# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 6 Conclusions  
Given that the vertical stress rises from small to large, the roadway in the supporting pressure area experiences different degrees of failure. The evolution law of pore and fissures in the coal sample under progressive loads is analyzed from the micro level, which is very important for how to control the coal wall and realize the safety of roadway. The low-field NMR can be used to finely characterize the evolution of pores and fissures in coal samples. The following conclusions are drawn specifically.  
1. The porosity of coal samples evolves differently in the compaction stage and the elastic stage. During the compaction stage, the porosity declines mildly, indicating that part of the primary pores and fissures close up. During the elastic stage, the porosity increases correspondingly as the load rises gradually; and the growth rate of porosity tends to be larger and larger, indicating that secondary fissures develop more and more drastically.
2. Before loading, AP, PP, and CP all develop in the coal sample, with AP and PP taking the overwhelming advantage. When the load increases to 3 MPa, the volume ratio of PP declines while that of AP increases significantly. When the load increases to 11 MPa, the volume ratio of PP witnesses a significant rise while that of AP decreases remarkably, indicating that the elastic stage is a crucial phase for the rapid expansion of fissures. Meanwhile, it is also discovered that compared with one-time loading, progressive loading accelerates the evolution of pores and fissures.
3. Compared with the situation before loading, in the compaction stage, the \(T_{2}\) distribution curve moves to the smaller value as a whole and PP displays some tendency to transform to AP; in the elastic stage, the \(T_{2}\) distribution curve moves to the larger value as a whole and AP displays some tendency to transform to PP. As the load increases gradually, the relaxation time becomes longer correspondingly as the \(T_{2}\) peak of AP starts to rise, revealing that part of the primary fissures close up when secondary fissures expand.
4. The NMR images show that as the load increases, the pores and fissures experiences three phases: wide distribution--expansion in the concentration areas--macro perforation in the end. This illustrates the processive and stage characteristics of fissure expansion on the one hand, while on the other hand, it also reflects indirectly that the instability of coal wall under the effect of progressive stress displays some stage characteristics. To guarantee the long-term roadway stability, reinforcing support should be provided to inhibit the full perforation of fissures in time.  
Data curation, Z.X. ([PERSON]), J.W., and Z.X. ([PERSON]); formal analysis, Z.X. ([PERSON]); funding acquisition, N.Z.; investigation, Z.X. ([PERSON]) and N.Z.; software, J.W. and C.Z.; writing--original draft, Z.X. ([PERSON]); writing--review and editing, Z.X. ([PERSON]), N.Z., J.W., Z.X. ([PERSON]), and C.Z. All authors have read and agreed to the published version of the manuscript.  
Figure 13: Fracture evolution process of coal sample.  
**Funding:** This work is financially supported by the Future Scientists Program of China University of Mining and Technology (2020 WLKXJ007) and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX20_2013).  
**Acknowledgments:** The authors are grateful to the staff at the Zhangjiamao Coal Mine for their assistance during the field measurements.  
**Conflicts of Interest:** The authors declare no conflict of interest.