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To study the spontaneous combustion characteristics of second oxidized coal, the proximate and elemental analysis, Fourier transform infrared spectroscopy, and temperature-programmed experiment system were used to investigate the micro-spontaneous combustion characteristics of raw coal samples and oxidized coal samples, which were from three coal samples of different mines. The findings indicate the content of carbon decreases, oxygen increases, and the moisture decreases. Moreover, because the specific surface area, pore volume, and porosity of second oxidized coal increase, the CO concentration of the coal samples with second oxidation become higher than those of the first oxidation in the earlier stage of oxidation. With temperature increase, the number of active functional groups,which are hydroxyl and aliphatic hydrocarbons in the coal molecules, change a lot, have a large amount of consumption and regeneration, and cracked in C2H4 gas in the later stage of oxidation. Hence, the concentrations of CO and C2H4 in the coal samples with the second oxidation are lower compared to the first oxidation. In addition, there are advanced characteristic temperature points of the second oxidized coal compared to raw samples. That is,the period of spontaneous combustion is getting earlier. In summary, the oxidation ability of the second oxidized coal is stronger than that at the first time and gets oxidized readily. [ABSTRACT FROM AUTHOR]

Physical simulations and field measurements were performed to study the movement of overlying strata and water conductive fractures (WCF) under conditions of repeated mining in short-distance coal seams. In addition, the feasibility of water resource conservation mining (WRCM) was analyzed in areas disturbed by repeated mining. The results of the simulation showed that the aquiclude remained intact after the mining of Coal Seam 11 (the 1st main seam) in Shigetai coal mine in northwest China. According to the findings, WRCM can be conducted in this area. After the mining of Coal Seam 12 (the 2nd main seam), the WCF in overlying strata above the central section of the mined-out area will gradually compress and close. However, due to the impact of repeated mining in short-distance coal seams, WCF in overlying strata at the edge of the mined-out area are connected to the Quaternary loose aquifer and are not likely to close, resulting in the loss of the water resource. Thus, WRCM in this area would be difficult to conduct. Moreover, field observations showed that the aquifer can likely remain intact after Coal Seam 11 is mined, but the aquifer's water level will probably not fully recover if Coal Seam 12 is mined. This is consistent with the simulation results. Therefore, appropriate technical mining measures must be taken to conduct WRCM at the 2nd main seam in the shallow-buried short-distance coal seams. These research results can be used as a reference for future WRCM endeavors in the arid and semi-arid regions of northwest China. [ABSTRACT FROM AUTHOR]

According to calculating ol the support strength when the repeated mining face passing through abandoned parallel gateway,a mechanical model for studying the principle of main roof fracture was established through spot investigation and analysis for fracture of roof of workface. Three factors such as the width of coal pillar between repeated mining face and abandoned parallel gateway, the distance between repeated mining face and main roof fracture line and the width of abandoned parallel roadways which may lead to the advanced fracture of main roof was put forward. Through the analysis of the three factors,the mechanical mechanism of advanced fracture of main roof was obtained. It shows that the coal pillars become destabilization when coal being extracted. The destabilization of the coal pillars leaves roof beam unsupported and increases the length of cantilever beam. When the length of cantilever beam is greater than or equal to cycle weighting interval, the cantilever beam breaks. Based on the Bieniawski formula, the critical width of coal pillar that becomes destabilization and the critical width of abandoned parallel gateway that main roof inevitably breaks in advance are calculated. The size of broken rock block of mechanical model of main roof weighting is adjusted on the basis of the mechanical model of main roof fracture. The adjustment is used to calculate the support strength of hydraulic support when repeated mining face passing through abandoned parallel gateway. It shows that the least support strength is 10 910. 77 kN. On-the-spot observation results show that the support strength is 9 656 kN when repeated mining face passing through abandoned parallel gateway. [ABSTRACT FROM AUTHOR]

• The influence of stope roof structure movement on direct roof is expounded. • The mechanism of roof fall under repeated mining was studied. • Different instability conditions of cantilever beam structure are obtained. • Reasonable support working resistance is an important reason to maintain the stability of cantilever beam structure. In view of the problem that the broken stope roof under repeated mining is prone to end face roof leaks, this paper's research background is the close distance coal seams mining of a mine in Guizhou province, China. The impact of stope roof structure movement on the immediate roof was elucidated from the analysis of stope roof structure movement under repeated mining employing a combination method of field measurement, physical simulation, theoretical analysis, and numerical simulation. As a result, the mechanism of end face roof leaks under repeated mining is investigated, and the corresponding control measures are proposed. The results show that the cantilever beam structure instability of the stope roof under repeated mining is an important reason for the end face roof leaks and support crushing accident. The fracture forms of cantilever beam structure are divided into direct fracture and hinged structure fracture. The direct fracture is affected by the support working resistance. The cantilever beam hinged structure is divided into sliding and rotational instability. The periodic weighting interval is small and the fracture degree is large under repeated mining, which leads to the sliding instability. It is concluded that reasonable support working resistance is the important reason to keep the cantilever beam structure stability. Therefore, the stope roof leakage control countermeasures under repeated mining are proposed. The UDEC numerical simulation is used to verify the above analysis again, so as to provide the basis for the end face roof leaks prevention and control under repeated mining. [ABSTRACT FROM AUTHOR]

