## 3 Experimental Results and Analysis  
### Stress and Strain Responses  
Figure 6a shows the stress-strain curves of coal rock at different bedding angles. The peak strengths of coal rock with bedding angles of 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) were 30.59 MPa, 19.61 Mpa, 11.69 Mpa, and 28.93 Mpa, respectively. The peak strength of coal rock initially decreases and then increases as the bedding angle increases, indicating that the peak strength of coal rock under creep loading is significantly influenced by the bedding angle. For coal samples with bedding angles of 0\({}^{\circ}\) and 90\({}^{\circ}\), the peak strengths are the highest, and the strength values for these two bedding angles are nearly identical. For coal samples with a bedding angle of 30\({}^{\circ}\), the peak strength is reduced by 35.9% compared to the 0\({}^{\circ}\) bedding angle specimens. For samples with a bedding angle of 60\({}^{\circ}\), the weakening of peak strength is even more pronounced, with a 61.8% decrease compared to the 0\({}^{\circ}\) bedding angle  
Figure 4: Experimental equipment used for creep digital image correlation testing of coal samples.  
Figure 5: Graded creep loading test scheme.  
samples. Therefore, in the process of strip mining in coal mines, attention should be paid to the influence of the bedding angle on the overall strength of coal pillars. It is recommended to retain coal samples with bedding angles close to 0\({}^{\circ}\) or 90\({}^{\circ}\) for use as supporting pillars.  
The post-peak behavior of the stress=-strain curve of coal samples is closely related to the failure mode of the coal body. Based on the post-peak characteristics of the specimens, the post-peak stage can be divided into two types: (I) linear type and (II) buffering type. The linear type post-peak failure mode is characterized by the specimen's stress rapidly decreasing along a straight line with a certain slope after reaching the peak stress, typically with a steep slope, as seen in coal samples with bedding angles of 0\({}^{\circ}\), 30\({}^{\circ}\), and 90\({}^{\circ}\). This failure mode involves significant damage, with the specimen losing stability rapidly within a short period and no plastic buffering time. In field engineering, this mode is the most destructive, making it difficult to maintain the coal body after the peak stress using engineering techniques. The buffering type post-peak failure mode refers to a stress drop after the peak stress that reaches the residual strength and then maintains this load-carrying capacity for a period of time, as shown in the stress-strain curve of the coal sample with a bedding angle of 60\({}^{\circ}\). In this mode, the coal body does not immediately and completely lose stability, offering the potential for reinforcement and stabilization through engineering measures in the later stages.  
The time-strain curves of coal with different bedding angles, as shown in Figure 6b, clearly demonstrate that the peak strain characteristics and creep loading stages of coal rock are also controlled by the bedding angle. For coal rock with bedding angles of 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\), the peak strains are 1.95%, 1.59%, 1.24%, and 1.79%, respectively, with 7, 5, 3, and 7 creep loading stages applied. Under the same creep stages, the coal samples are subjected to the same stress levels. Coal samples with bedding angles of 0\({}^{\circ}\) and 90\({}^{\circ}\) exhibit greater resistance to deformation, while the sample with a bedding angle of 60\({}^{\circ}\) demonstrates the weakest resistance to deformation, making it more prone to creep slip failure along the bedding planes.