## 3 Results  
### Deformation and Strength Characteristics  
From the stress-strain curves (Figure 3), it can be observed that the damage and failure behavior of the six types of samples, C, CSR (9:1), CSR (5:5), CSR (3:7), R, and CR (5:5), under uniaxial compression are broadly similar. They all undergo compaction, elastic, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Number** & **Height (d \(\times\) h/mm** & **Coal–Rock Ratio** \\ \hline Raw coal & C (10:0) & 49.58 (coal) \(\times\) 100.34 & 10:0 \\ Mudstone & R (0:10) & 49.74 (coal) \(\times\) 100.11 & 0:10 \\ Artificial coal–rock combination & CR (5:5) & 50.22 (coal) \(\times\) 100.23 & 5:5 \\ Primary coal–rock combination & CSR (5:5) & 49.86 (coal) \(\times\) 100.17 & 5:5 \\ Primary coal–rock combination & CSR (3:7) & 49.96 (coal) \(\times\) 100.16 & 3:7 \\ Primary coal–rock combination & CSR (9:1) & 50.11 (coal) \(\times\) 100.37 & 9:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sample information. Figure 2: Schematic diagram of experimental system. yielding, and post-peak failure stages. As the axial stress gradually increases to the peak stress, the load-bearing capacity of the samples gradually diminishes, and the axial stress rapidly decreases. Typically, either the secant or the tangent slope of the stress-strain curve near 50% of the peak strength is used. In this study, we adopt the tangent slope. For the detailed formula, please refer to Equation (6). In this experiment, the peak stress of a single-rock sample is 134.18 MPa, with an elastic modulus of 56.31 GPa. When the coal-rock ratio is 3.7, the peak stress of the sample is 25.53 MPa, with an elastic modulus of 14.93 GPa. When the coal-rock ratio is 5.5, the peak stress is 22.52 MPa, with an elastic modulus of 10.81 GPa. When the coal-rock ratio is 9:1, the peak stress is 18.92 MPa, with an elastic modulus of 7.32 GPa. The peak stress of a single-coal sample is 17.59 MPa, with an elastic modulus of 6.36 GPa. Compared with the sample with a coal-rock ratio of 0:10, the uniaxial compression strength of the composite samples with coal-rock ratios of 3:7, 5:5, 9:1, and 10:0 decreased by 80.97%, 83.22%, 85.9%, and 86.89%, respectively, while the elastic modulus decreased by 73.49%, 80.8%, 87%, and 88.71%, respectively. Therefore, with the increase in coal-rock ratio, both the uniaxial compression strength and elastic modulus (E) of the composite samples show a decreasing trend (Figure 4). The varying strengths of the samples indicate that the failure primarily occurs within the coal component. As the coal-rock ratio decreases, the overall strength and deformation resistance of the samples gradually increase. This phenomenon stems from coal being  
Figure 4: Peak stress and elastic modulus of single-rock sample, single-coal sample, and combinations. Figure 3: Stress–strain curves of single-rock sample, single-coal sample, and combinations. a unique type of rock characterized by numerous inherent defects, such as microcracks and micropores. These defects inherently influence the strength of coal. However, as the coal proportion diminishes within the coal-rock composite samples, the prevalence of defects decreases accordingly. Consequently, this leads to an enhancement in the composite coal-rock sample's strength and deformation resistance capabilities. Comparing the primary coal-rock composite CSR (5:5) with the artificial coal-rock composite CR (5:5), the peak stress of the artificial composite is 35.13 MPa with an elastic modulus of 11.9 GPa, both exceeding those of the primary coal-rock composite CSR (5:5), which measures 22.52 MPa for peak stress and 10.81 GPa for elastic modulus. This disparity arises from the presence of distinct coal-rock transition zones at the interface of the natural coal-rock composite, contrasting with the smooth interfaces of the artificial coal-rock composite.