Blast
Content
General (See Summary for main points) Surface mining and quarrying operations usually require the use of blasting for the preparation of rock for excavation. As urban areas spread and as the mines become deeper, the constraints on blasting increases. This has resulted in an increased requirement for well trained and experienced personnel for the design and implementation of the blasts. When appropriate precautions and safety procedures are implemented, blasting can be safe. The cost efficiency of blasting is often greater than that of mechanical methods such as ripping by a dozer or removing rock with hydraulic breakers. Blasts need to be designed and implemented so as to minimize:
the risk of damage to nearby structures disturbance of people who live and/or work near the blast area
This course describes methods of rock blasting and how they may be safely and cost efficiently employed. Included are the following topics:
Types of Explosives Initiators and Initiating Systems Mechanisms of Rock Fracture by Explosives Fundamentals of Production Bench Blasting
Blasting Techniques for Producing Stable Rock Slopes Preventing Damage From Ground Vibration, Air Blast and Flyrock Safety Procedures
Also included is a list of references (from which more details of the products and techniques described in this manual can be obtained), and a glossary of blasting and excavation terms.
Blasting Applications in Surface Mining and Quarrying (See Summary for main points) The following is a brief description of some of the types of blasts commonly used in surface mines and quarries: Production Bench Blasts This is the most common type of blasting. The goal is to fragment and loosen the rock in preparation for excavation by front end loaders, shovels, draglines or dozers. The amount of preparation or conditioning of the rock, to be done by the blasting, depends on the rock mass characteristics as well as the type, size and mode of operation of the excavation equipment. Wall Control Blasts
These blasts are designed to break the rock near or up to the final pit or quarry limit while causing minimal damage to the rock beyond this limit. A number of techniques are used to achieve this including: line drilling (?), presplitting (?), trim blasting (?), cushion blasting (?) and buffer blasting (?). Throw/Cast Blasts In many surface coal mines there is an economic incentive to move a significant portion of the overburden material to its final position by blasting. The throw or cast blasts are designed in such a way as to maximize the horizontal movement of the rock in the desired direction. Sinking Cut Blasts These blasts are usually the first blast in a new bench where a vertical free face is not available. They are designed to break and loosen the rock volume using the surface as the only free face. A ramp is dug through the muckpile to the floor of the new bench and a free face established.
Non-Explosive Methods of Rock Breakage (See Summary for main points)
There are some circumstances where nonblasting methods of rock breakage should be employed. For example, in locations where there is nearby equipment or structures which are extremely sensitive to vibrations, or in areas of soft rock where it is more economical to rip than to blast. The following is a brief description of some non-explosive methods of rock breakage. Ripping Weak rock can often be broken by ripping using an excavator or, in harder rock, a bulldozer. The most commonly used criterion for determining whether ripping is possible, and for selecting the appropriate equipment, is the seismic velocity of the rock. The Caterpillar Equipment Handbook provides information on seismic velocities and the required ripping equipment. The advantage of ripping over blasting in weak rock is that ripping can be carried out to close tolerances, and there is virtually no damage to the rock in the sides of the excavation. Ripping can also be done after dusk. Hydraulic Splitting For small excavations in brittle rock, an hydraulically powered splitter or wedge can be used to create a tensile fracture between closely spaced drillholes. This is a slow but highly controlled method of rock breakage. It is most often used for breaking concrete and in dimensional stone quarries, or heavily loaded foundation areas where overbreak from blasting would reduce the bearing capacity of the rock.
Hydraulic Breakers Hydraulically powered breakers, often mounted on excavator boom, can be used to break both strong and weak rock. This method of rock breakage can be carefully controlled, and is sometimes used in conjunction with ripping where lenses of harder rock are encountered. Disadvantages of this equipment are that it is noisy, and not effective in very hard bedrock with few cracks or discontinuities. Chemicals Another product on the market is a cementlike compound that expands when mixed with water. The expansion pressure is sufficient to break rock when the material is placed in a row of closely spaced holes. While this is a highly controlled method of breaking rock, it has the disadvantages that the material is costly and the expansion process takes 10 to 20 hours.
