Floor failure characteristics and safety evaluation of upper coal mining with underlying semi-goaf

Abstract

In the process of upper coal mining, the floor condition may be incomplete because of the lower coal goaf, and the change of the floor condition has an obvious influence on the floor failure in the upper coal mining. Based on the micro-seismic monitoring data of 15249S working face in Jiulong Mine, the activity of floor events, the ratio of roof to floor, energy and other parameters are statistically analyzed. The results show that: when the lower coal is a gob, the activity of floor events is strong and the disturbance depth is large when the upper coal is mined. With the advance of mining location and the change of underlying coal seam conditions, the frequency ratio of floor and roof events changes greatly, and its change rule is consistent with the formation of water channel. The proportion of micro-seismic events increased by 16.49% and decreased by 10.92% when the bottom floor failure depth was 0-5 m and 5-20 m respectively. In the process of overlying coal stoping approaching the underlying stoping contact roadway, the development location of large energy events extends from shallow to deep. The floor safety evaluation system is constructed by using the parameters of floor warning layer event activity, floor and roof event ratio, distance from the stoping line of the underlying coal seam and vertical energy density. The danger zone before and after the stoping position of the underlying working face is a high risk factor, which can be combined with the changes of safety evaluation parameters for early warning and strengthening hydro-geological observation.

Keywords

• semi-goaf
• floor warning layer
• ratio of floor and roof event frequency
• energy density
• floor failure depth