# Experimental Study on Shear Creep and Long-Term Strength of Clay-Type Muddy Interlayer
## 3 Results and Discussion  
### Shear Creep Curve of Clay-Type Muddy Interlayer  
The creep test curve of this shear-creep test is shown in Figure 7, and the curve shows the values of shear stress at all levels. Through the shear displacement-time curve, it can be seen that the muddy interlayer has obvious creep characteristics, which can be summarized as follows:  
1. Under the constant normal stress level, the muddy interlayer sample has transient deformation and initial creep stage after all the levels of shear stress are loaded, and the initial creep stage lasts for a short time. At lower stress levels, the samples exhibited a stable creep stage, characterized by a creep curve that approximates a straight line. When the shear stress is relatively low, the shear displacement basically remains unchanged (the creep rate is approximately 0), and when the shear stress is slightly higher, the shear displacement slowly increases (the creep rate is a small constant value). This suggests that shear stress affects the creep behavior of the mud stratification, a feature noted by some researchers in the shear creep curves of muddy interlayer [38; 39]. The accelerated creep stage usually occurs after the application of the last Shear stress, because the shear stress is greater than the critical strength of the impedance capacity of the specimen, during which the shear displacement increases rapidly with time until the specimen is destroyed.
2. The shear displacement in the initial creep stage increases rapidly with time, while the creep rate decreases rapidly with time, and the creep rate in the stable creep stage increases with the increase in shear stress, indicating that the creep of the muddy interlayer is very strong, and the creep is enhanced with the increase in shear stress.  
Figure 7: Shear creep curves under different normal stresses ((**a**) \(\sigma=100\) kPa, (**b**) \(\sigma=200\) kPa, (**c**) \(\sigma=300\) kPa, (**d**) \(\sigma=400\) kPa).  
3. After the last shear stress is applied, there are three obvious stages of creep (Figure 7b), which shows a typical ductile failure form, while in Figure 7a,c,d, it directly enters the accelerated creep stage and occurs, showing a brittle failure form. It shows that when \(\sigma\) = 200 kPa, the last shear stress is just within the range where three stages of creep can occur, while when \(\sigma\) = 100, 300 and 400 kPa, the last shear stress is outside this range. 4. According to the curve of test results, it can be seen that the shear failure value of muddy interlayer increases with the increase in normal stress level. This is mainly because the increase in the normal stress level enhances the compaction effect on the interlayer particles and increases the shear friction resistance of the sample, so the shear stress required for shear failure increases accordingly. As shown in Figure 8, even though the vertical compression displacement under \(\sigma\) = 200 kPa is slightly greater than under \(\sigma\) = 300 kPa during the 12 to 408-h period, the difference is quite small, falling within the range of 0.01 to 0.04 mm. In summary, the overall trend in the shear-creep test process shows that the vertical compression displacement of the specimens increases with the rise in normal stress and gradually grows over time.