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Before mining, coal is under 3D stress equilibrium at great depths. Mining disrupts the stress equilibrium, leading to redistribution of macroscopic stress and energy fields in the 3D space of the stope. However, the evolution of stress and energy fields brings about dynamic disasters [1]. Understanding the relationship between rupture morphology of rocks surrounding the coal mine stope and the stress field is the basis for predicting and controlling impact ground pressure, water bursting in mine, coal and gas bursting, and roof collapses [2][3]. To prevent the impact of ground pressure hazards occurring in a deep working face of a coal mine [4][5], the time-space Pol. J. Environ. Stud. Vol. 25, No. 6 (2016), 2633-2639 AbstractTo quantitatively characterize the evolution process of disaster-causing stress fields and to analyze the whole time domain characteristics of a stope from moving to stability, we constructed the four-dimensional time-space structure model of deep stope using PFC discrete element modeling software, and embedded transducers in the goaf area to monitor overlying strata movement characteristics. Targeting the gangue in the goaf area, the compression characteristics, energy absorption characteristics, and evolution of hulking coefficient over time during compaction are analyzed under different mining conditions. Results indicate that: 1. In the first stage of development of overlying strata, an intact time-space structure model of the stope cannot be formed. This means the stope structure has not reached final mechanical equilibrium. 2. Compression of the gangue fragments is an important mechanism of energy release of key strata as the strata are ruptured. The energy absorbed by the gangue reaches the maximum when the intact time-space structure model of the stope is formed. 3. The strength of the immediate roof is directly related to the development of the stope structure. 4. The development of the time-space structure of the stope is divided into two stages, which are marked by the time point when the advance distance is equal to the width of the working face. The above analysis can explain reasons for the delayed occurrence of dynamic disasters, laying a basis for reducing dynamic disasters.

To solve the problems of the rapid advance of the working face was delayed by complicated working procedure and high labor intensity, and the severe damage of roof bolt (anchor cable) induced by advanced hydraulic support, the deformation characteristics of surrounding rock, and the supporting principle of grouting truss anchor cable were analyzed theoretically by taking the roadway of 3_(down) coal seams 2326# working face in Sanhekou coal mine as the research object; then, the mechanical model of supporting structure of roadway under goaf was established. Based on this model, the optimal supporting scheme was determined, and the active advanced support technology scheme of “advanced grouting truss anchor cable” was proposed to take the place of the existing single pillar. The deformation and failure characteristics of surrounding rock of the working face leading roadway were observed and analyzed on-site. Within the allowable range of reading error, the results showed that the maximum displacement of medium-deep base point and shallow base point of two roadways was 15.2 cm and 10.9 cm, respectively; the pressure value had a more obvious jump increase when the distance between each measuring point and the working face was about 35 m, which means the range is strongly affected by the advance mining, and the area affected by advanced mining was 35 m ahead of the working face. It was observed that the lowest position of roof separation development ranged from 0.71 m to 2.73 m. The separation layer was generally distributed in the range of 0.73 m-2.49 m, and the fracture area was roughly distributed in the range of 0.01 m-0.62 m. Under the condition of overlying goaf, there was a complete stress structure, which can meet the requirements of suspension support.

Highwall mining, which is referred to the technique of extracting coal from the bottom of an exposed highwall, features safety, efficiency, and economy. According to existing highwall mining methods, the mining sequence has a great influence on highwall stability. Based on a highwall mining project in Australia, this study adopted the FLAC3D numerical simulation method to investigate the stability of coal pillars with different mining sequences. The results show that different mining sequences of boreholes exert a great effect on highwall stability. Compared with sequential mining, the skip mining method achieves higher speed of highwall stabilization and smaller plastic zone of coal pillar with its maximum strength decreasing by 12%. By adjusting the mining sequence scientifically, the coal pillar failure and roof collapse caused by the deviation of mining angle can be avoided. The results may provide a new angle for the studies on the coal pillar layout and stability design in highwall mining.

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