A rock burst is a sudden — and potentially violent — dislodging of a rock mass from the walls of a mine, tunnel or quarry. It is typically caused by the rapid or even instantaneous release of accumulated energy. Rock bursts are often present in the deep-level mining of hard, brittle rock deposits. Rock bursts may result in the closing of a mine opening, and are often accompanied by ground tremors or rockfalls.
Related: Safe Mining Practices
There are five main types of rock bursts:
- Strain bursts:
A strain burst is a violent failure around the highest stress point of stope that causes small pieces of rock to be expelled from the boundary of excavation. Strain bursts are usually relatively limited in the amount of damage they cause, since the amount of energy released is so small.
- Pillar bursts:A pillar burst is caused the local stress redistribution inherent in stoping and is marked by the violent failure of one of the pillars inside the stope. The amount of damage a pillar burst causes depends not only on the location of the burst, but also on the state of the surrounding pillars and rock formations. Since there is a large amount of stress redistributed to the remaining pillars due to the initial pillar failure, a domino effect may occur whereby several other pillars may fail violently in succession.
- Fault-slip burst:Fault-slip bursts are caused by the reduction of shear resistance and an increase in shear force across a fault line. Most fault-slip bursts occur in deep mines where the amount of stress acting on the fault is decreased due to nearby mining operations. Fault-slip bursts tend to release a large amount of seismic energy due to the fact that there is an instantaneous relaxation of the strain that was stored in the rock surrounding the slip or rupture.
- Shear rupture:A shear rupture rock burst is caused when the compressive stresses exerted on the rock exceed the shear strength of the rock. Once this happens, there is a violent propagation of shear fractures through the otherwise-intact rock mass, which releases seismic energy on a similar scale to a fault-slip burst, often causing large-scale rock bursts that exceed a 3.5 reading on the Richter scale.
- Buckling:Buckling is the result of a rock face being subjected to compressive stresses. It is characterized by the sudden release of locally stored energy, which results in the sideways deflection of part of a structure, column or pillar. Buckling damage is most likely to occur in mine openings that are composed of laminated or transversely anisotropic rock, but may occur anywhere around the periphery of a mine opening that has been subjected to enough compressive forces to cause the opening to buckle.
- Strain bursts:
The History of Rock Bursts
The first recorded incident of a rock burst occurred in the early twentieth century, in the gold mines in the Witwatersrand scarp in the Gauteng province of South Africa. After that first incident, there have been several other recorded rock blasts at the Witwatersrand mines, including one on April 21, 1928, at the City Deep mine and one in 1936 at the Simmer and Jack mines, which caused enough damage to halt ore removal operations for four months.
Rock burst incidents haven’t just been confined to the early twentieth century, either. According to a list compiled by the CDC, there have been several rock burst-related incidents at non-coal mines in the US вЂ” one in 1939, one in 1943 and two in 1952. While coal mines generally deal with incidents in the form of methane gas or coal dust explosions, there has been at least one recorded rock burst incident at a coal mine, which was the fall of a rock face at the Crandall Canyon Mine in Huntington, Utah on August 6, 2007, which claimed the lives of six mine workers.
The most recent non-coal rock burst incident occurred on December 14, 2011 at the Lucky Friday mine near Mullan, Idaho, where the rock explosion forced 20 Hecla mine workers to flee to safety. Seven of them were later hospitalized.
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Where Do Rock Bursts Occur?
Rock bursts can occur in many different kinds of mines. Of the incidents listed in the CDC chart referenced in the section above, one occurred in a zinc mine, one occurred in a lead mine, one in a gold prospect and the last one at a gold prospect in Herron, Michigan. The Crandall Canyon mine incident in Utah occurred in a coal mine, while the Lucky Friday mine incident occurred at a combination lead, zinc and silver mine.
The type of rock inside the mine is not necessarily the determining factor for whether or not a rock burst will occur inside of a mine. Rather, there are three conditions that need to be present in order for a rock burst to be triggered:
- There must be a high amount of stress present in the rock.
- There must be a high amount of stiffness in the rock formation.
- There must be a free surface available for the rock to burst into, such as those in front of rock faces or around rock pillars where mining operations take place.
While these conditions are present in most commercial mining operations, they become especially prevalent in deeper mines, which require more complex structures to keep the mine intact. For example, once the Wright-Hargreaves mine reached a depth of 7,272 feet, rock bursts began to occur in the gold-bearing porphyry that made up the mine walls.
