BRIEF EXPLANATION OF ROCK BREAKAGE

There are two principal explosive mechanisms namely:

a) Shock pressure

b) Gas pressure

Chapman and Jouguet proposed the first theory of detonation in 1899. This was modified by Zeldovich in 1943 and has gone through many improvements since then. Modern computers have made it easy to apply the theories to explosive compositions and calculate the expected detonation parameters. Calculated energy values based on thermochemical and hydrodynamic theories are commonly quoted by explosive manufacturers.

Different explosives exhibit different degrees of gas and shock pressure. The extent to which an explosive possesses these properties is described by the Aquarium test for bubble energy and shock energy.

On detonation, explosive energy is converted into mechanical energy, heat and incomplete reaction products. The initial detonation pressure Pd is quickly reduced to the explosion pressure Pe and as the gaseous reaction products expands to fill available voids, it reduces to blasthole pressure Pd. This pressure is maintained until venting occurs. The relationship between Pd and the explosion pressure Pe depends on the type of explosive used. In general the relationship between them is [Pd = n x Pe] where n lies between 1 and 2. The blasthole pressure Pd will depend on the extent to which the hole is filled.

The energy transferred from the explosive charge to the rock will depend on the extent to which the blasthole is coupled with the explosive charge. Any air-gap will cushion the blast and will reduce the efficiency of the energy transfer. This effect is called the decoupling effect. Decoupling factor D which is diameter of borehole divided by diameter of charge is always greater than or equal to 1. D is '1' when the explosive charge is fully coupled with the borehole wall. The smaller the decoupling factor, the greater will be the blasthole pressure and efficiency of the energy transfer.

The shock pulse transmitted to the rock creates stressing resulting in radial compressive strains which tend to crush the rock close to the borehole, and also tensile hoop stresses which are concentric to the borehole and cause local radial cracking. If the transmitted shock pulse is strong in relation to the confined strength of rock, then considerable initial crushing and fracturing near the explosive charge is bound to occur (Figure 1).

Figure 1: Process of Rock Breakage

Beyond the fracture zone, the pulse travels as an elastic wave until it reaches the free face, where the wave is reflected as a tensile wave. The amount of energy transferred to a given rock is a linear function of the product of density and the rate of detonation. This term is called the characteristic impedance of explosive.

The explosive must be selected to match with the operational aspects of blasting so that maximum energy could be transferred to the surrounding rock mass. The ideal one being that which breaks the rock in tension. If the compressive strength of the rock is exceeded by the shock pulse then annular rock around the borehole will be crushed to a fine size, and this powder will prohibit the propagation of the strain pulse. In addition, increased amount of dust will be formed.

Out of the energy available in the explosive, it is estimated that only 3 to 20% is transmitted into stress waves. The major portion comes out as heat, which is not wasted but expands the gas products of detonation.