below which the instrument does not respond. (What is the point in recording sound levels in the normal environmental range?) This base level value is then added to the computed value to give true sound level reading. The following example illustrates the procedure.

Sound level changes less than 3 dB are normally not noticeable to human ear.

5.3 Field Procedure and Operation Guides

Site selection is the first item of procedure. This is usually determined by a complaint or sensitive are which needs to be checked. If there is no such problem, then place the seismograph at the nearest structure that is not owned by or connected with the operation. The seismograph distance should always be less than or at most equal to the distance to the structure.

When dealing with residents or persons in the vibration affected are be factual and direct. Emphasize that the purpose of the seismograph measurement is to protect them and their property from vibration damage.

Place the sensor on solid ground. Do not place it on:
  1. Grass
  2. Isolated slab on stone or concrete
  3. Loose earth
  4. Any soft material
  5. Inside a structure except on a basement floor
  6. Concrete or driveway connected to a blast area

Failure to observe these precautions will result in distorted readings that are not representative of the true ground vibration. Level the sensor, some sensor have bull eye level on top for this purpose. Other can be leveled by eye. Make sure the sensor is solidly planted. In cases of large ground motion it may be necessary to cover the sensor with a sand bag, spike it down or dig a hole and cover it with earth otherwise the sensor may be decoupled from the earth and the vibration record will not represent the true ground motion. Remember that the ground displacement is usually only a few thousandths of an inch so do not expect to see the decoupling of the sensor.

Most sound measurement is made with a hand held microphone. Hold the mike at arm's length away to avoid reflection of the sound from your body. Regardless if the microphone is used set up on a stand or hand held, do not set it up in front of a wall. This will prevent sound reflection from the wall.

5.4 Practical Interpretations

The seismograph record can be used for much more than obtaining the peak particle velocity. It can be helpful in engineering the blast and provide information to the operator as to how to achieve the best vibration control as well as optimizing the use of the explosives energy to break rock. Assume one has a seismograph record that exhibits on large peak in the center of the wave trace. That large peak has a particle velocity of 2 in/s. Also assume that no other peak on the record is larger that 1.0 in/s. The one large peak is at 2 ms controlling how we design and execute all our blast in the future. In blasting, it is not the average vibration value that counts, it is the maximum. Therefore, common sense would dictate that if one could reduce that 2 in/s peak to 1 in/s, it would not only be better for the residence in the area but also would be more economical for the operator because he would not have to go extreme measures to try to reduce this one peak value.

What does this large peak mean from a practical standpoint? It was indicated that if it occurred in the center of the record, it is indicating that something occurred that was of unusual nature approximately half way through the blast. If one assumes that these peaks and the vibration record indicate energy release over time, then the record tells one that for some reason significantly more seismic energy was obtained approximately halfway through the blast. If all blastholes were loaded the same, this indicates there is inefficiency in the blasting process approximately half way through the blast. Now go back to the blasting pattern and determine approximately where the problem resulted. One might be able to then correct the problem so that one does not have to go to unusual measures to reduce vibration. A common problem which occurs is that if blastholes are wet, one commonly does not place as much energy in the wet portion of the hole as in a totally dry hole. This is because cartridge product is used instead of, for example, bulk ANFO. The smaller diameter cartridge product may not have as much energy as ANFO and therefore, the vibration level would increase. How one handles wet hole situations can greatly effect the vibration generated from the blast.

Another common problem is drilling inaccuracy. If a blasthole within the pattern has an excessive burden at the time it shoots, vibration levels go up. The seismograph record, therefore, can be used as a diagnostic tool to determine where within the blast the problem occurred which resulted in the higher vibration level.

Ideally, if one looks at vibration records and assumes that the peaks indicate energy release over time. Common sense would dictate the one would like to see all peaks near equal throughout the entire record. If this would occur, the explosive energy is being used efficiently and is reducing vibration to a minimum.

In the past, to expect a vibration record to have near identical peaks would have been considered an academic solution which was not practical in the field, however, today with advanced technology this type of vibration record can be achieved on blasts that are well engineered.


The unlucky instances in open pit blasting are misfires. The instances of misfire can be all but eliminated if well-trained, careful blasting crews and blasters are employed. When a blaster checks his blast area and states that a misfire does occur, the one who conduct the blasting activity (or the supervisor) has to still ask all blockers staying on position to block human and light vehicles.

