No abstract

The surrounding rock failure range of floor drainage roadway under the mining influence and its effect on the aquiclude are the key to determine the aquiclude thickness of the floor. This paper studied the distribution characteristics of the surrounding rock plastic zone by the numerical simulation when the floor drainage roadway was located at different positions under the working face and determined the rational position. Results show that (1) when the floor drainage roadway is staggered inward, the floor surrounding rock is prone to appear the butterfly plastic zone under single work face mining. And the butterfly plastic zone increases sharply after being affected by secondary mining of adjacent working face. (2) When the floor drainage roadway is staggered outward, the floor surrounding rock plastic zone extends gently affected by a single working face. And the depth of the plastic zone has no obvious change after being affected by secondary mining of adjacent working face. (3) According to the risk of water inrush, the three layout schemes can be ranked as follows: stagger   inward   25   m > stagger   inward   80   m > stagger   outward   15   m . (4) Considering the floor stress environment, gas extraction efficiency, and water prevention and control, the reasonable location of floor drainage roadway below the No. 11060 working face of Zhaogu No. 2 Coal Mine was finally determined. It was arranged in the sandy mudstone layer on the upper part of L9 limestone under the middle part of coal pillar and was drived along the seam floor.

Irregular coal pillars are often reserved in the upper coal seam in multiseam mining due to the limitation of geological conditions and mining methods. Diffused and transmitted stress in the pillars will form the stress concentrated areas in the lower coal seam and will increase the risk of rockburst. Based on the upper irregular pillars and fault encountered in the 7301 working face of the Zhaolou coal mine, this paper studies the evolution of stress and energy when the working face passed through the area affected by pillars. The adopted methods include numerical simulations and field monitoring. The change in stress concentration factor and stress gradient because of the mining activities in lower coal seam was analyzed by numerical simulation, indicating that the stress gradient reaches a peak when the working face is closed to the area under the edge and junction of pillars, which has the high risk of inducing rockburst. The sources’ location, variation rule of microseismic (MS) total energy and events, frequency spectrum distributions, and source parameters are discussed, respectively, based on the field monitoring data. The main conclusions were obtained as follows: (1) The total energy and event counts reach the peak when working face is close to the area under the edge and the junction of pillars. (2) The dominant frequency transfers from high frequency to low frequency, the stress drop reaches the peak value, the energy index decreases sharply, and the cumulative apparent volume increases sharply, which all are obvious precursory characteristics before rockburst.

No abstract

The Zhaogu No. 2 coal mine is a single thick coal seam mining mine with high gas content. Due to the lack of protective layer mining conditions, we can only arrange the floor gas extraction roadway (FGER) to extract the gas from the overlying coal seam of the FGER to ensure safe production. However, improper placement of FGER will cause water inrush from the floor of FGER. Given above contradictions, this paper analyzes the stress-strain relationship of the fractured rock mass in the caving zone and the stress-recovery characteristics of the goaf from the perspective of the structural characteristics of the overlying strata above goaf. Based on this, a FLAC3D numerical model of equivalent delayed filling of caving rock mass was established by using the double-yield model filling method. The distribution of floor stress under the influence of mining is obtained after the model calculation, i.e., with the increase of the distance from the floor, the ratio of bidirectional stress to the peak value, and the stability value decrease, but the decrease amplitude becomes smaller and smaller. Therefore, floor roadway used for gas extraction should be located 10-15 m below the floor. Combined with the distribution of floor strata in 11060 working face, it is finally determined that FGER should be located 13 m below the floor. Such arrangement of FGER can not only ensure the effect of gas drainage but also prevent the occurrence of water inrush from floor.