Background (See Summary for main points) The interaction of the explosive and the surrounding rock mass during and immediately after detonation is a function of the detonation (?) properties of the explosive and the dynamic physical properties of the adjacent rock mass. The theories of rock breakage and the mechanisms of muckpile formation are based on the interaction of the detonating explosive and the surrounding
rock. An understanding of the mechanism of rock breakage by explosives enables the blast designer to fragment the rock mass economically, while minimizing the damage caused by the blast beyond the excavation perimeter. The mechanisms by which rock is fractured by explosives are fundamental to the design of blasting patterns. They also relate to the damage that can be suffered by surrounding rock and structures and to the reactions of people living in the vicinity of a blast.
Theories of Rock Breakage (See Summary for main points) There are many theories and models that attempt to describe the process that occurs during and after the detonation of an explosive charge in a rock mass. In general terms this process involves the rapid release of energy by the explosive, the application of the energy to the rock and the subsequent response of the rock to the application of the energy. It is complicated by such things as the rate, type and amount of energy released by the explosive, the design of the blast and the properties of the rock mass. The mechanisms of rock breakage that have been identified (Hagan (1967), Hagan (1973) and Mercer (1980)) are:
crushing relative radial motion
release of load spalling gas extension of strain wave-generated and/or natural cracks flexural rupture shear fracturing along natural and strain wave generated cracks in-flight collisions
These mechanisms cause varying amounts of breakage depending on the characteristics of the explosive, rock properties and geometry of the rock mass and explosive charge. They can be split into two categories, those caused by the shock component of the energy from the explosive and those resulting from the gas energy (Brown (1956)).
Shock Energy Breakage Mechanisms (See Summary for main points) When an explosive is detonated, it is converted within a few thousandths of a second into high-temperature gases. When confined in a blasthole, this very rapid reaction causes pressures that usually exceed about 1800 GPa (approximately 260 kips/in2) to be exerted against the blasthole wall. This energy is transmitted into the surrounding rock mass in the form of a compressive strain wave, or shock wave, which travels at a
velocity of 2000 - 6000 metres per second (6500 - 20000 ft/sec) through the rock. The rock breakage mechanisms that can be attributed to the shock component of the energy released by an explosive are crushing, relative radial motion, release of load and spalling. Crushing occurs around a blasthole wall when the pressure in the detonation front exceeds the dynamic compressive strength of the rock (Hagan (1973) and Bauer (1978)). The out-going strain pulse generated by the high pressure detonation front disperses and loses energy rapidly. Crushing ceases when the strain level in the pulse drops below the elastic limit of the rock. This is usually very close to the blasthole wall. The rock that forms the wall of the blasthole outside the crushed zone is subjected to very sudden compression due to the dispersing strain pulse as illustrated in Figure 1. This compression (i.e. relative radial motion) results in tangential stresses which can cause cracks to develop radially from the blasthole (see Figure 1). The radial cracks initially develop in all directions from the blasthole wall and are not influenced by a local free face. The mechanism of fracturing caused by release of load occurs immediately after the strain or compression pulse passes, resulting in a local decrease in density with subsequent tensile stresses. These tensile stresses
produce fractures aligned perpendicularly to the direction of travel of the strain pulse (Clay et al. (1965)). Spalling occurs when a compression or strain pulse is reflected by a free surface. At this point two waves are generated, a tensile wave and a shear wave. The tensile wave may cause cracking and the rock to spall in the region of the free surface. Both the tensile and shear waves may extend pre-existing or new (i.e. formed by the initial out-going strain pulse) cracks.
Gas or Heave Energy Breakage Mechanisms (See Summary for main points) The mechanisms of fracturing described above are caused by the initial strain or compression pulse from the detonating explosive charge. A zone of very high pressure and temperature gases occupies the blasthole behind the detonation front. These gases penetrate the crushed zone around the blasthole and flow into the radial or naturally occurring cracks. The gas pressure tends to wedge open the cracks and cause them to extend. The pressure on the blasthole walls caused by the explosive generated gases and the stress field due to the pressurised cracks displaces the rock mass between the blasthole and the free face. Because of the geometry of the explosive charge and the rock mass (see Figure 2) the rock at the face bends causing fracturing by flexural rupture (Mercer (1980)). This has been observed in high-speed films
and still photographs of experimental and production blasts. An example is shown in Figure 3.