In most instances, misfires are not a result of poor product. Instead, they occur because of:
1) the negligent use,
2) cut-off of the surface delay,
3) defective product, or
4) ignorance in the proper use of the product.
When a misfire does occur, the degree of hazard depends on the the type of explosives used in the blasthole. In the general cases, if the blastholes are relatively intact, the explosive should be shot in place. Shooting in place is the least hazardous method of handling the misfire.

However if the blasthole is broken away and in-hole delay can not be connected again with the initiator, you can make a new borehole with radial distance 3 m of the misfired blasthole. Ather that, fill it out with explosives and detonate it.


Safety in blasting is extremely important because of the inherent danger in explosive use. These publications, from The Institute of Makers of Explosives, are made available to the blaster to ensure that blasting is accomplished in a safe, efficient manner. It is not the intent of this writing to recite safety rules of review material which the Institute of Makers of Explosives has published. It is the intent of this writing to describe commonly violated guidelines and the causes of accidents in transportation, storage, and use of explosives.

Storage of Explosives

Explosives should be stored in accordance with federal, state and local regulations. When explosives are stored in a magazine, the magazine should be clean, dry, cool and well-ventilated. The magazine should be constructed so that it is bullet-proof, fire-resistant, and meeting all federal or state codes. Magazines should be locked with shielded locks to discourage theft.

 Figure 1: Electric Blasting Caps

Initiators such as electric blasting caps, nonel, or any other type of blasting cap should not be stored in the same magazine with high explosives. Blasting agents require less stringent storage requirements than high explosives. However, if they are placed in a high explosive magazine, their weight would count toward the total weight allowable in that magazine. Explosives in magazines should be thoroughly marked as to the date of purchase and the oldest products should be used first. Hazardous conditions sometimes arise when explosives, especially those contain nitroglycerin, begin to deteriorate due to age and leak onto the floor of the magazine.

No source of fire flame should be brought near an explosive magazine, and the magazine should be located so that there is no grass, brush, or debris nearby. When explosives are brought to the job, they must be stored in day boxes that meet federal or state codes. They should be placed in a area that is not in any danger from falling objects, fire or heavy equipment.

Transportation of Explosives

When explosives are transported on the highway, they should be transported in vehicles in proper working condition and equipped with federal, state or locally approved containers for safe transport. If the load is an open body truck, it should be covered with waterproof, fire-resistant tarpaulin. Unless the explosives are in the proper approved containers, caps and explosives should not be carried on the same vehicle. Smoking should not be permitted while loading and unloading the vehicle containing explosives. Trucks and vehicles containing explosives should bypass cities, towns or villages, if possible. The explosive cargo should be gently unloaded and cases should not be thrown onto the ground. Sometimes accidents can occur when truck fires have occurred during explosive transit. Personnel, including fireman, should be evacuated when explosive begins to burn. When the fire has reached the cargo, many types of explosives will detonate.

Handling of Explosives

>>> Electrical Hazards

Premature initiation of electric cap circuits have occurred. The hazards associated with electric blasting should be recognized  and known by the blaster and all in charge on the job. Electrical hazards can be broken down into six categories: current leakage, lighting, static electricity, stray currents, galvanic action, and radio frequency energy.

Current leakage into the ground is a hazard, but not because it results in premature initiation. It can, however, cause a round to misfire or partially fire since a large portion of the current may flow into the ground and not through the cap circuit. Conductive material such as wet shale, clay, magnetite, galena, and ammonium nitrate are more prone to current leakage problems. The reduced current which reaches the cap may be insufficient to fire the entire round. If such environmental conditions occur, the cap series should be set up such that the ground resistance is at least 10 times that of the series resistance. A blasting multi-meter can be used to make the proper tests.

Lightning is a hazard to both surface and underground blasting. Should a lightning bolt strike the blasting circuit, a detonation would most probably result with both electric or non-electric initiators. The probability that a direct hit would occur is remote, but a lightning bolt striking a far away object could induce enough current into an electric circuit to cause a detonation. The danger from lightning is increased if a fence, stream, or power transmission line exists between the blasting site and the storm. Underground blasting is not safe from lightning hazards since induced currents large enough to cause detonations can and have been transmitted through the ground. All blasting operations should cease and the area should be guarded when a storm is approaching. Commercially available lightning detectors can be purchase in areas where electrical storms are common.