Both the mine depth and the increased complexity of mine support mechanisms increase the amount of stress the remaining rocks in the mine have to bear in order to keep their equilibrium, leading to an increased chance of a rock burst happening as the mine structures attempt to equalize the stresses borne by the removed portions of rock.
For example, before the Crandall Canyon incident, the use of room-and-pillar mining techniques created the conditions that made the incident possible. In the months before the August 6 rock face collapse, the miners hollowed вЂњroomsвЂќ and вЂњhallwaysвЂќ out of the coal deposits, leaving pillars of intact coal in between them. Following the initial room-and-pillar extraction of the coal in the mine, the miners began the practice of retreat mining, whereby they mined out the pillars in the mine from the back, working inward toward the mine entrance.
On March 10, 2007, the north pillar of the mine burst, causing a partial collapse of the area and forcing the miners to evacuate. Following the collapse, the mine was reduced to producing coal with a higher ash content. Because of this, Genwal Resources, Inc., which owned the mine, was unable to meet some of its contractual obligations. In order to meet those contractual obligations, Genwal convened a meeting on March 21 and decided to resume mining operations at the southern pillar.
Mining at the south pillar resumed on July 15, 2007 and continued until August 6, 2007, the date of the accident. At 2:48 a.m. on August 6, a coal outburst accident occurred at the south barrier section of the mine. Within seconds, pillar failures had propagated out to neighboring areas, trapping the six miners working in the area and leaving them stranded 1500 feet underground and more than three miles from the mine entrance. The barrier pillars to the north and south of the south barrier section also failed, releasing oxygen-deficient air into the trapped miners’ work area.
Rescue operations began later in the morning on August 6 and continued until August 16, at which time a second pillar burst accident occurred, costing the lives of two rescue workers and a Mining Safety and Health Administration (MSHA) coal mine inspector.
Controlling Rock Bursts and Protecting Miners From Rock Bursts
In order to ensure there is a procedure in place to control rock bursts like the ones that occurred at Crandall Canyon and the Lucky Friday mine, the MSHA requires all operators of metal and non-metal mines to devise and implement a rock burst control plan within a 90-day time frame after the first occurrence of a rock burst on site.
Plans should include:
- Mining and operating procedures which have been designed to reduce the occurrence of rock bursts
- Monitoring procedures with regard to where rock burst detection methods are used
- Any other measures deemed necessary by the mine operators to minimize the exposure of mine employees or other persons to areas which are prone to rock bursts
Rock burst control plans are required to be updated as conditions warrant and must be made available to MSHA inspectors and to mine employees.
Rock bursts can be controlled or even prevented through the use of several different techniques, such as destress blasting, rock reinforcement or the application of a liner or blast shield.
Destress blasting is, as the name implies, the use of blasting charges to fracture, vibrate or otherwise remove rock around the mine face in order to reduce the strength of the rock around the blast hole and reduce the friction of the joint planes inherent in the rock, which together reduce the stresses near the active mine face.
Rock reinforcement refers to shoring up or otherwise supporting an active mine face. The three main goals of rock reinforcement are to strengthen the rock mass and control bulking, to retain broken rocks in order to prevent the failure of fractured blocks and to hold fractured blocks and tie back both the fractured elements and the retaining elements to stable ground. The goal of strengthening the rock mass is to enable the rock mass to support itself and control the bulking process in order to prevent fractures from opening up or further propagating in places where they are already open.
There are several different ways to do this, with perhaps the simplest being to shore the rock up with a series of rock bolts to strengthen the rock, followed by the application of a thin spray-on liner (TSL), such as Mineguard or Rockguard, to hold any fractured blocks and tie them back to the non-fractured rock face. TSLs, which are made out of a variety of materials like polyurethane, latex, cement and polyurea, are designed to provide support over areas of substantial rock deformation so the fractured rock can be held in an interlocked position to generate frictional strength reinforcement.
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While TFLs have been shown to reduce the number of fractures that appear in a rock face following a rock burst event, liners can only resist shear displacements of up to a few millimeters. If a larger deformation occurs, an adhesive failure around the rock may occur, followed by a tensile rupture at the deformation point.
If a thin spray-on liner does not provide adequate rock burst protection, another option is to use a more solid liner, such as shotcrete or cast-in-place concrete. Shotcrete is concrete or mortar that is extruded through a hose that can then be applied to any number of other support methods, such as steel rods, wire mesh or simple fibers.