Shear fracturing occurs when adjoining rock is displaced at different times or at different rates. The displacement is caused by the high pressure gases. Some fracturing occurs when rock particles which are in motion collide. The amount of fracturing resulting from this mechanism depends on the geometry of the explosive charges, their order and relative time of initiation plus the physical properties of the rock. The previous four mechanisms of rock fracturing are a result of the high pressure and temperature gases acting on the rock mass. This gas energy also plays an important role in the displacement of the muckpile.
Mechanics of Muckpile Displacement (See Summary for main points) The movement of material in a blast is primarily a result of the high pressure gases produced by the detonation of explosive
charges (Edl (1983)). These high pressure gases flow into the cracks surrounding the blasthole forming a hydrostatically stressed region. The shape of this region depends on the geometry of the explosive charge and the blasthole, though in most mining applications the explosive charge is in the form of a long cylinder creating a cylindrically shaped hydrostatically stressed region. The high pressure gases in this region exert a force in all directions with material movement occurring in the direction of least resistance. The amount of displacement of material in a blast is a function of the physical properties of the material, blasthole orientation, burden distance, spacing between blastholes, sequence and relative time of initiation of charges, amount and distribution of the explosive and the properties of the gas generated by the detonating explosive. These factors all influence the length of time the high pressure gases are contained within the rock mass and therefore determine the amount of work performed by them (Lownds (1986) and Brinkmann (1990)). When these gases vent to the atmosphere they stop doing useful work.
Influence of Blast Design on Explosive/Rock Interaction (See Summary for main points) The fragmentation achieved by the process is highly dependent upon the degree of confinement and coupling (?) of charges within the blastholes, the amount of burden (?) and the sequencing of the blast. If
confinement of the charges by stemming is inadequate, some energy will be lost from the blastholes. Inadequate coupling results in poor transmission of the strain wave to the rock mass. Excessive burdens result in choking and poor movement of the rock, whereas inadequate burden results in a waste of explosive energy and excessive throw of blasted rock. Effective delaying of individual blastholes ensures maximum development and utilization of free surfaces by reducing effective burden, provides freedom for the rock to move toward the free face, and reduces the extent of damage to surrounding rock.
the risk of damage to nearby structures disturbance of people who live and/or work near the blast area
This course describes methods of rock blasting and how they may be safely and cost efficiently employed. Included are the following topics:
Types of Explosives Initiators and Initiating Systems Mechanisms of Rock Fracture by Explosives Fundamentals of Production Bench Blasting
Blasting Techniques for Producing Stable Rock Slopes Preventing Damage From Ground Vibration, Air Blast and Flyrock Safety Procedures
Also included is a list of references (from which more details of the products and techniques described in this manual can be obtained), and a glossary of blasting and excavation terms.
Blasting Applications in Surface Mining and Quarrying (See Summary for main points) The following is a brief description of some of the types of blasts commonly used in surface mines and quarries: Production Bench Blasts This is the most common type of blasting. The goal is to fragment and loosen the rock in preparation for excavation by front end loaders, shovels, draglines or dozers. The amount of preparation or conditioning of the rock, to be done by the blasting, depends on the rock mass characteristics as well as the type, size and mode of operation of the excavation equipment. Wall Control Blasts
These blasts are designed to break the rock near or up to the final pit or quarry limit while causing minimal damage to the rock beyond this limit. A number of techniques are used to achieve this including: line drilling (?), presplitting (?), trim blasting (?), cushion blasting (?) and buffer blasting (?). Throw/Cast Blasts In many surface coal mines there is an economic incentive to move a significant portion of the overburden material to its final position by blasting. The throw or cast blasts are designed in such a way as to maximize the horizontal movement of the rock in the desired direction. Sinking Cut Blasts These blasts are usually the first blast in a new bench where a vertical free face is not available. They are designed to break and loosen the rock volume using the surface as the only free face. A ramp is dug through the muckpile to the floor of the new bench and a free face established.
