Blasting Operations

Blasting Operations

 

operations in the national economy that are accomplished by blasting natural materials (rock, wood, or ice) or artificial materials (concrete, masonry and brickwork, metals, and so on) with the aim of controlled destruction and shifting or change of their structure and form. Blasting operations are accomplished by means of explosives and detonators, which create the initial impulse for triggering the blast (blasting caps with safety fuses; electric detonators), and for transmitting the initial impulse the required distance (for example, an instantaneous detonating fuse). A cavity (a drill hole, blasthole, or powder chamber) is made—usually by drilling—preliminary to placing the explosives inside the object to be destroyed; therefore the aggregate of the processes for producing explosions is often called drilling and blasting operations. The measured quantity of explosive, placed in the cavity or on the surface of the object to be destroyed and equipped with a detonator, is called the charge.

The range of application of blasting operations is extensive; their greatest volume of usage is in mining. Blasting is used for seismic exploration for minerals, for opening a deposit (for example, directed explosions for ejection or disposal), and for extracting hard minerals (the explosive breakout separates the rock from the mountain mass, in addition to finely crushing it and shifting it). In construction, blasting operations are used in laying out construction areas; for loosening frozen and rocky ground; for removing boulders and stumps; for excavating ditches, trenches, and earthen and rock-filled dams; for the construction of road and hydraulic-engineering tunnels; and for the destruction of temporary bridges. Blasting operations are used in reconstruction for the destruction of structures scheduled for demolition and for the destruction of equipment foundations inside functioning workshops.

In water conservation, blasting operations are used to deepen reservoirs and river navigation channels, to straighten and clear riverbeds, to eliminate rapids and shoals, to eliminate ice blocking when rivers freeze over in the fall, to permit ice to pass under bridges, and to protect structures from ice and eliminate ice blocking during the passage of ice downstream in the spring.

In arctic conditions, blasting operations are used for destroying icefields and ice hummocks and for freeing ships that are trapped in the ice. In the metalworking industry, blasting operations are used for hardening metal, for stamping complex parts from sheet metal, for cutting and welding metal, for placing rivets in hard-to-reach places, for cleaning scales and rust from castings, for destroying plugs of hardened metal, for crushing slag, and for dividing large scrap metal. In the chemical industry blasting operations serve for rooting out stumps, which are the raw material for rosin-turpentine factories.

In agriculture and forestry, explosive felling of trees is used to create protective zones, which prevent the spread of forest fires. Blasting operations are used to prepare arable areas by freeing them of rocks, stumps, and shrubbery; for deep plowing; for digging holes for planting fruit trees; for draining marsh-ridden places by breaking the waterproof stratum; and for the creation of ditches for irrigation and drainage works.

In the petroleum and gas industry, blasting operations eliminate damage to drilling instruments; they increase the petroleum yield from the stratum by means of an explosion of torpedoes in the drill holes; they aid the construction of dams and islands in places of underwater extraction; and they create underground reservoirs of oil by means of the condensation of clayey soils by an explosion. Blasts are also used to extinguish fires in oil and gas drill holes.

Explosives were first used for peaceful purposes in 1448-72, when the bed of the Neman River was cleared of stones and rapids by means of a gunpowder charge. According to the president of the Berg Kollegium, I. Shlatter (a contemporary of M. V. Lomonosov), blasting operations using powder for extracting ores were first used in Russia (1617) and became widespread in Europe—in Silesia (1627), Bohemia (1629), the Harz Mountains region (1632), Saxony (1645), England (1670), and France (1679). The invention by the Russian scientist P. L. Shilling (1812) of an electrical method of detonation, the creation of mobile boring machines (1861) and drilling rigs, the invention of dynamite (1860), the discovery of trinitrotoluene (TNT) (1863) and the explosive properties of a mixture of ammonium nitrate with carbonaceous substances, and the manufacture of detonator capsules (1867) promoted the further development of blasting operations.

