Blasting Theory

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Rock properties are the uncontrollable variables in surface blast design operation. Blast performance is influenced by the geological structure and the rock strength. Practically in every mining practice, rocks are non-homogeneous in nature. Geological discontinuities like joints, fracture, foliation, bedding planes, have a major effect on blasting performance. Joints are defined as planes of weakness within a rock mass along which there has been no visible movement. Tight joints do not affect the transmission of stress waves whereas open and filled joints introduce an acoustic impedance mismatch and reflect the stress waves (Obert and Duvall, 1950; Goldsmith, 1967). If the reflected wave is sufficiently strong, internal spalling takes place.…show more content…
Rock mass damage due to blasting is directly related to the level of stress experienced by the rock mass and its pre-blasting condition. The damage due to blasting is the combined effect of stress wave and gas pressure. This type of damage is a function of blast design characteristics as well as the rock mass characteristics. The mechanism of the initiation system and formation of cracks along the pre-split line was studied widely by (Chiappetta et al., 1987; Konya, 1990; Clark, 1987). Although the rock fails under tensile stress, the cracks are formed gradually by crack extension and propagation. Stress concentration is the very important part in the control direction of propagation (Jiag, 1996). The explosion load exerted at the weak planes such as joints and bedding planes in sharply attenuates and does not break rock masses. These weak planes may produce the internal damage of rock masses and reduce their load-bearing capacity and stability. In surface blasting operation, the effect of blasting area have been difficult and yet not solved (Zang and Chang, 1999). Thus, it is important to study the micro cracking mechanism for blasting in rock masses and simulation. This result will contribute in selecting the blasting parameters. The mechanism of rock breakage depends mainly on rock mass properties, physic-mechanical aspects of rock and environment related to mining operations…show more content…
The sudden impact of gases at high pressures on the wall of blast hole causes the shock wave to be transmitted into the rock mass. The outgoing shock wave generates the fracture in the surrounding rocks, provided the intensity of the stress is greater than the dynamic compressive strength of rock (Hagan, 1973). It also develops radial fractures due to tangential and tensile stresses (Johansson and Persson, 1970). Upon meeting an open joint or a free face, the compressive stress wave gets reflected and spalling occurs if the intensity of the tensile wave exceeds the dynamic tensile strength of the rock (Hino, 1956; Duvall and Atchison, 1957). The reflections of the compressive wave generate tensile and shear waves, which may propagate through pre-existing cracks and further widens them apart (Fourney, 1993) due to the phase difference between the two wave types. After stress wave, propagations and gases with high temperature and pressure expand the original borehole by extending radial cracks and penetrate into it. The high-pressure gases tear these cracks open and extend them. The explosive gases entrapped in the rock mass push the rock mass forward and bend the face causing flexural rupture (Ash, 1973). Shear fracturing occurs when the adjoining rock is displaced at different times or at different rates by the

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