Experimental study on the evolution characteristics of bituminous coal pore-fissure structure under multiple vibration loads

Abstract

Frequent mining disturbance or periodic rupture of far-field roof in coal seam mining will produce multiple vibration loads, which have an important influence on the micro pore-fissure structure and macro mechanical behavior of coal. In order to explore the evolution characteristics of pore-fissure structure of coal under vibration load, the SHPB test system was used to carry out a multiple vibration load impact test on bituminous coal. With the help of low-field nuclear magnetic resonance analyzer, the T2 spectrum of coal after each impact was tested, and MRI was performed to analyze the law of pore distribution and evolution, and the damage evolution characteristics of coal pore-fissure structure were studied. The results demonstrate that the peak stress and dynamic elastic modulus of coal show a linear decline trend with the increase of vibration load, and the impact effect of vibration load significantly weakens the bearing capacity and deformation resistance of coal, so it is necessary to conduct in-depth analysis on the evolution characteristics of coal pore cracks. According to the T2 spectrum and MRI information, the total pore volume of coal increases greatly under the first action of vibration load, in which the adsorption pores volume increases by 5.0 times. With the increase of the number of vibration loads, the volume of seepage pores begins to increase significantly, while the volume of adsorption pores remains basically unchanged until the sample is completely destroyed. During the multiple action of the vibration load, the internal damage of the coal body gradually accumulates from the initial point distribution to strip distribution until micro-cracks are formed. With the multiple action of the vibration load, the microcracks of the coal sample begin to connect and converge to form macrocracks, which greatly improves the connectivity between the seepage pores, and the overall porosity of the coal sample reaches a peak value, about 6 times higher than the original porosity. During the whole process of coal body being damaged and destroyed by vibration load, the connectivity of seepage pores is gradually increased and improved, and its fractal dimension shows a linear decline trend. The MRI reveals the mechanism of vibration load on coal pore-fissure, and the results show that the central region of the coal first develops and gradually forms microcracks. Under the action of reflection and stretching of subsequent vibration waves, the damaged areas gradually extend to both sides until penetrating the sample.

Keywords

• vibration load
• nuclear magnetic resonance technology(nmr)
• energy dissipation
• pore-fissure evolution
• damage evolution mechanism