## 3 Creep Characteristics of the Strongly Weather Argillaceous Sandstone
### Analysis of the Experimental Results  
#### 3.2.1 Creep Curve  
Creep tests allowed the full creep process curve of the strongly weathered argillaceous sandstone to be obtained under different confining pressures, as shown in Figure 7. The creep curve was determined in four steps, and the specimen was damaged under the fifth load (\(1.0q_{f}\)). Accordingly, the creep curve was stepped once applying each shear load level under which the specimens show instantaneous displacement. Over time, the deformation speed gradually decreased and the curve tended to flatten.  
Figure 6: Large-scale SZLB-4 triaxial apparatus and a specimen of the strongly weathered argillaceous sandstone. (**a**) Triaxial apparatus, (**b**) Specim.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Moisture Content** & **Confining Pressure/kPa** & **Deviator Stress/kPa** \\ \hline \multirow{4}{*}{13\%} & 100 & \(125\to\,250\to\,375\to\,500\to\,620\) \\  & 200 & \(180\to\,360\to\,540\to\,720\to\,880\) \\  & 300 & \(210\to\,420\to\,630\to\,1050\to\,1050\) \\ \hline \multirow{4}{*}{saturated state} & 100 & \(65\to\,130\to\,195\to\,260\to\,320\) \\  & 200 & \(110\to\,220\to\,330\to\,440\to\,530\) \\ \cline{1-1}  & 300 & \(150\to\,300\to\,450\to\,600\to\,750\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Triaxial creep test scheme.  
The full creep process curve (also called the graded loading curve) obtained from the test was converted into separate loading curves using [PERSON]'s loading method, as shown in Figure 8. Under each level of loading, the specimen first generated instantaneous strain, which then gradually flatted upon transitioning into creep strain. The instantaneous strain was mostly completed within 30 min and occupied the main part of the curve displacement. Although the displacement was relatively small, the creep strain represents the key part of the curve and can be used to determine the specimen's damage tendency. Research suggests that when the creep stress is less than the long-term strength, the soil sample exhibits a decaying creep mode; when the creep stress is greater than the long-term strength, the soil sample exhibits a creep or nondecaying creep mode as the strain rate increases [35; 37].  
Figure 8: _Cont._  
Figure 7: Graded loading curve. (**a**) Natural state, (**b**) Saturated state.  
The full creep process can be divided into three stages (Figure 9): decay creep; steady creep; and accelerated creep [38]. The creep rate gradually decreased during the decay creep stage and remained approximately constant during the steady creep stage. Stable deformation occurred when the velocity was nearly constant. At lower stress levels, the specimens were dominated by instantaneous strain and stabilized at 3000-4000 min. The creep curve later approximated a horizontal steady state and the creep rate tended to zero. At this time, the specimen was in the decay creep stage and no damage occurred. As the stress level increased, the creep curve began to linearly increase at a lower creep rate and the specimen entered the steady creep stage. Upon maintaining this state, the specimen would have eventually been destroyed. The transition from decay creep to steady creep marked a turning point in the specimen's creep characteristics, and the corresponding deviator stress at this point was the specimen's long-term strength [39]. When a specimen was in the accelerated creep stage at higher stress levels, the strain rate increased rapidly and the sample rapidly sheared, causing damage.