Non-Explosive Methods of Rock Breakage (See Summary for main points)
There are some circumstances where nonblasting methods of rock breakage should be employed. For example, in locations where there is nearby equipment or structures which are extremely sensitive to vibrations, or in areas of soft rock where it is more economical to rip than to blast. The following is a brief description of some non-explosive methods of rock breakage. Ripping Weak rock can often be broken by ripping using an excavator or, in harder rock, a bulldozer. The most commonly used criterion for determining whether ripping is possible, and for selecting the appropriate equipment, is the seismic velocity of the rock. The Caterpillar Equipment Handbook provides information on seismic velocities and the required ripping equipment. The advantage of ripping over blasting in weak rock is that ripping can be carried out to close tolerances, and there is virtually no damage to the rock in the sides of the excavation. Ripping can also be done after dusk. Hydraulic Splitting For small excavations in brittle rock, an hydraulically powered splitter or wedge can be used to create a tensile fracture between closely spaced drillholes. This is a slow but highly controlled method of rock breakage. It is most often used for breaking concrete and in dimensional stone quarries, or heavily loaded foundation areas where overbreak from blasting would reduce the bearing capacity of the rock.
Hydraulic Breakers Hydraulically powered breakers, often mounted on excavator boom, can be used to break both strong and weak rock. This method of rock breakage can be carefully controlled, and is sometimes used in conjunction with ripping where lenses of harder rock are encountered. Disadvantages of this equipment are that it is noisy, and not effective in very hard bedrock with few cracks or discontinuities. Chemicals Another product on the market is a cementlike compound that expands when mixed with water. The expansion pressure is sufficient to break rock when the material is placed in a row of closely spaced holes. While this is a highly controlled method of breaking rock, it has the disadvantages that the material is costly and the expansion process takes 10 to 20 hours.
Background (See Summary for main points) The interaction of the explosive and the surrounding rock mass during and immediately after detonation is a function of the detonation (?) properties of the explosive and the dynamic physical properties of the adjacent rock mass. The theories of rock breakage and the mechanisms of muckpile formation are based on the interaction of the detonating explosive and the surrounding
rock. An understanding of the mechanism of rock breakage by explosives enables the blast designer to fragment the rock mass economically, while minimizing the damage caused by the blast beyond the excavation perimeter. The mechanisms by which rock is fractured by explosives are fundamental to the design of blasting patterns. They also relate to the damage that can be suffered by surrounding rock and structures and to the reactions of people living in the vicinity of a blast.
Theories of Rock Breakage (See Summary for main points) There are many theories and models that attempt to describe the process that occurs during and after the detonation of an explosive charge in a rock mass. In general terms this process involves the rapid release of energy by the explosive, the application of the energy to the rock and the subsequent response of the rock to the application of the energy. It is complicated by such things as the rate, type and amount of energy released by the explosive, the design of the blast and the properties of the rock mass. The mechanisms of rock breakage that have been identified (Hagan (1967), Hagan (1973) and Mercer (1980)) are:
crushing relative radial motion
release of load spalling gas extension of strain wave-generated and/or natural cracks flexural rupture shear fracturing along natural and strain wave generated cracks in-flight collisions
These mechanisms cause varying amounts of breakage depending on the characteristics of the explosive, rock properties and geometry of the rock mass and explosive charge. They can be split into two categories, those caused by the shock component of the energy from the explosive and those resulting from the gas energy (Brown (1956)).