Static electricity, both that occurring in nature and man-made, is a hazard to blasting. Electrical storms are not only dangerous because of lightning, but also because of the build up of a static electric front at some distance from the storm's center. The static charge can be stored on any ungrounded object. The insulations on the cap wires will not prevent the cap, whether shunted or unshunted, from detonating from static electrical discharge. The movement of particles under dry conditions can generate static charges. Particles of dust or snow driven by high winds, escaping steam under pressure, or motor driven belts can accumulate a static charge. To throw the wires of blasting cap into the air to straighten them in a snow or sandstorm can be dangerous. To minimize the hazards of static electric buildup from man-made sources, the equipment near the loading site should have all moving parts grounded. All metallic parts of any machinery should be kept away from the blasting circuit. Moving equipment should be shut down in the immediate area when the blasting circuit is being connected. Pneumatic loading of explosives constitutes a possible hazard. Some materials act as capacitors and become charged from small static charges and cause a premature detonation.

Stray currents can result from any power source. Electric current originating from a battery, transformer, or generator will always return to the source by any available path. Normally, it is expected that the current will return along insulated transmission lines or by a ground, which is the earth itself. If the return path is interrupted by a broken line or a blown fuse, high ground currents can result in an earth-grounded system. Under normal operating conditions, the return is continuous, the resistance of the earth is usually sufficiently high and the potential difference between two points close together on the ground is usually low. Exceptions can occur when two highly conducted beds are separated by a narrow bed of low conductivity material.

Dangerous currents in excess of 0.05 amps can be produced when leg wires contact rails, pipes, or ventilation ducts in underground operations. The maximum current which can be tolerated is 0.05 amps or 1/5th of the minimum firing current for one EBC which is 0.25 amps.

Power transmission lines are another source of current which can be hazardous. The cap wires or lead wires can be thrown by the blast. If this occurs and the wires touch the power transmission line, electrocution of the blaster can result. Ground currents can also exist near high power transmission lines, therefore, ground currents should be checked.

Galvanic action occurs when two dissimilar metals are in the presence of a conducting fluid. Premature explosions have occurred in the case where aluminum tamping poles were used in steel casing in the presence of an alkaline mud. The above situation can be compared to a crude battery.

Radio frequency (RF) transmitters which include television, radar, and A.M. and F.M. radio create powerful electromagnetic fields which decrease in intensity at distance from the transmission point. Tests have demonstrated that under certain conditions, electrical blasting wires may receive enough electrical energy to cause them to detonate. Although the possibility of premature detonation due to radio waves exists, the chance that they will occur is remote.

Commercial A.M. broadcast transmitters 0.555 to 1.605 MHz are potentially the most hazardous. They combine high power and low frequency resulting in small loss of radio frequency energy in the lead line. Mobile radio transmitters are a potential hazard because they can be brought directly into the blasting areas. The leg wires of blasting caps whether shunted or unshunted can act as a radio receiving antenna. The most hazardous condition exists when the circuit wires or leg wires are elevated a few feet above the ground.

>>> Blast Area Security

Blasting accidents have occurred due to the failure of the operator to clear the blasting area. Failure to clear blasting area can be broken down into other functions such as failure to follow instructions, inadequate guarding, having personnel under insufficient cover, or at an unsafe location. The end result can be injuries or fatalities from flyrock in the blasting area.

>>> Flyrock

An additional cause of fatalities, injuries, and property damage outside of the blasting area is flyrock. Occasionally flyrock can travel thousands of feet from poorly designed blast. Even with the best care and competent personnel, flyrock may not be totally avoided. The majority of flyrock problems which exist today are due to carelessness in the loading, execution, and design of blasts.

>>> Disposal

Accidents can  occur during the routine disposal of explosives. Safety rules are violated and workmen are often too close to the fires at the time of burning. When detonation occurs, workmen can be injured.


Figure 1: Elements of Rotary Bit (1)


Cones make up the cutting elements of the rock bit and are comprised of the following:
  1. Tungsten Carbide Inserts - which are pressed into the softer steel material with interface fit to hold item in place.
  2. Cone Thrust Button - made of a wear resistant material used to take axial bearing loads.
  3. Outer Cone Shell - insert land's and cone grooves.
  4. Cone Bore - internal ball and roller bearing races.
Tungsten Carbide Insert Rows:
A >>> Nose
B >>> Inner
C >>> Next to Gage
D >>> Gage
E >>> Gage Bevel

Figure 2: Elements of Cone


Coupled in threes, by 120 degree to form the bit body and the pin connection, the lugs are machined to hold the nozzles and a journal-bearing surface.


