Simply put, rock blasting is the art of utilizing explosives for the breaking of rock. The science behind it involves detonation of a spherical charge which releases a detonation wave that expands from the surface of the rock, to give rise to shock, plastic and elastic waves. The wave forms generated in rock blasting depend on the behavior of the material which supports the wave propagation. If for small and moderate amounts of explosives, the material behavior is linear or non-linear elastic, these waves are termed elastic or non-linear elastic stress waves. If some plastic work is associated with the propagation of these waves, the waves are called inelastic waves. If the energy input into the material due to explosion is beyond a certain, the material is deformed. For rock formations that are homogeneous the stress wave propagation is in all directions with the same speed. Inhomogeneous rock formations such as layers of rock with different mineral compositions causes the stress wave to change, accelerate or retard its progress and hence the wave front will be distorted. Homogeneity is usually found in bed rock or competent rock. Structural geological features such as jointing, faulting have a decisive influence on wave propagation. In a highly competent rock several types of waves will be transmitted; volume waves, surface waves and interface waves with each of these groups playing an important role under certain circumstances. This article focuses on the science behind rock blasting, underground and quarry blasting whilst wrapping up with industry best practice for dust control in rock blasting operations.
What is the science behind rock blasting?
In rock blasting the most important form of waves are volume waves. These waves have two varieties namely primary or longitudinal waves and secondary or shear waves. Primary waves simply known as P-waves, are the fastest waves similar to acoustic waves in air. These waves typically propagate at speeds of 1000 m/s for weak rock up to 7000 m/s for highly competent rock. Secondary or simply S-waves have a speed of about 50% of the speed of P-waves. Typically in the order of 500 m/s for weak rock up to 4100 m/s for very competent rocks. The most important characteristic parameter of an explosive used in rock blasting are the density and the velocity of detonation. The velocity of detonation depends on the diameter of the cylindrical charge in the borehole. Upon initiation and detonation of a linear charge, two stress waves will emerge from the detonation front, a longitudinal P-wave and a shear S-wave. For homogenous rocks, there are three different modes of sonicity, namely supersonic, transsonic and subsonic detonation. Supersonic detonation has detonation velocity is larger than both the P-wave and S-wave. Conical Mach fronts will form and sustain as long as the detonation velocity is larger. The result yields optimal to very good rock fragmentation with comparatively low vibration levels. Transsonic detonation the velocity of detonation is sandwiched between the P-wave and S-wave of the rock with only one Mach front being formed. The results yield average to acceptable rock fragmentation within the near region around the blasthole and causes medium level vibrations. Subsonic detonation the velocity of detonation is smaller than both wave speeds with no Mach cone fronts produced. The results are poor fragmentation around the blast hole accompanied by very large and intense unacceptable vibrations.
What is underground blasting?
Underground blasting is the process of detonating explosives in an underground operation following (1) drill (2) initiation and blast patterns for underground works. When blasting underground, the dimensions of the cavity to be blasted, the size of the stope face and the stress triaxiality play an important role. This also includes factors such as rock strength and explosive types. The specific consumption of explosives is dependent on the stope face and rock quality. The selection of the explosive is controlled by maximizing the sonic effect. The active boreholes of the break-in section and those within the region of the auxiliary holes should be uniformly loaded with appropriate explosive. The peripheral ring blastholes and the bottom blastholes ought to be designed and treated like the holes in a split blast. Their purpose is to limit the fragmentation zone by arresting any uncontrolled radical cracks that would destabilize the immediate surrounding of the stope. The firing pattern should be designed properly such that break-in should occur in a harmonic fashion with the firing sequence following a spiral pattern from inside the periphery. Many underground mines have production blasts consisting of a fan-shaped blastholes in each row. Most production blast holes are often drilled upward instead of downward. The upward blast holes make it difficult to apply stemming. The sublevel caving method is widely used in various types of underground mines. The main disadvantage of sublevel caving are high ore loss and high dilution. However, ore loss can be reduced by improving rock blasting.
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What is quarry blasting?
Quarry blasting is the use of explosives in the operations carried out in a quarry to gain the material for further processing. It starts with a detailed survey of the quarry face, which will enable the explosives engineer to design the blast and to plot where the shot holes should be drilled, enabling the task to be carried out safely and efficiently. In essence, quarry operations are a process of converting solid rock into smaller particle sizes of a desired fragmentation distribution. The process of quarry blasting is attained through application of energy in one form or another. Typically, in blasting the chemical energy in the explosive is used to fragment and displace the rock. Too much energy gives increased fines, high air overpressure and a risk of fly rock. On the contrary too little energy leads to poor fragmentation, high vibration and poor diggability. Each blast should be backed with a quarry blast design which has the perfect energy balance giving a safe blast with both desired fragmentation and minimum environmental impact. Achieving the balance is part of the blast optimization process involving close control over blast design, explosive and detonator performance and detail performance monitoring. Specifications for quarry blasting are designed to minimize the risk of fly rock, misfires, enable the location of any misfired shots to be accurately determined and ensure where possible, that faces are left in a safe condition after the blast.
What does the future hold for blasting?
The future of rock blasting seeks improvements in challenges associated with high-stress state, such as borehole instability, high confining pressure in blasting, effect of blasting on muckpile and ore flow and achieving optimum fragmentation. Some of the efforts to achieve optimum fragmentation have been tackling energy distribution when using only one type of explosive in the blast hole. The disadvantage is that energy at the toe being optimized results in the energy concentration in the upper part of the face being too high. To overcome this problem, bulk emulsion technology has been applied in the lower portion of the blast holes that enables density variations as a blast hole is being loaded which gives variable energy up the column.
Improving health and safety in drill and blast is GRT’s focus. In fulfilling our mandate to eliminate dust generated from rock blasting operations at the source, Global Road Technology offers GRT: 12X for blast and drill dust control and GRT DC Binder for blast pattern dust control.
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REFERENCES
Balasubramanian, A. 2017. Rock Blasting For Mining. Technical Report.
Quarry drill and blast: a vital first operation. Retrieved 03/04/21
Zhang. Z-X. 2016. Rock Fracture and Blasting. Theory and Applications. Elsevier.
