A systematic planning and designing of destress rock blasting
resulted in safe longwall mining in a rock-burst prone area. The amount of explosive used at different stages of the destress rock
blasting varied from 1550 kg up to 3450 kg. A simple statistical
analysis of nearly 1000 field data (consisting amount of explosive
and observed seismological monitoring results) was used to derive
the coefficient KOKC, which represents natural conditions of the
overlying rock mass. Efficacy of the blasting for the stress release
from the overburden strata is evaluated through a parameter called
seismic effect (SE). A proposed classification of the destress blasting
on the basis of the value of the seismic effect is also validated
through different field observations. Out of total eighteen stages of
the destress blasting in the longwall panel, five stages experienced
very good (SE varied from 3.6 to 5.3), 7 stages experienced
extremely good (SE varied from 6.4 to 11.2) and the remaining six
cases experienced excellent (SE varied from 13.1 to 52.4) categories
of the stress release.
Adopted design of the destress blasting also reduced the range
and amount of mining induced stress concentration ahead of the
longwall face. As per the existing nomogram, the range of
influence of the induced stress for the site is calculated to be
93 m, but the actual field measurement by the CCBM method
found it to be only 50 m. The observed reduction in the range of
the influence is mainly due to dilution of competency of the
overlying strata by the blasting. Field observations of mining
induced stress by the probe L3 also showed the stress releasing
characteristic of the destress blasting.