Nozzles are used to create back-pressure in the bit to force air through the bearing airways and increase the "air-blast" force to remove and flush cuttings from the bottom of the hole. Too large of a nozzle will cause insufficient volumes of air to be delivered to the bearings, while too small of nozzle will increase the back-pressure above the compressor modulation setting. When the compressor's modulation setting is reached, it will then reduce it's volume output causing a decrease in air volume to the bit.

 Figure 3: Elements of Rotary Bit (2)

Tungsten Carbide Inserts

Inserts are the actual physical elements that spall and break the rock. Inserts are made from tungsten carbide powder and a cobalt binder material, which is pressed into the designed shape then sintered. Depending on the application, the tungsten carbide inserts in a given bit will have a shape and physical properties best suited for the rock being drilled.

>>> Conical

Figure 4: Conical Insert

The conical insert is used primarily in medium/medium-hard rock.

>>> Chisel

Figure 5: Chisel Insert

The chisel insert is used in soft/medium-soft rock. It is the standard insert in soft bits.

>>> Ogive

Figure 6: Ogive Insert

The ogive insert is used in areas where aggressiveness of the conical insert is required with additional toughness.

>>> Super Scoop

Figure 7: Super Scoop Insert

The super scoop is used in very soft rock. With the patented offset tip, digging and gouging help in sticky material.

>>> Round Top

Figure 8: Round Top Insert

The ovoid or round top insert is used in the hardest formations. Its blunt geometry gives it the most fracture resistant design. The round top is the standard insert in hard bits.

>>> Trimmer

Figure 9: Trimmer Insert

The trimmer insert, used in soft to medium brittle rock formation, enhances the gage rows ability to cut the bore hole wall.

>>> Wedge Crested Chisel

Figure 10: Wedge Crested Chisel Insert

Wedge crested chisel insert are used exclusively on the gage rows of very soft to hard bits. This shape gives a fracture resistant insert that is much tougher than conical or regular chisel inserts on gage.

>>> Serrated Flat Top

Figure 11: Serrated Flat Top Insert

Serrated flat top inserts are used on shirttail lips and along the lug as "armor" to protect against shirttail and lug wear.

>>> Double Angle Conical

Figure 12: Double Angle Conical Insert

Double angle conical inserts belong hardmetal retard erosion and are able to increase ROP.


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The Theory of Air-Decking

As long ago as 1940, Russian scientist came up with the idea that by using explosive charges spaced with air gaps the efficiency of the blast could be improved. Further research carried out in Russia during the 1970's was confirmed by work in Australia and the USA.

When a charge is detonated in a blasthole the tremendous initial pressure greatly exceeds the strength of the rock. A strong shock wave begins to propagate into the rock, crushing and breaking it into extremely small particles. A large portion of the blast energy is spent unproductively in the area nearest the charge (the so-called 'crush zone') as the pressure produced by commercial explosives are far in excess of that required to simply fracture the rock. The pulverized region can often be observed in the area immediately around a half-barrel hole in the face.

By introduction an air gap (airdeck) within the explosive column, secondary or multiple stress waves are produced which extend the duration of their action, thus increasing the extent of crack propagation. The reduced blasthole pressure caused by the air-deck is still capable of creating an extended fracture system and there is sufficient high-pressure gas to obtain the desired amount of ground movement. The lower peak blasthole pressure reduces the loss of explosive energy associated with excessive crushing of the rock adjacent to the hole. This process adds only microseconds to the event and an observer would not notice anything different about the blast.

Creating Air-Decks

The most convenient way of creating an air-deck is to use a gas bag. The earliest types were no more than a development of a football bladder. They were simply lowered into the hole and than inflated from the surface using a small compressor or gas bottle. They could only be used near the top of the hole since they were not capable of sustaining more than the weight of stemming. The second-generation models were of the chemical type. A sachet of vinegar and some bicarbonate of soda were placed inside a sealed plastic bag. The sachet was broken, causing the two ingredients to react., producing carbon dioxide. This type is still widely used in large-diameter holes (greater than 200 mm). The third generation incorporate an aerosol within the plastic bag. Pressing the cap down on top activates a time delay of about 20s before the gas emerges. This allows sufficient time to place the bag anywhere in the hole. The bag fully inflates in a further 30s. The high-strength plastic used for the third generation one allows the bag to bear the heavy load of an explosive column for long periods of time.

