# Investigation of Red Clay Internal Stress Anisotropy and Influence Factors
## 1 Introduction  
Red clay, resulting from the intense chemical weathering of carbonate rocks, is a highly plastic clay with well-developed fractures. During rainy seasons, water easily infiltrates through these cracks, causing deformation and instability in the roadbed [1; 2; 3]. In the process of engineering design, construction, or use, understanding the permeation characteristics of the foundation soil is of great significance for solving engineering problems [4; 5; 6]. If the geological model is simplified to a saturated, homogeneous, and isotropic porous continuous medium, ignoring the fundamental reality that soil in nature often displays anisotropy due to geological actions and its own weight, it would be highly unfavorable for estimating the long-term serviceability of engineering structures [7]. Currently, researchers have conducted studies on the anisotropic permeability behavior of soils in different regions [8; 9; 10]. [PERSON] et al. [11] conducted indoor variable head permeability tests on undisturbed loess at different burial depths to measure the vertical and horizontal permeability coefficients. They found that as the burial depth increased, the anisotropic permeability performance decreased successively, and they believed that the contact and arrangement of soil particles were the primary reasons for the inherent anisotropy. [PERSON] et al. [12] used three types of permeability coefficient anisotropy ratios to compare and analyze thecharacteristics of slope stress under rainfall infiltration conditions in terms of the magnitude of horizontal and vertical permeability coefficients. The results showed that the greater the anisotropy ratio of the permeability coefficient, the shallower the infiltration depth of rainwater and the smaller the increase in pore water pressure. [PERSON] et al. (2013) used Seep/W software for numerical simulation analysis, discussing factors such as the direction of anisotropy, degree of anisotropy, and distribution area of anisotropy. The results indicated that factors like the direction, degree, and distribution of fracture-induced anisotropy all have a certain impact on slope rainfall infiltration. Furthermore, the impact of fracture-induced anisotropy only becomes apparent when the rainfall intensity exceeds the permeability coefficient in the direction of the fractures. [PERSON] (2014) utilized a permeameter for variable head permeability tests to determine the permeability coefficients of peat soil samples at various cut angles. The findings suggest a specific relationship between the permeability coefficient of highly decomposed peat and the angle of cutting, where the coefficient increases with the cutting angle. The anisotropy of permeability was most distinct at moderate levels of organic content. [PERSON] (2015) conducted indoor permeability tests on silty soil, finding that the permeability coefficient ratio was generally less than 2. [PERSON] (2016) measured the permeability ratio of loess to be 1.3. From the above studies, it can be seen that different types of soil have varying degrees of permeability anisotropy. Additionally, international researchers have conducted tests on general soft soils. For instance, [PERSON] (2017) and [PERSON] (2018) reported permeability coefficient ratios for soft clay falling within the range of 1.0 to 2.0. While previous research has acknowledged that the inherent structure of soil significantly affects its stress characteristics, it is an essential factor in the calculation of stress and stability in engineering soil (2019, 2020). Regarding red clay itself, its soil structure is relatively loose, with developed pores and low shear strength. Therefore, soil is more prone to deformation under external forces, meaning that the particle structure inside the soil is more prone to undergo a reshaping effect under external loads. Moreover, the emergence of osmotic pressure is also an important objective factor that cannot be ignored for cohesive soil (2017).