# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 2 Engineering Geological Background
### Characteristics of Gradual Coal Wall Failure in the Supporting Pressure Area  
As can be known from Figure 2, the coal pillar wall in different positions of the supporting pressure area is analyzed. The upper part of the figure displays the in situ photos of coal wall and the lower part shows the corresponding sketches of coal wall failure. As the distance to the working face decreases gradually, the coal wall displays the characteristics of gradual failure. When the distance to the working face is 24 m (corresponding to Sketch 1), failure occurs firstly at the upper corner of the coal wall. The failure range is smaller than 1 m and the failure mode is mainly small pieces of coal peeling off. When the distance to the working face reduces to 19 m (corresponding to Sketch 2), the failure extends downward along the coal pillar wall. The failure range is about 2 m and more vertical coal lumps are peeled off. When the distance to the working face further shortens to 15 m (corresponding to Sketch 3), the failure range is further enlarged to 3 m, approximately two thirds of the total roadway height. When the distance to the working face is 8 m (corresponding to Sketch 4), the failure mode changes correspondingly. The failure range ceases to extend downward; instead it tends to bulge as a whole and appears to be a V-shape. In the working face (corresponding to Sketch 5), the coal wall failure is the most severe. The opening of the V-shape coal wall shrinks, and the maximum bulging depth reaches up to 1 m.  
Figure 1: Mining layout of the Zhangjiamao Coal Mine.  
The recovery of the working face causes the stress readjustment in the roadway surrounding rock. The coal pillar bears greater supporting stress when it is close to the working face, thus leading to the severe failure in the coal pillar wall. By contrast, the coal pillars bear relatively smaller supporting stress when they are farther away from the working face, which in turn lessens the failure correspondingly. Numerical simulation was conducted to obtain the evolution law of the stress exerted on coal pillars. Based on the geological occurrence characteristics and mining environment of LCF 15208, the characteristics of vertical stress distribution in the coal pillar were simulated with FLAC3D numerical analytical software. The numerical model is \(230\times 180\times 60\) m in length, width, and height. It is assumed that LCF 15207 has been recovered fully, and LCF 15208 has been recovered by half. In other words, there are still 90 m left unrecovered the vertical stress distribution at the position of 0 m on the roof of AHR 15208; meanwhile the vertical stress curve is drawn within the range of 90 m ahead of the working face and 0.5 m to the inner side of coal pillar. As can be known from Figure 3, the vertical stress of coal pillar displays a remarkable gradual evolution characteristic: the vertical stress increases gradually from the front side of the working face to its rear. Simultaneously, within the area of advance face, the vertical stress evolves from 3 MPa to 10 MPa, increasing first mildly and then rapidly as the measuring point gets closer and closer to the working face.  
Figure 3: Vertical stress distribution.  
Figure 2: Failure diagram and sketch of the coal pillar wall in the supporting pressure area.  
As the working face advances constantly, the supporting pressure above the coal wall has a periodic evolution [28]. Suppose that one tiny unit of the coal wall is set as the research object, the vertical stress exerted on it would experience an evolution process of gradual increase, which in turn causes different failure modes in the coal wall. Therefore, it is of crucial significance for the control of coal wall instability to study the evolution of pore and fissures in the coal sample under progressive loads.