Weakening treatment technology of hard roof above fully-mechanized coal mining face under permafrost overlying strata

Based on the overburden strata in Kawazhang Mine as the permafrost overburden strata,in order to ensure a safety mining of the fully-mechanized coal min ing face under the hard roof,roof borehole drillings within the open-off cut were applied to the roof breaking blasting in the coal mining face. Meanwhile ,a fan-shaped boreh ole pattern was made in the roof of the transportation gateway and the air returning gateway of the coal mining face for the cutting and breaking blasting. Under the permaf rost overburden strata,a roof blasting pre-fracturing weakening test was conducted in the coal mining face. KJ27 online monitoring and measuring system installed in the fu lly-mechanized coal mining face was applied to the monitoring and measuring of the mine pressure behavior after the roof weakening treatment conducted. The monitoring and measuring results showed that the initial roof weighting and the periodical roof weighting step distance of the coal mining face was 19 meters and 15 meters individuall y,the periodical roof weighting step distance was reduced by 50% in comparison with the roof weakening not conducted. Thus,the danger of the large area suspended roof above the coal mining face could be avoided and the roof accident occurred in the coal mining face could be prevented.