# Investigation of Red Clay Internal Stress Anisotropy and Influence Factors
## 4 Conclusions  
This study, based on indoor variable head permeability tests, obtained the permeability coefficients of undisturbed red clay in both vertical and horizontal directions under different vertical loads. The study deeply discussed the variation rules of permeability coefficients in different directions within the soil body under load, clarified the anisotropy of permeation characteristics in red clay, statistically analyzed the relationship between permeability coefficients and porosity ratio, and constructed a predictive model for permeability coefficients based on regression fitting.  
1. The stress within undisturbed red clay exhibits obvious anisotropy, and the clay cannot be considered an isotropic homogeneous body. Under the dual effects of different overburden loads, the vertical permeability coefficient kV is always greater  
Figure 8: The change curve of permeability coefficient ratio and osmotic pressure.  
than the horizontal permeability coefficient kH, and the permeability coefficients k in both directions are negatively correlated with lg\(\sigma\).
2. With the gradual increase of overburden load, the permeability coefficient ratio kV/kH shows a decreasing trend. When the overburden load does not exceed 150 kPa, the value of kV/kH decreases linearly. However, after the overburden load exceeds 150 kPa, the rate of decrease gradually slows and eventually stabilizes, approximately at 1.77. The porosity ratio e has a linear relationship with lgkV and lgkH, based on which a predictive formula for the anisotropy of permeability coefficients in undisturbed red clay was established.
3. Permeation pressure can expand the internal stress channels of red clay, increasing its permeability coefficient, and the vertical permeability coefficient is more sensitive to permeation pressure; moreover, permeation pressure can weaken the anisotropy of seepage in red clay to a certain extent.
4. The anisotropy of stress depends on the anisotropy of pores and their connectivity; the inherent anisotropy of pores in red clay leads to the anisotropy of stress movement. However, based on the structural characteristics of the soil, the permeation characteristics of the soil in different directions are also sensitive to feedback from the external environment (such as permeation pressure) and load.  
Based on the stress characteristics of red clay under different osmotic pressure and overburden load conditions, this study provides a detailed discussion, clarifying the evolution laws of the anisotropy of stress in red clay under different stress and osmotic pressure conditions, and exploring the intrinsic mechanisms of stress anisotropy inside the soil. The research findings are of significant importance for determining the permeability strength and consolidation settlement laws of red clay. Of course, there are some shortcomings in this study, and in the future, we will use CT technology to further explore the evolution of the internal voids and structure of red clay under the action of different influencing factors from a fine viewpoint.  
[PERSON] was responsible for planning the experiments, directing the research project, and writing this paper. [PERSON] was responsible for some experiments and data sorting. [PERSON] was responsible for project administration, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the financial supports of the Planned Project of Gansu Science and Technology Department [grant No. 21]R7R4310] and the Youth Science Foundation of Lanzhou Jiaotong University [grant No. 2021029].  
Not applicable.  
Data availability Statement: Data are contained within the article.  
Author [PERSON] was employed by the Gansu Shunda Road and Bridge Construction Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.