Date Published: June 18, 2019
Publisher: Public Library of Science
Author(s): Gaowei Yue, Minmin Li, Lu Wang, Weimin Liang, Ming-Zhong Gao.
Compared with the idealized isotropic coal seam, the effectively cracking distances of deep-hole pre-splitting blasting in different directions have significant difference in the structural anisotropy of coal seam. In this paper, based on the test results of coal mechanical parameters in different coring directions, the blasting crack distances in these coring directions of coal seam are theoretically calculated and analyzed, and then a new method of blasthole layout in coal seam is designed and the blasting crack effect is investigated according to the test data of gas drainage. The research results show that both compressive strength and tensile strength of the direction of perpendicular to bedding are larger than that in other directions, especially parallel to bedding. The blasting crack distances of calculation results are in great agreement with that of underground in-situ measurement, so the blasting crack zone caused by the differences of mechanical properties in different directions can be approximately treated as an ellipse. Comparing the conventional method of blasthole layout, the blasting crack effect with new method has greatly enhanced the methane concentration and gas drainage quantity. Pure methane quantities with the new method and conventional method are about 2.8 times and 1.67 times as large as that before blasting, and pure methane quantity with the new method is about 1.46 times that with the conventional method. Therefore, considering the anisotropic characteristics of coal seams, the reasonable blasthole layout plays an important role for enhancing gas drainage in low permeability and outburst working face.
Coal mine methane (CMM) occurs naturally in coal seams as the adsorbed and free state, which is a kind of green energy with high burning value and little emissions after burning . However, CMM is one of the threats for underground coal mine because the accumulated methane in the entry has the potential to trigger methane explosive, and high methane content in coal seams is one of the sufficient conditions for coal and gas outburst disaster [2–5]. In order to solve gas utilization and prevent gas disaster in coal mine, gas pre-extraction before coal seam mining is one of the important measures, however, the permeability of coal seams is low in most of high gas and outburst mines, and as the coal mining gets deeper and deeper, low-gas mines are gradually replaced by high-gas mines and the geological conditions are becoming increasingly complex. For example, over 70% of state-owned coal mines in China feature very low gas permeability making it immensely difficult to implement direct gas drainage, the essential measure of gas control. So conventional pre-pumping method is not able to solve these problems, which seriously restricts the safe production of coal mine and the development and utilization of coal mine methane .
Before designing blasthole layout in coal seam, it needs to investigate correctness of calculation results. So field test is arranged at No. 16 mining area of Second-1 coal seam in Jiulishan coal mine, Jiaozuo, China. The thickness of coal seam is between 6.1m and 8.1m, and the average thickness is 7.1m. Gas extraction is carried out after deep-hole blasting.
Deep-hole pre-splitting blasting technology is applied widely to enhance gas drainage in low permeability and high gassy seam, and the effective crack distance of blasting is the key for blasthole layout. Considering the structural anisotropic characteristics of coal seam, the mechanical parameters of coal samples in different coring directions are obtained by the tensile and compressive tests. And the calculation results of the blasting crack distances are good agreement with the field test results. So according to the blasting crack distances in different directions, a new method of blasthhole layout in anisotropic coal seam is designed and applied, the gas drainage effect is investigated underground in-situ measurement.