The replacement in dynamites of a still greater part of nitroglycerin by ammonium nitrate, reducing the cost of explosives and decreasing the danger of handling them, had the effect of broadening the scope of blasting operations and improvement in the technology of their manufacture. Beginning in the mid-19th century, blasting operations became widespread for eliminating ice blocks (on the Neva River in 1841), deepening navigation channels (the Bug River, the Dnieper estuary in 1858, and the Neva in 1860), pulling stumps (near St. Petersburg, 1873), destroying underwater reefs (New York harbor, 1885), and clearing forest plots into arable land (Irkutsk Province, 1913). The increase in the scale of mining production in the early 20th century, especially with the development of open mining, demanded an increase in the depth of placement and the size of the explosive charge. To accomplish this, the bottom parts of deep blastholes (5-6 m) were widened by the detonation of small charges until they assumed the shape of a kettle with a capacity of a few dozen kg (the so-called kettle charge, adopted in 1913 for extracting iron ore in the Krivoi Rog area). The method of chamber charges (with a mass of up to several thousand tons of explosives) placed in a mine (chamber) that is being sunk from pits, tunnels, and so on has been in use in Soviet opencut mines since 1926. Thanks to an increase in the quantity of explosives per unit volume of rock to be blasted (with kettle and chamber charges), the ejection of the rock, with the formation of finished excavations (ditches, canals, and trenches), as well as the crushing of the rock, became possible.

Priority in developing a method of detonating chamber charges for ejection belongs to the USSR. The scale of such blasts grew constantly: 257 tons of explosives were used for the creation of a railroad cut in Barkhatnyi Pass in 1933, 1,808 tons for the construction of a split trench with a volume of 800,000 cu m at the opening of the Korkino coal deposit in 1936, 3,100 tons to create a canal 1,150 m long for diverting the Kolonga River around the coalfield of the Pokrovskii mine (March 1958), and 5,300 tons for the first line of a rock-filled flood-control dam with a volume of 1,670,000 cu m near Alma-Ata (October 1966).

Chamber charges were also widely used in the underground exploitation of large underground deposits of hard ore by systems of mine breaking in the Krivoi Rog area (charges of 100 to 5,000 kg are placed as close as possible to the plane of the breakout), in the exploitation of pillars, and for the elimination of underground cavities by collapsing the ceilings. The methods of calculating the size of such charges, developed by M. M. Frolov and M. M. Boreskov on the basis of the experience of mine warfare during the defense of Sevastopol’ (the Crimean campaign of 1853-56) and later developed in the work of G. I. Pokrovskii (1950’s), furthered the varied application and improvement of the method of chamber charges. In 1957 the Academy of Sciences of the USSR conducted test explosions with charges of 0.1 to 1,000 tons for the experimental verification of the influence of the position of the center of gravity of the mass being shifted on the effectiveness of ejection blasts. These experiments were used as the basis of calculating ejection charges, taking into account the force of gravity and the determination of the maximum depth of placement.

The perfection of drilling rigs made possible an increase in the diameter and depth of the holes in opencut mines, and it became feasible to turn away from concentrated chamber charges to blasthole charges. In the USSR this method was first adopted in 1927 in the exploitation of hard granite de-posits at the construction site of the Dnieper Hydroelectric Power Plant and quickly became widespread in opencut mines. Since 1935 the method of blasthole charges has been used in the underground exploitation of large ore deposits. At first, vertical holes arranged in a row were used in opencut mines. In this case the uniformity of the rock crushing by the explosion was inadequate, and outsize pieces that exceeded the size of the excavator scoop required a second explosion. The second explosion was perfected by means of a sharp decrease in the size of the charge, by filling the empty space in the blasthole with water (the so-called hydroexplosive method), covering the external charge with a plastic water-filled packet, or by the use of external charges with an end charge hollow. In all cases a significant decrease in the radius of dangerous fragments is obtained. The use of water as a medium (which transmits the energy of the blast to the object to be deformed) and the use of hollow charges have also been applied in blasting operations on metals. Beginning in 1923 in the USSR, blasting operations were used for the crushing of large metal parts, particularly for cutting sheet metal; subsequently the effectiveness of the cutting was increased by using explosives in chucks with a longitudinal charge hollow.

The introduction of breakout of rock with drill-hole charges served as the first step toward the intensification of blast crushing by reducing the quantity of oversize pieces in the ruptured rock. The development of mining technology posed the problem of obtaining uniform lumpiness, which made possible the transition of extraction operations to flow technology. Questions of blast crushing were first dealt with in the USSR by M. V. Machinskii (1933), N. V. Mel’nikov (1940), and O. E. Vlasov (1962). M. A. Sadovskii and A. F. Beliaev (1952), who had established the dependence of the crushing action on the total impulse of the explosion, did research on the influence of the characteristics of explosives on the various forms of work done by an explosion. Intensification of the blast crushing is achieved by the adoption of the millisecond delayed explosion, which increases the duration of the impulse, by the transition to multiple delayed explosions with an intensity reaching several million tons, by the improvement of the program of millisecond delayed blasting (the use of kinetic energy of the movement of pieces of ruptured rock for supplementary crushing when the pieces collide), by the dispersion of drill-hole charges with axial air spaces (which reduce the peak pressure of the blast and increase the duration of the blast impulse), by applying the method of explosion on the rock mass that has been partially cleared from the previous blast or at a height of two to three benches, by breaking the blasthole charge into parts detonated with blasthole delay, by slanted charges that are parallel to the side surface of the bench, by paired placement of the blasthole charges (which reduce the energy losses on the front of the blast wave), and by the improvement of the placement parameters of blasthole charges in the bench.

