Blast vibration damage to underground coal mines from adjacent open-pit blasting

A study was conducted to evaluate the effect of heavy blasting in open-pit coal mines on the stability of adjoining underground coal mine workings. Investigations were carried out at seven coal mines in India. Strata monitoring instruments, viz. borehole extensometers, convergence indicators, strain bars, stress capsules and load cells were installed in the roof and pillar. Monitoring of strata behaviour was carried out before and after the blasts. Arrangements were made to mount the transducers of seismographs in the roof and pillars to monitor vibration. Altogether, 202 production blasts were conducted and 622 vibration data were recorded. The maximum peak particle velocity (PPV) recorded was 372.8 mm/s with the associated frequency of 78 Hz. At this magnitude of vibration, a coal block of 0.38 m(3) detached from the roof. Minor damage in the form of loosened coal chips falling from the roof and the pillars was noticed at PPV level of 113 mm/s. Major damage was observed when the magnitude of PPV exceeded 181.9 mm/s. The damage is classified into three groups, viz. major damage, minor damage and no damage. The vibration levels in no damage zone are taken as safe level of vibration, The threshold value of vibration for the safety of underground workings is recommended based on the RMR of the roof rock. It was also observed that the roof of underground roadways vibrated with 1.1-2.58 times higher amplitude of vibration compared to the pillars, The amplification was further higher at junctions. Monitoring of underground strata behaviour indicated, in all the mines, some amount of divergence between the roof and floor just after the blast, but, later on it was followed by convergence and most of the divergence that occurred was restored. (C) 2002 Elsevier Science Ltd. All rights reserved.