*4.2. Analysis of the Fracture Development Characteristics of a Coal Pillar Dam under the Stress of Overlying Strata*

In order to analyze the vertical bearing capacities of coal pillar dams of different sizes, after mining the 2-2 coal seams, the load was applied on top of the two coal pillars. The stress monitoring sheet embedded in the coal column was used to monitor the stress of the coal pillar dam and its top and bottom plate. A high-definition camera was used to take regular pictures of the surface of the coal pillar dam to monitor the development characteristics of and displacement of cracks on the surface of the coal pillar dam. According to the observation results of stress, displacement, and fracture development, the fracture development evolution law and the failure instability characteristics of the coal pillar dam were further analyzed.

(1) Analysis of the stress failure characteristics of a 15-m 2-2 coal pillar

The overlying rock layer of the 2-2 coal 15-m coal pillar was locally compressed, as shown in Figure 13; the stress inside the coal pillar and the top monitoring point increased linearly with the increasing upper load. When the stress of the monitoring point adjacent to the reservoir side at the top of the coal pillar reached 0.62 MPa, the stress value dropped sharply, and no obvious damage occurred on the roadway side, while the roof plate of the reservoir side was completely destroyed, resulting in the stress loss of pressure at the monitoring point. The stress at the monitoring points inside the coal pillar and adjacent to the roadway side at the top continued to fluctuate and increase linearly, and subsequently showed the characteristics of slow growth, during which the original fracture of the coal pillar dam expanded and the fracture opening and length increased. When the stress of the monitoring point reached 0.69~0.73 MPa, the cracks between the top and bottom plates of the coal pillar dam (reservoir side) gradually expanded and penetrated each other. When the stress of the monitoring point reached 0.78~0.89 MPa, the coal body of the coal pillar dam (reservoir side) formed a semi-oval spalling body, which was separated from the coal pillar, and the crack could be observed macroscopically at up to about 5 mm. The coal body on the other side of the roadway also produced long cracks that ran through the top and bottom plates and basically formed a semi-oval spalling. When the stress of the monitoring point reached 0.85–0.91 MPa, the coal body of the coal pillar dam (reservoir side) had a wall caving, which was completely scattered in the roadway, and the coal body spalling on the other side was separated from the coal pillar, forming a crack of about 4 mm. When the stress of the monitoring point reached 0.96–1.04 MPa, the coal body on both sides of the roadway had a wall caving, and after cleaning the roadway, it could be seen that the effective support width of the coal pillar dam was getting smaller, and the stress of the monitoring point also tended to be stable, and the coal pillar dam recovered stability. With the continuous increase in the upper load, the stress at the monitoring point suddenly dropped sharply from 0.9 MPa directly to approximately 0.3 MPa.

**Figure 13.** Stress failure characteristics of a 15-m coal pillar of 2-2 coal.

As shown in Figure 14, externally, the coal pillar dam produced obvious deformation, indicating that at this time, the internal cracks of the coal pillar dam had a high degree of development. The internal structure had been completely destroyed, as it could not bear the load of the overlying rock layer, and the load was transmitted to the outer coal pillar and the gravel in the reservoir, so far indicating that the coal pillar was unstable and damaged. The final effective bearing width of the coal pillar dam body was 11.5 m.

**Figure 14.** Final width of the coal pillar dam after instability and failure.

(2) Analysis of stress failure characteristics of a 30-m 2-2 coal pillar

The overlying rock layer of the 30-m coal pillar of 2-2 coal was locally compressed, as shown in Figure 15, and the stress in the internal and top monitoring points of the coal pillar dam increased linearly over a period of time. After reaching 0.62–0.69 MPa, with the continuous increase in the upper load, the stress of the monitoring point did not change significantly, and after a long time until the upper unloading, obvious cracks and wall caving did not appear in the coal pillar. Only a certain deformation appeared in the coal pillar. These results showed that the 30-m coal pillar could remain stable for a long time under the state of top pressure, that it did not strip the wall caving, and that its bearing capacity was better than that of the 15-m coal pillar. The increase in the width of this coal pillar weakened the stress concentration benefit, the coal pillar was not easily "eroded" by stress and cracks, the overall bearing capacity was enhanced, and the coal pillar dam was safer.

**Figure 15.** Stress failure characteristics of a 30-m coal pillar of 2-2 coal.

(3) Analysis of the crack development characteristics of a coal pillar dam

According to the experimental results, the fracture development characteristics of the 15-m coal pillar dam of the 2-2 coal seam were more obvious. Therefore, taking the 15-m coal pillar dam body of the 2-2 coal seam as an example, the digital image processing and analysis technology DAVIS was used to further analyze the fracture development characteristics of the coal pillar dam under vertical load conditions. As shown in Figure 16, under

the vertical load at the top, the coal pillar dam body went through four stages—fracture generation, propagation, roadway roof failure, and coal pillar dam damage—from the initial stage before compression. In the fracture generation stage, with the increasing pressure at the top, cracks began to appear in the top corner and floor of the roadway of the coal pillar dam. In the fracture propagation stage, visible cracks also appeared directly above the roadway, and cracks were generated in the middle of the coal pillar. In the stage of roof failure of the roadway, cracks appeared continuously in the top rock mass, flakes appeared at the top corner, and cracks in the middle of the coal pillar gradually developed towards the bottom corner. In the damage stage of the coal pillar, the roof plate collapsed sporadically, the coal pillar appeared to have wall caving, the crack expanded to the upper rock formation of the roadway, and the crack in the coal pillar developed and extended to the side of the goaf.