Shock Energy Breakage Mechanisms (See Summary for main points) When an explosive is detonated, it is converted within a few thousandths of a second into high-temperature gases. When confined in a blasthole, this very rapid reaction causes pressures that usually exceed about 1800 GPa (approximately 260 kips/in2) to be exerted against the blasthole wall. This energy is transmitted into the surrounding rock mass in the form of a compressive strain wave, or shock wave, which travels at a
velocity of 2000 - 6000 metres per second (6500 - 20000 ft/sec) through the rock. The rock breakage mechanisms that can be attributed to the shock component of the energy released by an explosive are crushing, relative radial motion, release of load and spalling. Crushing occurs around a blasthole wall when the pressure in the detonation front exceeds the dynamic compressive strength of the rock (Hagan (1973) and Bauer (1978)). The out-going strain pulse generated by the high pressure detonation front disperses and loses energy rapidly. Crushing ceases when the strain level in the pulse drops below the elastic limit of the rock. This is usually very close to the blasthole wall. The rock that forms the wall of the blasthole outside the crushed zone is subjected to very sudden compression due to the dispersing strain pulse as illustrated in Figure 1. This compression (i.e. relative radial motion) results in tangential stresses which can cause cracks to develop radially from the blasthole (see Figure 1). The radial cracks initially develop in all directions from the blasthole wall and are not influenced by a local free face. The mechanism of fracturing caused by release of load occurs immediately after the strain or compression pulse passes, resulting in a local decrease in density with subsequent tensile stresses. These tensile stresses
produce fractures aligned perpendicularly to the direction of travel of the strain pulse (Clay et al. (1965)). Spalling occurs when a compression or strain pulse is reflected by a free surface. At this point two waves are generated, a tensile wave and a shear wave. The tensile wave may cause cracking and the rock to spall in the region of the free surface. Both the tensile and shear waves may extend pre-existing or new (i.e. formed by the initial out-going strain pulse) cracks.
Gas or Heave Energy Breakage Mechanisms (See Summary for main points) The mechanisms of fracturing described above are caused by the initial strain or compression pulse from the detonating explosive charge. A zone of very high pressure and temperature gases occupies the blasthole behind the detonation front. These gases penetrate the crushed zone around the blasthole and flow into the radial or naturally occurring cracks. The gas pressure tends to wedge open the cracks and cause them to extend. The pressure on the blasthole walls caused by the explosive generated gases and the stress field due to the pressurised cracks displaces the rock mass between the blasthole and the free face. Because of the geometry of the explosive charge and the rock mass (see Figure 2) the rock at the face bends causing fracturing by flexural rupture (Mercer (1980)). This has been observed in high-speed films
and still photographs of experimental and production blasts. An example is shown in Figure 3.
Shear fracturing occurs when adjoining rock is displaced at different times or at different rates. The displacement is caused by the high pressure gases. Some fracturing occurs when rock particles which are in motion collide. The amount of fracturing resulting from this mechanism depends on the geometry of the explosive charges, their order and relative time of initiation plus the physical properties of the rock. The previous four mechanisms of rock fracturing are a result of the high pressure and temperature gases acting on the rock mass. This gas energy also plays an important role in the displacement of the muckpile.
Mechanics of Muckpile Displacement (See Summary for main points) The movement of material in a blast is primarily a result of the high pressure gases produced by the detonation of explosive
charges (Edl (1983)). These high pressure gases flow into the cracks surrounding the blasthole forming a hydrostatically stressed region. The shape of this region depends on the geometry of the explosive charge and the blasthole, though in most mining applications the explosive charge is in the form of a long cylinder creating a cylindrically shaped hydrostatically stressed region. The high pressure gases in this region exert a force in all directions with material movement occurring in the direction of least resistance. The amount of displacement of material in a blast is a function of the physical properties of the material, blasthole orientation, burden distance, spacing between blastholes, sequence and relative time of initiation of charges, amount and distribution of the explosive and the properties of the gas generated by the detonating explosive. These factors all influence the length of time the high pressure gases are contained within the rock mass and therefore determine the amount of work performed by them (Lownds (1986) and Brinkmann (1990)). When these gases vent to the atmosphere they stop doing useful work.
Influence of Blast Design on Explosive/Rock Interaction (See Summary for main points) The fragmentation achieved by the process is highly dependent upon the degree of confinement and coupling (?) of charges within the blastholes, the amount of burden (?) and the sequencing of the blast. If
confinement of the charges by stemming is inadequate, some energy will be lost from the blastholes. Inadequate coupling results in poor transmission of the strain wave to the rock mass. Excessive burdens result in choking and poor movement of the rock, whereas inadequate burden results in a waste of explosive energy and excessive throw of blasted rock. Effective delaying of individual blastholes ensures maximum development and utilization of free surfaces by reducing effective burden, provides freedom for the rock to move toward the free face, and reduces the extent of damage to surrounding rock.
