Long-hole destress blasting for rockburst control during deep underground coal mining

Abstract

The Lazy Colliery in the Ostrava–Karvina Coalfield of the Upper Silesian Coal Basin adopted modern longwall technology for an underground extraction of coal seam No. 504. This coal seam is located at a cover depth of around 700m. The seam thickness varied from 3.1m to 5.0m in the selected longwall panel. Two overlying coal seams, Nos. 512 and 530 experienced mining at average heights of 58m and 75m, respectively, from the planned working horizon of the seam No. 504. The proposed longwall panel was adversely situated below goaf edges of the workings in these two overlying extracted seams. An analysis of the inter-burden rock mass among these coal seams showed the presence of strong, massive strata of sandstones and conglomerates with uniaxial compressive strength values between 70MPa and 120MPa. The stress is measured at different mining stages by Compact Conical-ended Borehole Monitoring (CCBM). A simple laboratory test of the coal sample found a high value of the ratio of the elastic deformation to the total deformation (>0.8), indicating the energy-storing characteristic (prone to burst/bump) of the coal seam. Under the existing geo-mining conditions of the site a suitable destress blasting (long-hole drilling and blasting) design is adopted to pre-fracture the identified competent strata from both gate roads in advance. The total length of the panel could be extracted without any bump/rockburst after the destress blasting. The efficiency of the adopted destress blasting at the different mining stages is evaluated in terms seismic effect (SE), which is calculated through the available seismic monitoring data and weight of the charged explosive. A systematic adoption of the destress rock blasting led the 300m long longwall panel to be smoothly extracted without any further rockbursts.