## 2 Experimental Study on Adhesion Properties of Raw Coal
### Research Results and Discussion  
#### 2.3.1 Calculation of Raw Coal Adhesion Properties  
Based on the designed test plan and method, typical post-pre-shear stress-displacement curves for raw coal were obtained, as shown in Figures 3 and 4. The curves reveal that the shear stress-displacement behavior for both coal-coal and coal-wall interfaces is fundamentally similar. In the initial shearing phase, shear stress increases gradually, which is attributed to the incomplete formation of contact points between coal particles and the adjustment and rearrangement of particles. As the shear displacement continues to grow, the shear stress rises rapidly and exhibits irregular fluctuations. This phenomenon is related to the diversity of raw coal particles, localized particle breakage, variations in interface friction, and the complex mechanical transmission paths between particles. Eventually, the shear stress stabilizes, and the coal material reaches its limit stress state, at which point the contact forces or interface forces between coal particles achieve equilibrium. Furthermore, the experiments demonstrated that shear strength increases significantly with higher moisture content and normal stress. This finding suggests that the enhancement of liquid \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Factor** & **Shear Interface** & **Moisture Content \(w\)/\%** & **Compaction Force \(p\)/kPa** \\ \hline Level 1 & Coal-Coal & 0 & 300 \\ Level 2 & Coal-Wall & 1 & 400 \\ Level 3 & - & 3 & 500 \\ Level 4 & - & 5 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Influencing factors and their levels. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compaction Force \(p\)/kPa** & \multicolumn{3}{c}{**Normal Stress \(\sigma_{n}\)/kPa**} \\ \hline
300 & 200 & 250 & 300 \\
400 & 200 & 300 & 400 \\
500 & 300 & 400 & 500 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Compaction Force and Corresponding Normal Stress. bridge forces and normal stress contributes to the bonding and friction effects at particle contact surfaces. The compacted raw coal and the coal at a fixed depth within the coal bunker exhibit the same mechanical characteristics. Their shear yield trajectory is composed of shear failure points under various normal forces. Raw coal under the same compaction force shares an identical shear yield trajectory. As depicted in Figure 5, a linear fit of the three shear yield points A, B, and C yields the shear yield trajectory for the compacted raw coal, providing parameters that characterize the adhesion properties of raw coal. The shear yield behavior of raw coal adheres to the Mohr-Coulomb criterion [31,32]. \[\tau=\sigma_{n}\tan\varphi+c \tag{1}\]  
where \(\tau\) is the shear strength (kPa), \(\sigma_{n}\) is the normal stress (kPa), \(\varphi\) is the internal friction angle of raw coal (\({}^{\circ}\)), and \(c\) is the cohesion of raw coal (kPa). Figure 3: Relationship curves for the coal-coal interface under different conditions. Linear fitting was performed on the results of the raw coal shear test, yielding fitting curves for different compaction forces and moisture contents at the coal-coal and coal-wall interfaces, as shown in Figures 6 and 7. The correlation coefficients \(R^{2}\) of these fitting curves all exceed 98%, indicating a high quality of fit. Using these curves, the cohesion and internal friction angle of raw coal under various conditions at the two interfaces were calculated, as presented in Tables 4-7. Figure 4: Relationship curves for the coal-wall interface under different conditions. Figure 5: Principle of raw coal shear test. Figure 6: Fitting curves for the shear test results of the coal-coal interface. Figure 7: Fitting curves for the shear test results of the coal-wall interface.