Directional Blasting

Directional Blasting

 

an explosion in which the surrounding medium (usually rock) is displaced primarily in one particular direction and over a given distance.

The general mechanism of a directional blast is as follows. In the first stage, upon explosion of a charge, a shock wave propagates in the medium, thus causing the elements of the medium to move radially. The gaseous products of the explosion form a gas cavity, which expands laterally toward the boundary of the medium (the free surface), increasing the speed of displacement of the broken rock. Subsequently, gas erupts from the cavity and pieces of rock are ejected from the mass. A directional blast can be produced by appropriate arrangement of the explosive charge relative to the boundary of the medium, by using shaped charges, and by selecting the sequence of detonation of the charges. The charges are usually placed in chambers or boreholes within the rock mass.

An arbitrary distinction is made between ejection blasting and collapse blasting. Directional blasting with a horizontal mass surface is called ejection blasting; displacement of the rock in the desired direction is attained by using a system of inclined blast-hole charges (Figure 1,a) or a system of two or more chamber charges (Figure l,b). In the latter case, the charges are not exploded simultaneously, and the rock is ejected mainly toward the charge that was exploded first. Directional ejection blasting is used in the construction of canals and excavations (for example, the formation of the Chusovaia River bypass channel, 1935) and for stripping mineral deposits, when the mass of rock must be ejected to one side of the trench (for example, the stripping of the Krasnaia Shapochka bauxite deposit in the Urals, 1936).

Directional blasting in cases where the surface of the mass is inclined or vertical is called collapse blasting. In this case, a system of blasthole charges (Figure l,c) or one or more chamber charges (Figure l,d) are used. Directional collapse blasting is effective for the erection of levees and dams, since the heap of rock ejected by the explosion can cover a river with a considerable water discharge. Directional collapse blasting has been used for the reconstruction of the Volga-Isad branch of the Oka River (1931) and the construction of unique hydraulic-engineering projects, such as the dam on the Terek River (1958); the fulcrum of the upstream slope of the Nurek Hydroengineering Complex dam on the Vakhsh River (1966); the mud dam (height, about 100 m) in the Medeo natural landmark (first-phase explosion in 1966, total weight of explosives about 5, 000 tons; second-phase explosion in 1967, total weight of explosives about 4, 000 tons); the dam of the irrigation system in Baipaz on the Vakhsh River (1968, total weight of explosives about 1, 800 tons); the bridge dam (height, 90 m) in the Akhsu Gorge in Dagestan (1972; total weight of explosives, about 550 tons). Directional blasting is used successfully in open-pit mines for dumping the overlying rocks into the worked-out area of the pit.

Directional blasting may also be used under other conditions, such as in underwater explosions.

In the future, directional blasting using nuclear devices may be used in large-scale operations in hydraulic-engineering and transportation construction.

REFERENCE

Pokrovskii, G. I. , and I. S. Fedorov. Vozvedenie gidrotekhnicheskikh zemlianykh sooruzhenii napravlennym vzryvom. Moscow, 1971.

G. I. POKROVSKII

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Rock Failure Mechanism of Directional Blasting Using Slit Charge
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