A new blast damage index for the safety of underground coal mine openings

Abstract

In order to assess the damage to openings in underground coal mines due to blasting in nearby surface mines, vibration and roof strata behaviour were monitored in seven mines having a different rock mass rating (RMR) in India. In total, 202 blasts were examined and 603 vibration data were recorded. The data were analysed to define the threshold values of vibration for different levels of damage. Vibration up to 372·8 mm/s were recorded. Extent and levels of damage were found to vary with the level of vibration produced and site-specific characteristics of the underground openings. At three experimental sites, there was no visual damage observed whereas at four experimental sites falling of loosened chips and spalling of coal from roof and pillars were observed. The damage was classified in three categories - no damage, minor damage and major damage. In three instances, coal blocks of more than 0·25 m³ were detached from roof at vibration levels of 296·69, 359·4 and 372·8 mm/s. Rock/coal blocks of 0·1-0·25 m³ were observed in 13 instances. Threshold values of vibration for the safety of underground openings were defined for different RMR values. An attempt is made to define a new blast damage index (BDI) by combining the entire site-specific parameters and vibration recorded at different levels of damage observed in the mines. It was observed that a BDI value greater than 2 corresponds to major damage. At BDI values less than 1, there was no damage. Minor damage was observed at BDI values ranging from 1-2.