# Investigation of Red Clay Internal Stress Anisotropy and Influence Factors
## 3 Results and Discussion
### The Influence of Permeation Pressure on Stress Anisotropic  
Figure 7 illustrates the variation curves of permeability coefficients in different directions relative to permeation pressure under assorted overburden loads. It is observable that under consistent overlying load, the permeability coefficients for both directions incrementally increase with rising permeation pressure, demonstrating a strong correlation between permeability coefficient and permeation pressure. Moreover, when the overburden load is low, the slopes of the curves are steep. As the overburden load increases, the slopes of the curves gradually become stable, indicating that the impact of permeation pressure on the red clay's permeability coefficient is most intense under low-level overburden load conditions, and the increase in overburden load gradually suppresses this effect. On the other hand, it can be seen that overall, under the same overburden load condition, the slope of the kV-P curve is steeper than that of the kH-P curve, indicating that the vertical permeability coefficient is more sensitive to permeation pressure. This also indirectly reflects that the vertical fractures in the native structure of the red clay in this study are more developed. With the gradual increase in permeation pressure, internal flow pathways in the red clay widen, potentially leading to the creation of secondary fractures, further intensifying the soil's inherent stress field.  
Figure 7: Dynamic correlation between vertical and horizontal permeability coefficients and osmotic pressure (**a,b**).  
Figure 8 illustrates the change in the permeability coefficient ratio kV/kH in different directions as a function of permeation pressure P under different overlying load levels. The figure indicates that with increasing permeation pressure, the permeability coefficient ratio kV/kH decreases sequentially under each level of overlying load. Notably, the decrease in kV/kH is most pronounced under an overlying load of 50 kPa. Once the overburden load exceeds 50 kPa, the value of kV/kH only changes within a small range; in particular, when P = 300 kPa, kV/kH stabilizes at almost 1.7.  
It is worth noting that although the permeation pressure increased the stress strength of the red clay itself, it also led to further expansion of fractures, and might even cause originally independent pores to connect, thereby weakening the anisotropy of its stress.  
Additionally, it was found that when the osmotic pressure exceeded 50 kPa, the permeability coefficient ratio reversed under overburden loads of 200 and 300 kPa, which further proves that osmotic pressure and overburden load simultaneously have a dynamic interaction effect on stress anisotropy. Under continuously increasing osmotic pressure, although the bias flow effect inside the soil was reduced, weakening the stress anisotropy, the permeability coefficient ratio under the two loads gradually tended to be consistent. However, different overburden loads simultaneously have a reshaping effect on the internal structure of the soil. Larger overburden loads make the soil more compact, gradually reducing the interconnected pores inside, decreasing the intensity of stress, and eventually causing certain differentiated characteristics in stress properties under the two load.