# Study of the Internal Rebreaking Characteristics of Crushed Gangue in Mine Goal during Compression
## 3 Results
### Confined Compression Deformation Characteristics of Fractured Gangues with Different Lithologies
#### 3.1.2 Stress-Strain  
The stress and strain data during the compression of crushed gangue samples were recorded, and the stress-strain curves of sandstone, sandy mudstone, and a combination of both under confined compression were obtained (Figure 6). The following can be seen from Figure 6:  
(a) The stress and strain of the crushed singular gangues and a combination of both were nonlinear during confined compression. As the strain increased, the axial stress increased steadily, while the strain growth rate decreased toward the end. However, in different stress stages, the variation in the strain was different. According to the evolution process of the slope change rate of the stress-strain curve of fractured gangue samples, the compaction deformation process of fractured gangue samples was divided into three stages:  
(I) The rapid compaction stage. (II) The slow compaction stage. (III) The stable compaction stage. Specifically, in Stage I, the crushed gangue was in a loose state, with many voids inside the crushed gangue, and the overall bearing capacity was very weak. The block  
Figure 5: Strain-time curves of sandstone, sandy mudstone, and composite lithological crushed gangue samples during confined compression. Figure 6: Stress–strain curves of sandstone, sandy mudstone, and composite crushed gangue samples during confined compression. was extremely prone to deformation under axial pressure. In Stage II, the pores between crushed gangue blocks in mine goaf were gradually filled with crushed blocks as the axial stress increased, their resistance gradually increased, they were not prone to deformation after being stressed, and the strain growth rate gradually decreased. The boundary between Stage I and Stage II gradually shifted to the right as the elastic modulus of the lithology decreased; in Stage III, the blocks mainly meshed with each other at the point of contact. As the stress continued to increase, the strain value decreased due to the sliding, fracturing, and structural reorganization between the blocks. The larger the block particle size, the smaller the strain value, and the longer the duration. The pores between the blocks were filled by the crushed blocks. As the stress increased, the pores between the blocks decreased, and the blocks were gradually compacted to form a more stable overall structure. The strain curve gradually trended to a constant value. The boundary between Stages II and III gradually shifted to the right as the strength of the original gangue increased. (b) The compression deformation characteristics of different crushed gangue samples varied. In the rapid compaction stage, the strain curves of single and combined lithologies were close. In the slow compaction stage, the strain curves of single and combined lithologies gradually separated. Under the same stress conditions, the strain of sandy mudstone was greater than that of the combination and that of sandstone. In the stable compaction stage, the strain of the confined compaction of crushed single and combined lithological gangues gradually increased, and the increase amplitude gradually decreased. After basic compaction, the strain of crushed sandstone was 0.33, that of the composite was 0.34, and that of sandy mudstone was 0.36. This indicated that the combined lithology of fractured sandstone and sandy mudstone improved its overall strength compared to a single lithology of sandy mudstone, changing the compaction characteristics of sandy mudstone to a certain extent and affecting the evolution process of sandy mudstone strain with stress. (c) The strength values obtained in lateral compression tests for crushed gangue samples with higher hard crushed gangue content are also higher than those of gangue samples with lower hard gangue contents. Under lateral compression conditions, the compression deformation of the crushed gangue combination samples in goaf will decrease with an increase in the hard crushed gangue content.