Stability of underground mine workings due to surface blasting

Abstract

Blasting in the mining industry is being transformed, and can no longer be treated as an art or a practice purely dependent upon experiences of individuals. Examples of key drivers that have demanded consistent and more accurate blast results are initiatives to control pit-wall damage to help mine operators for steeper pit slope angles, to control damage to adjoining underground mine opening due to open-pit mining. Such situation exists at Rampura Agucha open-pit lead zinc mine which is producing 5.7 Mt/a of ore and has started its underground part which is slated to produce initially 2-2.4 Mt/a and 4.5 Mt/a in near future. The open-pit mine is currently working at 370 m depth and is designed to reach 400 m deep. The underground operations have been commenced successfully maintaining 50 m parting from the planned ultimate level of the open-pit mine. Presently, open-pit and underground mines are being operated simultaneously. The present average depth of underground working lies between 370 and 410 m below the surface level. Sometimes explosives up to 95,978 kg are being detonated in a blast round in open-pit mine for speedy removal of overburden. These blasts are often apprehended as a danger to the safety and stability of underground openings. It was decided to document the impact of open-pit blasting in the openings of underground workings, so that preventive measures may be taken for the safety and long term stability of the underground mine. The impact of 86 open-pit blasts has been documented in the underground openings and 258 blast vibration data has been recorded. The vibrations were recorded simultaneously in the roof, side walls and at floor levels. The highest levels of vibrations were recorded in the roof and the lowest levels at the corresponding floor levels. The pillars experienced lower level of vibrations than those of the roof. Blast designs were optimised at open-pit mine to control vibration in view of long term safety and stability of the underground openings. The underground development face blasts were also optimised for the safety and stability of underground workings. Vibrations generated due to detonation of explosives with shock tubes and electronic delay detonators have been recorded in underground openings for comparative assessment. Ground vibration recorded at roof, pillar and floor were analysed separately and threshold value of the vibration for the safety of underground workings has been determined based on the Rock Mass Rating (RMR) of the roof rock and accordingly blast designs have been optimised which helped in exploitation of precious minerals from openpit as well as from underground mines simultaneously.