# Creep Characteristics of Soil in the Sliding Zone of Huangtupo Landslide
## 5 Discussion  
The long-term strength of a soil sample refers to the turning point at which soil creep is converted from stable creep to unstable creep with increasing time under a constant load [41], also known as the third yield stress value of a soil sample. The theory of long-term strength has not yet been unified. In the engineering field, it is generally believed that the long-term strength of rock-soil masses is less than the short-term strength [42]. At present, rheological research on soil is mostly focused on the creep phenomenon, and there is little research on long-term strength and its determination method. In China, the long-term strength is generally determined based on the three boundaries proposed by [PERSON] [43], while it is determined by the Vyalov method [44], the Shlbeta method, and other methods abroad [45]. Currently, the isochronous curve method and creep rate-stress relationship curve are most commonly used to determine the long-term strength; the steady-state creep rate method was selected in this study (Figure 17a). Figure 17: (**a**) The steady-state creep rate method to determine the long-term strength. (**b**) Creep process curve; A–D represents the dividing point of each creep stage. According to the creep curve characteristics, the stable creep stage is the key process by which to distinguish the creep acceleration stage. As shown in Figure 17b, when the shear stress is less than the yield strength of the sliding zone soil, the strain curve tends to change horizontally with time, and the rate gradually decays to zero, as in section AB; as the shear stress increases, the strain rate decays to a stable value, and the strain changes with a certain slope, as in section BC; when the shear stress is greater than the yield strength of the sliding zone soil, the creep rate increases rapidly with time, resulting in the deformation and destruction of the sample, as in section CD. The shear stress of the BC section is taken as the long-term strength of the soil. The latter half of the creep rate-stress curve was fitted in order to determine the long-term strength, and the intersection point of the fitting line and the stress axis was used as the reference value of the long-term strength. The steady-state creep rate-shear stress curve of each specimen was plotted according to the isochronous curve, as shown in Figure 18, where the blue section is the tangent of the latter half of the curve, and its intercept in the transverse axis is regarded as the long-term strength value. The long-term strength of the sliding zone soil was found to be approximately 71%-72% of the peak strength. [PERSON] at al. also believe that the long-term strength is about 70% of the peak strength [35]. The long-term strength of sliding zone soil under different stress states was obtained using the steady-state creep rate method, which was 85.69 kPa (a), 107.22 kPa (b), 128.57 kPa (c), and 155.83 kPa (d). It was found that with the increase in the loading-unloading amplitude \(R\), the long-term strength value increased nonlinearly. The reason for this phenomenon may be related to the porosity ratio of the specimen. [PERSON] et al. also found  
Figure 18: The steady-state creep rate versus shear stress curve. that when the soil structure changed from loose to dense, the porosity ratio of the soil decreased, and the bonding force between the soil particles became weaker, which greatly reduced the shear strength of the soil [46]. As shown in Figure 6, it was found that the porosity ratio under the initial consolidation state was the largest, the height of the sample after consolidation was the highest, the cohesion between the particles on the shear surface was low, and the long-term strength was the smallest. When the consolidation pressure of the specimen increased, the sample particles became denser, and the porosity ratio of the specimen decreased accordingly, reflecting that the long-term strength of the sliding zone soil increased.