Damages to underground openings due to surface blasting

A programme of study was taken up to investigate the influence of surface blasting on underground coal mine openings and to suggest optimal blast design to avert any damage there to. A number of opencast blasts at all the operating faces i.e. shovel benches, dragline bench and coal benches were conducted in close vicinity to underground openings. The strata monitoring equipments were installed in the underground mine to monitor the strata behaviour. Altogether 78 blasts were performed with varying blast designs and charging patterns and 257 ground vibration data were generated. The analyses of vibration data indicate that roof of the underground openings vibrated with higher amplitude compared to pillar of the underground mine. Choked face blasting generated much higher ground vibrations than the free face blasting. It was observed that spalling of coal chips from roof started at peak particle velocity of 121 mm/s. A coal block of size 0.25 m 3 got detached from roof at the peak particle velocity of 243 mm/s. Strata monitoring study was continued for 11 months but it did not indicate any abnormal change in strata behaviour. Based on the analysis of data blast designs were recommended ensuring the safety of underground workings.