## 3 Creep Instability Mechanism of the Soft Coal Roadway  
### Creep Characteristics of Soft Coal under Triaxial Compression  
The numerical simulation of the creep test on soft coal specimens under triaxial compression was conducted using UDEC 7.0. The study focused on the deformation and fracture characteristics of microparticles and contacts in soft coal under confining pressure. Due to the exclusion of the specimen's discrete nature in the numerical test, the triaxial loading process can be executed through gradual increments in stress levels using constant loading. The peripheral pressure is set at varying values of 0 MPa, 0.2 MPa, 0.4 MPa, and 0.6 MPa. In instances where the loading load reaches the last stress level and remains without accelerated creep for over 12 h, a 5% increase in peak strength relative to the level difference is applied for the subsequent round of constant level loading. This procedure is repeated until accelerated creep is observed in the specimen. For precise details of the experimental program, refer to Table 3.  
By leveraging the FISH function inherent in the UDEC 7.0 software and extending its capabilities through secondary development, the accelerated creep rate curve of the specimen as it enters the accelerated creep stage under varying peripheral pressures was extracted. This extraction facilitates the analysis of the deformation characteristics exhibited by soft coal within the non-steady state creep condition across different peripheral pressures. Please refer to Figure 6 for a visual representation of these findings. The analysis results are as follows:  
1. The creep deformation of soft coal exhibits a confining pressure effect within a certain range. Specifically, when the confining pressure is 0, 0.2 MPa, or 0.4 MPa, the initial strength of accelerated creep increases by 6.2% and 5.5%, respectively. This suggests that an increase in surrounding rock results in a higher stress level required for accelerated creep to occur. However, even when the confining pressure reaches 0.6 MPa, the specimen still undergoes accelerated creep under an axial pressure of 3.27 MPa. This indicates that as the axial pressure increases, the confining pressure effect on the creep process of soft coal gradually weakens.
2. Increasing the confining pressure can delay the onset of accelerated creep in soft coal. For instance, when the axial pressure is 3.27 MPa, the time of accelerated creep with a confining pressure of 0.4 MPa is delayed by 0.25 h compared to the case with a confining pressure of 0.6 MPa. From a deformation rate perspective, the overall deformation rate of the latter is lower than that of the former, resulting in a longer constant creep period. The start time of accelerated creep is also delayed. These observations suggest that under high confining pressure, the constant creep stage is longer, the accelerated creep is more moderate, and both the deformation amount and deformation rate are reduced.
3. Increasing the confining pressure has a restraining effect on the creep deformation of soft coal. As the confining pressure increases from 0 to 0.6 MPa, the proportion of instantaneous strain to total strain increases from 17.2% to 19.0%. However, the proportion of creep increment in each deformation stage following the instantaneous strain decreases from 82.8% to 81%. This indicates that higher confining pressure  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test Scheme** & **Confining Pressure (MPa)** & **Stress Level (MPa)** & **Ratio of Ultimate Strengths (\%)** \\ \hline \multirow{6}{*}{Constant stage loading} & 0.34 & 10\% \\  & 0.69 & 20\% \\  & 1.38 & 40\% \\  & 2.06 & 60\% \\  & 2.75 & 80\% \\  & 2.92 & 85\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Numerical simulation scheme of the triaxial creep test.  
leads to a larger portion of instantaneous strain in the total deformation, while the contribution of creep deformation decreases slightly.