Abstract
With the characteristics of gradual instability in the supporting pressure area of roadway as the engineering background, this paper aims to explore the evolution law of pore and fracture in the coal sample under progressive loads. The low-field nuclear magnetic resonance (NMR) test was designed and conducted with the coal sample under different axial loads (0, 3, 5, 7, 9, and 11 MPa). The characteristic parameters such as the porosity, the pore size distribution, the transverse relaxation time (T2) distribution curve, and the magnetic resonance image (MRI) were obtained. As the test results show, significant difference in the NMR characteristics of the coal samples can be observed throughout the compaction stage and the elastic stage. In the compaction stage, the porosity of the coal samples decreases slightly; the T2 distribution curve moves to the smaller value as a whole, and the percolation pore (PP) displays a tendency to transform to the adsorption pore (AP). In the elastic stage, the porosity of the coal samples rises gradually as the load increases; the T2 distribution curve moves to the larger value as a whole, and the AP tends to transform to the PP. The MRI shows that some pores and fissures in the coal sample close up and disappear as the load increases gradually, while the main pores and fissures expand and perforate till the macro failure occurs. Compared with one-time loading, the progressive multiple loads can ensure the fracture of the coal sample to develop more fully and the damage degree higher. It indirectly reflects that the instability and failure of the coal under the progressive load has the stage characteristics, verifying that the coal in the supporting pressure area needs to be controlled in advance.
Highlights
In recent years, with the progress of coal mining technology and the improvement of mechanical equipment performance, highly efficient intensive coal mining has become the dominant trend of major coal mining countries in the world, such as China, the United States, and Australia [1,2,3]
The three samples falls to 16.5783%, 16.5165%, and 17.2258%, respectively; and the decrease rate is 1.59%, porosity of the three samples falls to 16.5783%, 16.5165%, and 17.2258%, respectively; and the
Stress plays a crucial role in the evolution of pores and fissures in the coal sample
Summary
With the progress of coal mining technology and the improvement of mechanical equipment performance, highly efficient intensive coal mining has become the dominant trend of major coal mining countries in the world, such as China, the United States, and Australia [1,2,3]. The high-intensity coal mining activities would incur the enlargement of roadway cross-section and the drastic strata pressure behaviors. Under the complex disturbance of multiple stress, severe roof fall and floor heave are highly likely to occur in the roadway. Coal wall collapse is highly probable in the roadway, which poses an extreme threat to the safety of production and miners [4,5]. When large-scale coal wall collapse occurs in the roadway with hard roof, it can trigger active movements in the top covers, which in turn are inductive to the overall rock burst [6,7]. Coal wall instability does not happen overnight; instead, it is the consequence of gradual evolution
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