In terms of the stress concentration problems at pillar support goaf in thick coal seam, combining with recovery method and geological conditions at pillar support face, using mechanics theory and stress characteristics of pillar support goaf, this study established a mechanical model and derived stress calculation formula. In addition, a comparative analysis on stress distribution law, influence scope and the main influence factors of single left-over pillar and multiple left-over pillars was conducted to obtain the concentrated stress influence scope of single left-over pillar, which is 2. 5-3. 0 times larger than that of multiple left-over pillars. The increase rate of influence scope in vertical direction gradually reduces with the increase of the number of left-over pillars. The influence range tends to be stable when the number of left-over pillar reaches 20. The influence range at horizontal direction has less variation with the number of left-over pillars changes. Therefore, the influence range of vertical stress is determined as the criteria for assessing leftover pillars stress influence range. In a case study at Yangjian mine, combined with mining practices at left-over goaf working face, the influence range is calculated as 10.6 m in vertical direction, which will cause impact on the repeated mining face at No. 9 seam. Also, the rational design principles and left-over goaf stress release measures are proposed for the repeated mining face. [ABSTRACT FROM AUTHOR]

Abstract: Filling repeated mining technology of small coal mines mainly researches on the gob or cavity in the coal seam formed by the mining method of small coal mines. Adopting the means of field exploration, indoor test, the simulation experiment, pumping filling material that is stable in mechanical property and low in price into the gob, it can meet the needs of normal mining and achieves the purpose of mining safety and efficiency. The keys of this technology are detecting, the stability and controlling method of the surrounding rock, management and safe-efficient mining process technology. The gob of small coal mines in B909 working face of Anjialing No.2 underground coal mine affected the advancing of the working face. In view of this situation, the detecting method and solution were researched. The results showed that, combining geophysical exploration and the trinity of detecting, releasing, controlling, ae specific location of the goal could be mastered accurately and the harmful factors could be removed. According to the characteristic of recovery, the suitable filling material and scheme were chosen to deal with the gob by grouting. This guaranteed the stability of the surrounding rock and the coal in the period of mining. These methods mentioned above realized the safely and efficiently mining of the coal seam affected by the gob of small coal mines and provided experiences to the coal resource exploitation of similar condition. [Copyright &y& Elsevier]

The western region of China is rich in mineral resources. The vigorous development of mineral resources has exacerbated the environmental and safety problems in the region. One of the important links to solve this problem is to control the development laws and distribution characteristics of the overburdened cracks in the mining of this area. In this paper, the Xiashijie coal mine 3-2 coal seam and 4-2 coal seam are examples of repeated mining, and are examined as the background, through theoretical analysis to optimize the size of the coal pillars in the lower section, using the 3DEC numerical simulation experiment method and the rise of the cracks in the short-distance coal seam. Repeated mining monitoring and analysis of the development law are used to ascertain distribution characteristics of overburdened cracks caused by the repeated mining process of the working face. The results show that: (1) By establishing a mechanical model of the overlying strata structure under short-distance coal seam group mining, and carrying out the force analysis of the double section coal pillar under repeated mining, the reasonable size of a lower section coal pillar was determined to be 70 m. (2) As the development height of a fracture progresses with the working face, its expansion rate undergoes four obvious changes: fluctuations within a certain range, the expansion rate reaches the peak after the rock formation is concentrated and broken, the cyclical change gradually decreases, and the expansion rate is zero after complete mining. (3) The fracture zone height of 222 and 224 face under repeated mining in the 4-2 coal seam was 19.56–22.31 times and 22.38–24.54 times larger, respectively, and the post-mining fracture extension of the face with larger width and deeper burial under repeated mining was higher than that of the adjacent face. This study provides scientific guidance for the rational division of coal pillars and the solution of the problem of water conservation mining under repeated mining in the adjacent face of a short-distance coal seam. [ABSTRACT FROM AUTHOR]

Aiming at the serious problems caused by coal mine mining activities causing the rock burst accidents, this paper is based on rock mechanics and material mechanics to establish the key layer breaking by the double-key layer beam breaking structural mechanics model of a single working face and double working face under repeated mining. The theoretical calculation formula of the angle was used as the theoretical basis for the elevation angle of the pre-reloading hole of the hard roof. The rationality and reliability of the formula were verified by the physical similarity simulation experiment and the 3 Dimension Distinct Element Code numerical simulation experiment, revealing the rock formation under the influence of repeated mining. The results show that the derived key layer breaking angle formula is suitable for the theoretical calculation of the breaking angle of the key layer of a single coal seam when the repeated disturbance coefficient is λ = 1; when it is λ = 2, it is suitable for the repeated mining of the short-distance double-coal mining. The rationality and reliability of the theoretical formula of the breaking angle of the double key layer of single coal seam and double coal seam were verified by the physical similarity simulation experiment. Through the 3DEC numerical simulation results and theoretical calculation results, the W1123 working face hard top pre-cracking pressure relief drilling elevation angle was 78°. The drilling peeping method was used to verify the results. The results show that the theoretical formula of the critical layer breaking angle is well applied in engineering practice. [ABSTRACT FROM AUTHOR]