While shotcrete and cast-in-place concrete do not actually reinforce the mine face themselves (a process which is done by the bolts put through both the concrete and the rock face), they act to retain the fractured rock following a rock burst event. However, since they are poured, hardened materials, neither shotcrete nor pour-in-place concrete handle deformation well, being subjected to the possibility of cracking, much like regular concrete.
In addition, shotcrete and pour-in-place concrete can be expensive, costing about 87 cents per square foot for just the shotcrete alone, which does not factor in the cost of the work teams needed to mix, cure and isolate the shotcrete once it has been sprayed onto the mine face.
Backfill is another option to shore up a mine face. Simply put, backfill is the use of previously excavated material waste to support the areas around the active mine. While backfill has evolved from the sand and water mixtures used fifty years ago, the end goal remains the same: to provide a cost-effective measure that can perform as required. However, it still has its disadvantages. Filling an area with backfill can be a highly labor-intensive process that, at the end of the day, may just end up adding another complex structure to the mining area, which then may need to be reinforced in the same way the mine is.
In addition, depending on the type of backfill used, the amount of machinery required to transport the backfill to the mining site, such as locomotives or tramcars, can be both expensive and a major cause of gas pollution.
The fourth and probably most flexible option would be the installation and use of a blast shield or blast mat. While the main purpose of a blast shield is to protect mine personnel during blast-hole drilling or the opening of pit mines, blast shields can also be deployed as a method to shore up a mine face, prevent or control the damage to a mine that is caused by the various types of rock bursts, or protect miners during rock burst control methods such as destress blasting.
To protect miners from rock burst events, blast mats are anchored around the active mine face, either through the use of rock bolts such as in the TSL example above, or through the use of soil or backfill. Once the blast mats are anchored around the rock face, they act not only as a way of protecting miners from flying debris, but also as a way of capturing and retaining fractured rock.
Most standard blasting mats are made out of rubber — usually sliced-up rubber tires bound together with rope or cabling. While this has the advantage of making them cheap to manufacture and purchase, the use of rubber as a base material in blasting mats has its downsides. Not only is rubber flammable, but the mats must also be covered in soil, anchored to the ground or otherwise mated to the area that they are enclosing. These two properties mean rubber blasting mats can pose just as much of a hazard as the explosion or rock burst event they are protecting from, since that selfsame explosion may send the mats up in flames or flying through the air.
Once the mats perform their purpose, there is another drawback to consider, as well: waste. Since rubber blasting mats are, for the most part, made out of old tires, they cannot be re-sold and must be disposed of at the purchaser’s expense.
As an alternative, consider the woven rope design of the blast shields and walls manufactured by TM International, which makes them better-suited to the purpose. Constructed out of 5/8-inch galvanized IWRC wire rope and anchored to the mine support structures with rock bolts, TM International, LLC’s blast shields are designed to be able to withstand 6,500 joules of energy per square centimeter while still allowing gases to vent through holes in the weave.
The shields and walls come in several stock sizes, including 14′ by 14′, 12′ by 12′ and 10′ by 15′, as well as a variety of custom sizes ranging from 4′ by 6′ all the way up to 14′ by 16′. The shields are incredibly light for their construction, weighing in at only 16.5 pounds per square foot, which means that a 12′ by 12′ section of blast shield weighs only a little over a ton. A comparable rubber blasting mat weighs 35 pounds per square foot, meaning the same 12′ by 12′ blast shield, if made out of rubber, would weigh 5,040 pounds — more than double the weight of one of TM International, LLC’s blast shields.
Not only do the shields weigh less than conventional rubber mats, but they can also be transported easier, as well. Since they are made out of 5/8″ wire, they are much thinner than the foot-thick regular rubber mats, meaning 2,500 square feet of TM International, LLC blast wall can fit in a truckload, compared to about 1,200 feet of conventional rubber blast mats.
The blast shields are also capable of reducing the peak blast pressure of an explosion — its primary destructive element вЂ” anywhere from 50 to 80 percent and are tested to ensure this by being subjected to 9000-psi blasts of air, which are designed to simulate a bomb detonation or the impact of shrapnel or other flying debris at high speeds.
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In addition to these tests, our products are also military-tested against everything from Composition B warheads to several kilograms of TNT and C4. They have passed every test, even retaining their protection after ten separate detonations in the case of the TNT tests.
Whether it’s rock burst control or simple rock reinforcement measures, consider TM International, LLC for all of your blasting mat needs.