140 / 2021-10-05 17:45:05

A Novel Approach of Destress Blasting for Ground Control in A Coal Mine

destress blasting

A case study of coal mine ground control is presented. The mine has 17m thick coal seam. The mine has adopted bord and pillar mining method. The mine is in practice to extract coal with multi lift operations. Backfilling by river sand is used to fill the successive lifts starting from the seam bottom. The development takes place in the second lift which is about 4m from the coal seam bottom. The first lift at seam bottom is first depillared with split and slice method. The sequence of extraction proceeds to upper lifts permitting geo-mining conditions. Mine workings are near a century old. A vast area already extracted over the period and a single thick seam contributed to ground conditions amenable for coal mine bumping. The mine has history of bumping and fatality due to the same. The major issues in the mine are coal bumping, sidewall spalling in level bords and cutter failure in dip-rise bords.

A novel destress blasting program has been conceived for the mine for ground control. The novelty in the destress blasting was use of air-decking and light charge for destressing holes. The usual practice adopted is no air-decking and heavy charging for the destress blasting. The magnitude of vertical stress and horizontal stress is estimated based on the stress measurements in Indian coalfields elsewhere and same are corroborated from the filed experience. Horizontal stress direction is estimated from plotting the cutter failure.

Sidewall destress blasting pattern with 3.0m deep holes is conceived for the bords already developed and experiencing spalling and cutter failure. These blastholes are kept at 1.5m apart, drilled about 0.3m below to the roof line, aimed to fracture 1.5m deep in the sidewall around the bolting horizon, stemmed up to 1.6m depth from the borehole collars and charged with a single cartridge of 185gm permitted explosive with keeping about 1.0m toe section of blasthole as air-decked. Similarly, a destress blasting pattern has been developed for the advancing faces. It contained four 3.0m deep blastholes. Two holes are kept face parallel and positioned around centerline at mid of the advancing face. Two holes were angled and drilled at both the corners of the face, the blastholes positions were 0.3m below to the roof line and respective sidewalls, and they are also drilled to 3.0m deep. The destressing holes were detonated with such delay and care that no misfire is generated due to the concurrent production blasting.



Application of destress blasting reduced the occurrences of coal bumping in the mine, reduced spalling and cutter failure. Some failed experiments of destress blasting also gave evidence of stress shifting. Another advantage achieved was the face production raised from 14t per round of blast to 20t per blast round. The borehole camera was used to measure fracture patterns due to destress blasting. The rock bolt load cells were used to measure change in rock load conditions due to destress blasting.