 Figure 1: The Fist-Generation of Gas Bag

Figure 2: The Second-Generation of Gas Bag
(Stem Lock Gas Bag 8 - made for 3 to 16 in diameter hole)

Figure 3: The-Third Generation of Gas Bag
(Infladeck Gas Bag - made for 4 to 12.25 in diameter holes)

Air-Deck Volume

Figure 4 shows the results of some research work down in Australia. The air-deck volume is expressed as a percentage (or proportion) of the explosive volume plus the air-decked volume. In effect, this is the amount of explosives that can be removed from the blasthole and substituted with air (or water).

 Figure 1: Mean Fragment Size vs. Air Deck Volume

The graph indicates that as much as 30-40% of the explosive charge can be replaced by an air-deck before a significant deterioration in fragmentation is experienced. These results were produced from laboratory experiments and have been widely reported. Experience within the UK has confirmed that air-deck volumes of 25-30% can be employed in all rock types without noticeable loss of fragmentation.

Standard Air-Deck Techniques

The earliest application (and one still widely used in large-diameter holes) was simply to reduce the quantity of explosives in the hole by placing a gas bag at the base of the stemming. Figure 2 indicates how a UK quarry drilling 200 mm diameter holes saves 150 kg of ANFO from each blasthole. The air-deck volume is 33%.

 Figure 5: Earliest Application of Gas Bag

Table 1 illustrates the quantity of explosive that can be removed from a hole with  a 20% air-deck volume using 110 mm , 125 mm and 150 mm diameter holes 16 m deep.

Table 1: 20% Air-Deck Volume

Figure 6: Reducing Stemming Length by Application of Gas Bag

Reducing Oversize from The Stemming Area

The production of oversize material is an inevitable consequence in blasting operations. A typical blast may be produce 10-15 % of oversize material. The majority of oversize is produced from the stemming area and the rule of thumb in conventional blasting is that stemming depth should be [(0.7 till 1.0) x the burden]. Reducing it further may increase the risk of stemming ejection and flyrock. By placing a gas bag in the stemming area, allowing the explosive gasses to do useful work higher up without the risk of stemming ejection. Figure 6 illustrates such an example. The quarry has created a 5 m air-deck by stopping the explosive column at 7 m (as opposed to the normal 5 m) and by reducing the stemming from 5 m to 2 m. This represents a 28% air-deck, through which a saving of about 25 kg of ANFO has been achieved. By reducing the stemming depth the quarry has dramatically reduced the quantity of oversize produced in blast from around 10% to 2%.

Water-Deck Techniques

In order to reduce costs, most quarries try to maximize the use of ANFO, which is a relatively cheap and highly effective explosive. Where water is present it is necessary to resort to waterproof, packaged explosives.

Figure 7: Alternative 1 of The Water-Decked Volume

Some quarries prefer to place their normal waterproof base charge in the hole. This may still leave water above it. Rather than continue with a waterproof charge, Figure 8 shows how a gas bag can be placed on top of the water. There may be additional initiating costs.

 Figure 8: Alternative 2 of The Water-Decked Volume

Reduction in Maximum Instantaneous Charge

If there is a need to reduce ground vibrations from blasting operations, the first option is usually to employ multi-delay techniques. Two delays per hole cope with most situations, but as the site boundary or property are approached, more drastic measures may have to be considered. Reducing the borehole diameter and/or splitting the face are both costly and create logistical problems. The next option is to go to three delays per hole. Such an arrangement greatly increases the cost and complexity of the initiation system.

Figure 9 shows how a hole was loaded in a quarry where vibration control was essential. In order to maintain an agreed peak particle velocity (PPV), two delays per hole were used to achieve a maximum instantaneous charge (MIC) of 96 kg.

Figure 9: Application of  The Air-Decked Volume with Two Delays per Hole

The site management were considering three per hole, but decided to try air-deck techniques first. The diagram shows how they reduced the MIC from 96 kg to 68 kg. Based on data, there was no deterioration in the fragmentation or throw of the blast and they achieved a satisfactory reduction in PPV levels.

