## 3 Creep Characteristics of the Strongly Weather Argillaceous Sandstone
### Analysis of the Experimental Results
#### 3.2.3 Particle Fragmentation Analysis of the Creep Specimens  
Most contemporary studies concerning the strength characteristics of rock and soil are analyzed via macro-analyses, with few observations of the internal microscopic characteristics. However, studying the particle fragmentation in triaxial tests can help reveal the strength mechanism of soil-rock mixtures [40; 41]. The particle fragmentation condition of the specimen was observed at the end of the tests completed in this study. Particle sieving analysis tests were performed to compare the changes in the particle gradation of the creep specimens before and after testing, and to analyze the influencing factors of the creep characteristics from the particle fragmentation characteristics. The particle fragmentation of each creep specimen is given in Table 5. The percentage of particle breakage (\(B_{g}\)) index proposed by [PERSON] (1967) was used to describe the particle fragmentation before and after testing [42]. The percentage of particle breakage was calculated as follows:  
\[B_{g}=\Sigma\Delta W_{k}=\Sigma(W_{ki}-W_{sf}), \tag{1}\]  
where \(\Delta W_{k}\) is the difference between the content of each grain group before and after the test, \(W_{ki}\) is the content of a certain grade of grain group before the test, and \(W_{sf}\) is the content of the corresponding grain group of the same grade after testing. The grain size distribution curves of the specimens after the triaxial creep test are shown in Figure 9.  
We analyzed the effect of particle fragmentation on creep based on three aspects: moisture content; confining pressure; and stress level. (a) Moisture content Figure 10 shows that the percentage of particle breakage in the saturated creep specimens was significantly lower than those in the natural state owing to the lubricating effect of water on the particle movement. Higher water content in the specimens was associated with more favorable movement, rotation, and overturning of the soil and stone. In the saturated state, water molecules effectively reduce the rigid contact between particles. The degree of particle fragmentation is significantly reduced when water molecules exist as a buffer between particles. (b) Confining pressure Figure 10 shows that the percentage of particle breakage gradually increased with increasing confining pressure. When the moisture content was 13%, the percentage of particle breakage at 300 kPa was 1.7% higher than that at 100 kPa. Under saturated conditions, the crushing rate of the creep specimen at 300 kPa was 0.9% higher than that at 100 kPa. This is because increasing the confining pressure compresses the specimen and restricts the movement of soil and stone. The simultaneous increase of the contact surface between particles and contact force thus results in an overall increase in the degree of fragmentation. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Moisture State**} & \multirow{2}{*}{**Strength**} & \multicolumn{2}{c}{**Confining Pressure**} & \multicolumn{2}{c}{**Strength Parameters**} \\  & & **100 kPa** & **200 kPa** & **300 kPa** & **Cohesion** & **Angle of Internal Friction** \\ \hline \multirow{3}{*}{13\% moisture content} & Long-term strength & 405 kPa & 557 kPa & 662 kPa & 94 kPa & 23\({}^{\circ}\) \\  & Instantaneous strength & 617.5 kPa & 876.8 kPa & 1048.2 kPa & 130.9 kPa & 32.1 \\  & Proportionality & 66\% & 64\% & 63\% & 72\% & 72\% \\ \hline \multirow{3}{*}{Saturated state} & Long-term strength & 198 kPa & 347 kPa & 471 kPa & 21 kPa & 22.9\({}^{\circ}\) \\  & Instantaneous strength & 317.1 kPa & 522.7 kPa & 745.1 kPa & 30.5 kPa & 30.6\({}^{\circ}\) \\ \cline{1-1}  & Proportionality & 62\% & 66\% & 63\% & 69\% & 78\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Long-term strength and instantaneous strength.