The great significance of the ratio between the removal of the charge from the free surface (the so-called line of least resistance) and the distance between simultaneously detonated charges was revealed through the geometrical parameters during blasting operations. An increase in this ratio, increasing the tension gradient along the front of the blast, facilitates intensification of the crushing, and a decrease leads to the separation of the rock by the blast simultaneously along the line of the position of the exploding charges. The combination of the latter method with the decrease of the maximum pressure of the blast by means of air spaces led to the development, at first in Sweden (1953) and then in the USA, Canada, and the USSR, of the method of shaped explosions, which makes it possible to achieve an equal surface of separation of rock along a predetermined profile. This method was successfully adopted in sinking mine shafts (hydraulic tunnels) and on open works (hydraulic channels, road excavations, and so on). Problems of the so-called flameless explosion, which makes it possible to safely conduct blasting operations in mineshafts that are dangerous because of gas or dust, are of particular importance in the underground excavation of coal deposits.

The reduction of the danger in dealing with explosives was achieved by the development in 1934 of very simple explosives in the form of a mixture of ammonium nitrate with combustible additives (dinamon in the USSR) or with paraffin (“nitramone” in the USA). In 1941 the solid combustible additive began to be partially replaced by a liquid additive(kerosinit in the USSR). Subsequently, the transition to granulated ammonium nitrate and a liquid combustible additive of increased viscosity (fuel oil) led to the creation of a new type of very safe, friable, granulated simple explosives (igdanit in the USSR; ammonium nitrate-fuel oil in other countries) that were suitable for mechanized charging. Over a period of ten years the volume of use of such explosives grew sharply; in particular, by 1965 in the USA it reached 60 percent of the entire quantity of industrial explosives. They facilitated the solution of the problem of the mechanization of charging explosives both in open and subterranean operations, particularly owing to the use of compressed air; pneumatic devices for mixing ammonium nitrate and fuel oil and for transporting and loading them have been developed. The stickiness of the ammonium nitrate-fuel oil granules and the increase of their density of packing owing to the speed with which they are blown into the charging cavity made possible mechanized charging of even rising holes (placed at an angle of 90°), with the filling of all the sections of the hole with explosives. Various factory-made granulated explosives suitable for mechanized charging were created after igdanit (ammonium nitrate-fuel oil). An increase in the density of charging and in the concentration of energy of the explosive is achieved through the use of water explosives, which were first introduced by N. M. Sytyi in the construction of a hydroelectric power plant in Frunze in 1943 (15 years earlier than in the USA).

The method of formation of underground cavities through blasting operations is highly promising for exploiting large deposits of ore lying at great depths by means of the application of nuclear explosions. The volume concentration of energy in them attains the order of 4,000 terajoules per cu m (109 calories per liter); in this case it is sufficient to drive the blasthole to a depth of several hundred meters for the placement of the atomic charge. As a result of the blast, evaporation of the surrounding rock takes place, with the formation of a cavity whose sides are broken by cracks of considerable length; as the pressure inside the cavity decreases, its sides and opening collapse. A cone of demolition is created, and the cavity is filled with broken rock. Subsequent extraction of useful components of the ore can be carried out by the method of subterranean lixiviation. At a lesser depth of placement of the nuclear charges the process, which is similar to the crater-forming activity of the blast of chemical explosives, is accompanied by the swelling and rupture of the surface, a cone of dispersion, and the formation of a cavity. The cost of the energy discharged by a nuclear device equivalent to over 50,000 tons of TNT is approximately three times less than that of explosives based on ammonium nitrate; the necessary volume of drilling—in view of the exceptionally high volume concentration of energy—is correspondingly less, and therefore, under conditions of dependable protection from radioactive fallout, the method is promising for the construction of large canals and harbor waterways and for the opening of deep ore deposits.

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G. P. DEMIDIUK

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