## 3 Results and Discussion  
### Compressive Deformation Characteristics  
The stress-strain curves of waste rock specimens with various particle sizes under axial loading are illustrated in Figure 9.  
Analysis of Figure 9 reveals that, under stress, the stress-strain curves of specimens with various particle sizes follow a similar trend with increasing axial stress. This can be divided into two stages: The first shows low concavity and entails quasi-linear deformation, which occurs because some voids present between waste rock particles are closed under the external force as the axial pressure grows. The second stage entails large concavity and is non-linear. At the beginning of the stage, some voids in the waste rock specimen are filled with pre-existing fine particles in the specimen, while other voids are not filled due to the support of the skeleton structure formed by waste rock particles and a small number of fine particles. As the axial pressure increases, large waste rock particles begin to break and form fine particles, the skeleton structure is broken, and voids are further filled.  
Figure 8: Appearance of the compressed backfill material specimen y-3.  
The compressive deformation properties of specimens are strongly influenced by particle size distribution. The strain of specimens with continuous particle size distributions of 0-5, 0-10, and 0-20 mm containing fine particles is lower than that of specimens with single particle size distributions of 5-10, 10-15, and 15-20 mm. Moreover, the strain increases with particle size in both cases. This is because specimens with continuous particle size distributions under natural crushing contain pre-existing fine particles that can fill in voids. As a result, voids that can be filled in by these specimens during the deformation process are smaller than those in specimens with a uniform particle size.  
Figure 10 shows the stress-strain curves of waste rock with particle size distributions of 0-10 mm placed in compressors with different inner diameters.  
Analysis of Figure 10 shows that the inner diameter of compressors greatly affects the strain of waste rock specimens during axial loading. Under the axial stress of 20 MPa, the strain of specimens in the compressor with an inner diameter of 50 mm is only 0.13107, while that in the compressor with an inner diameter of 250 mm reaches 0.15781, increasing by 20.4%. The strain is positively correlated with the inner diameter of compressors. This is because, under a random distribution of waste rock particles, the larger the inner diameter of a compressor, the lower the probability that stable skeleton structures will form from hinged hard waste rock particles, and thus the weaker the overall compressive behavior of  
Figure 10: Stress-strain curves of waste rock specimens in compressors with different inner diameters.  
Figure 9: Stress-strain curves of waste rock specimens with various particle sizes.  
the structure. Moreover, due to the random distribution of defects in the specimens, the larger the inner diameter of a compressor, the higher the probability of low-strength waste rock particles occurring in the specimens, and therefore the weaker the overall compressive strength of the structure.