# Study on Creep Behavior of Silty Clay Based on Fractal Derivative
## 2 Test Procedure and Results
### Test Results and Analysis  
The creep curves of the shear strain versus the time of undisturbed silty clay under different stress states and confining pressures were obtained through the triaxial creep tests, as shown in Figure 6. The corresponding relations of the creep strain rate versus time are shown in Figure 7 (taking the confining pressure of 200 kPa and the approximately deviatoric stress of 200 kPa as an example). Figures 6 and 7 show that the instantaneous and total strain of the soil is more significant when the confining pressure is low, indicating that the higher confining pressure can improve the instantaneous stiffness of the soil. Additionally, as the soil gradually consolidates, the creep deformation is also reduced. Furthermore, the creep deformation of the soil is mainly composed of instantaneous deformation, and the creep deformation characteristics under low stress is negligible but gradually become obvious when the stress increases. Nevertheless, the total creep deformation is relatively not significant and eventually tends to an asymptotic value by time, showing nonlinear characteristics. The creep strain rate also increases with the promotion of deviatoric stress, decreases with the increase in confining pressure, and gradually tends to be stable with time. Figure 5: Stress-strain curves of the silty clay samples in the CD triaxial test. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test No.** & \(\sigma_{3}\) **(kPa)** & \(q_{f}\) **(kPa)** & \(q\) **(kPa)** & **Test Time (h)** \\ \hline
1 & & & 57 & 170 \\
2 & & 116 & 190 \\
3 & & 169 & 190 \\
4 & & & 243 & 170 \\ \hline
5 & & & 117 & 170 \\
6 & & 220 & 170 \\
7 & & & 330 & 170 \\
8 & & & 459 & 170 \\ \hline
9 & & & 197 & 170 \\
10 & & & 422 & 170 \\
11 & & & 617 & 170 \\
12 & & & 863 & 170 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Loading application scheme of the triaxial creep test. In general, the failure deviatoric stress increases with increasing confining pressure. Meanwhile, the deformation trends of soil under different confining pressures and deviatoric stresses are similar. In the case where the deviatoric stress loading value during the test is always lower than the failure level, the creep deformation presents the following two stages. In the initial transient creep stage, the creep deformation increases significantly,  
Figure 6: Relation of axial strain versus time under different confining pressures and stresses: (a) \(\sigma_{3}=100\) kPa; (b) \(\sigma_{3}=200\) kPa; (c) \(\sigma_{3}=400\) kPa; (d) approximately \(q=200\) kPa. Figure 7: Relation of axial strain rate versus time: (a) \(\sigma_{3}=200\) kPa; (b) approximately \(q=200\) kPa. accompanied by a remarkable change in creep rate. As time goes on, it gradually enters the steady-state creep stage, in which the creep rate tends to zero in the low-stress state or remains constant in the high-stress state, as is consistent with the findings of [PERSON] [35] and [PERSON] [39] in coastal soft soils in South China. To further comprehend the creep characteristics of silty clay, the stress-strain isochronous curves of different confining pressures and deviatoric stresses are given as shown in Figure 8. The respective curves follow similar nonlinear trends under different deviatoric stress conditions and are related to the loading time. In addition, as the isochronous curves deviate from the straight line to a greater extent, the degree of nonlinearity becomes more obvious with increasing deviatoric stress and time. For the sample with constant deviator stress, the strain is negatively correlated with the change in confining pressure, which illustrates that the creep behavior of soil is the result of the combined action of time, stress, and confining pressure. Figure 9 shows the relation of volumetric strain versus time under different confining pressures and deviatoric stresses. Figure 10 is the relation of volumetric strain rate versus time (taking the confining pressure of 200 kPa and the approximately deviatoric stress of 200 kPa as an example).