# Factors Affecting the Stability of Loess Landslides: A Review
## 2 Relevant Measured Parameters
### Creep  
While landslide creep has been studied extensively, there is a lack of specific investigations on creep in loess landslides, primarily due to the complex nature of the landslide medium, which includes expansive soils, red clay fills, variations in rock composition, and more. The combination of creep action and external factors increases the likelihood of failure. To determine the long-term strength of loess landslide specimens under creep conditions, [PERSON] et al. [80] proposed the steady-state creep rate slope method (SCRSM). At constant moisture content, the deformation at the steady-state creep stage increases dramatically with the increasing deviatoric stress. In addition, to describe the accelerated creep behavior in the loess medium, an improved Burgers model was introduced. Higher moisture content reduces the preload required for creep damage to occur, while higher axial strain increases the time required for each load [97]. Precipitation is the most important catalyst for the acceleration of creep in landslides [26]. However, the need for further research is highlighted by the paucity of studies investigating the effect of creep on the stability and formation mechanism of loess landslides. To describe the creep properties of rocks and soils prior to accelerated creep, the Burgers model is well suited. The creep equation of the Burgers model can be expressed as follows [80]:  
\[\epsilon(t)=\frac{q}{E_{1}}+\frac{q}{E_{2}}\bigg{(}1-e^{-\frac{E_{2}}{\eta_{1 }}t}\bigg{)}+\frac{q}{\eta_{2}}t \tag{1}\]  
Figure 6: Relationships between shear strength parameters and moisture content for different slip zone soils of five sampling points in Lanzhou, China. (Black solid symbol represents the peak state, and white hollow symbol belongs to residual state). (**a**) Peak cohesions of slip zone soils decrease continuously with increasing moisture, while residuals appear insensitive to moisture; (**b**) the variations of peak and residual internal friction angles with moisture content for slip zone soil. Reprinted/adapted with permission from Ref. [94]. 2021, Springer Nature. where \(\varepsilon\) is the total strain; \(E_{1}\) is the instantaneous modulus of elasticity; \(E_{2}\) is the viscoelastic modulus; and \(\eta_{1}\) and \(\eta_{2}\) are the viscoelastic coefficients. Different approaches have been used to study loess landslides, especially in the Loess Plateau region. In order to find a more appropriate method for studying loess landslides, a comparison of different landslide research methods will be made. These methods include the use of different geophysical instruments, the development of inversion-based models, and the study of the influence of changes in water content. However, these methods have limitations. Specific conditions are required for accurate interpretation of the results. The methods used to obtain measured parameters for landslide stability analysis are summarized in Table 1. In this case, analytical factors are those that relate to the specific situation we want to analyze. They are mainly qualitative. The measured parameters are those that we can obtain by some means. They are mainly quantitative. The measured parameters include those mentioned above. They are obtained by direct and indirect measurements. The directly measured parameters refer to the data measured by instruments. Indirectly measured parameters have to be inferred, converted, modeled, and inverted by intermediate parameters. And it is these intermediate parameters that can be measured directly by the instrument. For example, water content is an essential parameter in the analysis of the collapsibility of loesses and can be measured directly by the instrument. Similarly, to analyze the internal structure of a landslide, the size and number of fractures must be considered, and these parameters can be inverted by seismic methods.