From : Quarry Management, April 1997 



Role of Stemming

Traditionally, the accepted procedure for directing the explosive energy into the surrounding rock mass is to load the blast hole with explosives and the remainder of the hole is filled with drill cuttings or imported aggregate. Drill cuttings are the most convenient stemming material, but are generally inadequate to fully contain explosive gasses if used with the optimum charge height for maximum blast efficiency. The stemming length is usually increased in an attempt to compensate for the loss of explosive energy. This result in mediocre blast result, usually with oversize material at the top of the shot.

Figure 1: Traditionally Stemmed Blasthole

  • To little stemming will allow the explosive gasses to vent, creating fly rock and air blast problems as well as reducing the effectiveness of the blast.
  • Too much stemming will result in poorly fragmented rock near the top. This is especially apparent with hard cap rock formations.
  • It is generally accepted that the shock from initial detonation of explosives in a blasthole is responsible for the cracking, spalling and weakening of the rock around a blast hole. The following rapid expansion of gasses provides the heave and resultant fragmentation. Thus, confining the gasses in the hole for as long as possible is important in maximizing the blast efficiency. This has been substantiated by studies indicating an inverse relationship between stemming ejection velocity and face velocity.

Product Description

The Stem Plug is a cone-shaped device constructed of high impact polystyrene. This material has a 15,000 psi compressive strength and is highly resilient. Stem Plug are available in 12 standard diameters ranging from 76 mm to 311 mm.

Table 1:Diameter of Stem Plug Based on Standardized Borehole Diamaters

Each plug is designed to occupy approximately 90% of the actual borehole diameter for the following reasons: (1) allowing space for stem plug to freely pass down wires and (2) compensating for drill bit wear. For maximum effectiveness always use the correct nominal size plug for stated borehole diameter range. For example, use 6-inch stem plug for a 6-inch diameter borehole.

Figure 2: Stem Plug

Method of Application

The first step in utilizing the Stem Plug is to create a buffer between the explosive column and the plug. This buffer be 1 1/2 times the diameter of the borehole and consist of a component stemming material. The purpose of the buffer is to protect the plug from superheated gas while still allowing the plug to provide the desired energy confinement.

Figure 3: Installation Step 1

Table 2:  A Good Rule of Thumb of Stemming Material (Standardized-Crushed Stone)

Next, the Stem Plug is lowered onto the buffer with the appropriate insertion tool. Tamp the stem plug on the buffer to ensure that it is properly seated.

Figure 4: Installation Step 2

Prior to disengaging the insertion tool from the Stem Plug, add at least one borehole diameter of stemming material to the borehole. This will secure the plug in place, allowing the insertion tool to be removed from the borehole while leaving the stem plug properly positioned within the stemming column.

Figure 5: Installation Step 3&4

Continue stemming the charged borehole to the collar or designated height. The stemming column is now equipped with the best available technology in blast energy confinement.

Figure 6: Installation Step 5

Upon detonation, explosive energy drives the stem plug upwards into the stemming column, the typical path of least resistance, and engages the stemming material in the borehole wall. Essentially a self-driving wedge, the stem plug will consistently replicate the "clogged gun barrel" effect to confine blast energy.

Figure 7: Stem Plug Mechanism

Insertion Tools

There is a pole which is available in a variety of 3-foot sections that may be joined together to accommodate most applications. Note the reverse thread on the tip. This allows the plug to be disengaged from the tip without loosening the pole sections.

 Figure 8: Standard Loading Pole Sections

Figure 9:Another Alternative Loading Pole for Use With 76 to 200-mm-stem plug

Blasting Benefits

The stem plug has one primary function is to seal the stemming column upon detonation. When applied correctly to a properly functioning shot, the benefits of this plug can be tremendous.

1) Higher Powder Columns
With improved stemming efficiency, the powder  column can be safely raised to utilize more of the borehole. The improved energy distribution reduces oversize generated by excessive stemming column lengths.

2) Improved Decking Efficiency
Studied show that maximum utilization of energy is obtained with a combination of the longest air deck and the shortest stemming deck that will provide adequate confinement of borehole gasses.

3) Air Blast and Dust Reduction
Lower stemming ejection velocities and reduced venting translate to reduce air blast. Reductions between 8 to 25 decibels may be achieved, depending on the rock type.

4) Flyrock Control
In all applications, it has been found that eliminating the venting of explosive force through the borehole collar greatly reduces or even eliminates fly rock. Thus, in addition to improving blasting safety in general, it is possible to extend the safe use of explosives into areas in which it would otherwise be marginal.