**Figure 16.** Characteristics of crack development in a coal pillar dam.

Based on the above analysis, the following points can be noted. (1) The smaller the size of a coal pillar dam is, the smaller the pressure that can carry the overlying rock layer is, the easier it is to form a stress concentration area in the coal pillar, and the more likely the stress concentration benefit is to produce the phenomenon of an "eroded" coal pillar, resulting in a decrease in the coal pillar's effective width. (2) According to the experimental results, after the continuous compression of the coal pillar dam, the roof plate near the reservoir side is the first to be damaged, followed by the coal body on the roadway side. Then, the crack development degree of the coal pillar dam increases and the smaller size of the coal pillar will eventually become damaged. (3) The larger the size of the coal pillar dam is, the less obvious the stress concentration effect is and the stronger its bearing capacity is; moreover, there is no obvious stripping of the wall caving, thus ensuring the long-term safe operation of the reservoir.

#### *4.3. Analysis of the Fracture Development Characteristics of a Coal Pillar Dam under the Dual Action of the Overlying Rock Layer and Lateral Water Pressure*

In order to analyze the lateral bearing capacities of different sizes of coal pillar dam, after mining in 3-1 coal seams, the lateral load was applied to two coal pillars to simulate the lateral water pressure. The linear displacement sensor was used to monitor the horizontal displacement of the coal pillar, and the surface of the coal pillar dam was photographed

regularly to monitor the fracture development characteristics of the coal pillar dam. According to the observed results of applied load, displacement, and fracture development, the fracture development and evolution law and the failure slip characteristics of the coal pillar dam were further analyzed.

(1) Analysis of the stress-failure slip characteristics of a 15-m 3-1 coal pillar

The lateral load was applied to the 15-m coal pillar of 3-1 coal, and the displacement of the roadway outside the coal pillar dam was recorded. As shown in Figure 17, after the loading started, the load increased linearly, but the amount of displacement was always 0. After loading for 5 min, the load did not increase, and was stable at about 480 kg. At this time, which was the elastic deformation stage, the coal pillar dam had not yet been displaced. When loading was done for approximately 6 min, the crack began to develop and the coal pillar produced obvious deformation. At the same time the coal pillar produced horizontal displacement, which increased linearly. When loading was done for approximately 12 min, the load suddenly decreased, and then the displacement also stopped at 9.15 mm. At this point, the coal pillar crack had been highly developed for the plastic failure stage, and the coal pillar roof plate had obvious separation cracks. When loaded to 23 min, the load dropped abruptly again, the coal pillar displacement suddenly increased linearly, and the coal pillar dam body was unstable and slipped.

**Figure 17.** Stress failure characteristics of a 15-m coal pillar of 3-1 coal.

(2) Analysis of stress-failure slip characteristics of a 30-m 3-1 coal pillar

We apply the load laterally to the 30-m coal pillar of 3-1 coal. As shown in Figure 18, after the start of loading, the load increased in volatile linear growth, but the amount of displacement was always 0. After 10 min of loading, the load reached a peak of approximately 1250 kg, after which the coal pillar dam was displaced; this was the elastic deformation stage. When the displacement reached about 7 mm, the crack of the roof of the coal pillar dam had been preliminarily penetrated and the coal pillar had obvious deformation. Within 10–12 min of loading, the load continued to decrease, and the displacement increased linearly with a large increase; this was the plastic failure stage. When loaded to 12 min, the load dropped, the coal pillar dam suddenly slipped and became unstable, and the cracks between the top and bottom plates of the coal pillar had been completely penetrated.

**Figure 18.** 3-1 Stress failure characteristics of a 30-m coal pillar.

Based on the above analysis, the following can be seen: (1) The size of a coal pillar dam is proportional to the lateral maximum water pressure it can bear. (2) Under the action of lateral water pressure, the coal pillar dam undergoes the process of "elastic deformation– plastic failure–instability slip", and the fracture develops most rapidly in the plastic failure stage—generally until the top and bottom plate cracks are completely penetrated—and instability slip occurs. (3) Experimentally, the maximum water level that the coal pillar dam can withstand can be roughly determined. For example, the maximum water level that can be withstood by a 15-m coal pillar is 50 m, which is the safe water storage height.

#### **5. Conclusions**

In this paper, the fracture evolution law and the critical conditions of failure instability of a coal pillar dam under different load conditions were studied, and the optimum size and safe water storage height of coal pillar dams were studied by establishing mechanical models, numerical simulations, and similar simulations. The following conclusions were obtained:


damage–instability slip" under the action of lateral water pressure. By using the experimental method, the maximum water level that the coal pillar dam can withstand can be roughly determined. The maximum water level that a 15-m coal pillar can withstand in the Shangwan coal mine is 50 m, which is the safe water storage height.

(4) Through interpreting the results of numerical simulation, it was determined that the optimal size of a coal pillar dam in the Shangwan mine is 30 m, and that the fracture development of the coal pillar at this size does not penetrate inside and outside the reservoir under the action of the overlying rock layer and lateral water pressure, thus ensuring the long-term safe operation of the underground reservoir in the coal mine.
