# Investigation of Red Clay Internal Stress Anisotropy and Influence Factors
## 2 Materials and Methods
### Experimental Method  
At present, the indoor measurement of the permeability coefficient k utilizes diverse instruments and methodologies, predominantly the constant head and variable head  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Natural** & **Optimum** & **Maximum Dry** & **Specific** & **Liquid Limit** & **Plastic Limit** & **Plasticity Index** \\
**Moisture** & **Moisture** & **Density** & **Gravity G\({}_{\mathbf{s}}\)** & **\(\omega_{\mathbf{L}}\)/\%** & **\(\omega_{\mathbf{r}}\)/\%** & **\(\mathbf{I_{P}}\)** \\
**Content (\%)** & **Content (\%)** & **(g/cm\({}^{3}\))** & **(g/cm\({}^{3}\))** & **(g/cm\({}^{3}\))** & **(g/cm\({}^{3}\))** & **(g/cm\({}^{3}\))** \\ \hline
36.7 & 18.6 & 1.89 & 2.65 & 57.8 & 25.9 & 31.9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main physical indexes of lateritic clay.  
Figure 1: Grading curve of lateritic clay.  
Figure 2: Acquisition of undisturbed samples in different directions.  
methods. The constant head method is typically suitable for sandy soils and non-cohesive soils that contain little to no gravel. In the case of clay, given its low permeability coefficient and the scant water flow through the sample, its k-value is challenging to measure directly and accurately, thus the variable head method is commonly employed. The variable head test's benefit lies in its ability to minimize reading errors due to evaporation and temperature, and as the water head difference changes over time during the test, it enables more accurate measurement of the permeability coefficient for low-permeability materials. For these reasons, this experiment employs the variable head method, depicted in Figure 3, to conduct stress experiments on undisturbed red clay with the TT-APP2C type dual variable head automatic permeameter from Suzhou Toptesting Instruments Co., Ltd in Suzhou, China. This device comes equipped with an automatic data collection system, capable of instantaneously reading water head heights and automatically calculating the permeability coefficient using built-in software and Formula (1).  
\[k=2.31\frac{L\cdot A_{1}}{A_{2}(t_{2}-t_{1})}18\frac{h_{1}}{h_{2}} \tag{1}\]  
Herein, L represents the length of the permeation path; A\({}_{1}\) is the cross-sectional area of the water column; A\({}_{2}\) is the cross-sectional area of the sample; \(t_{1}\) and \(t_{2}\) are the start and end time of the measuring head respectively; and h\({}_{1}\) and h\({}_{2}\) are the water head heights at different times [24]. And k represents the permeability coefficient, with kV and kH respectively denoting the vertical and horizontal permeability coefficients in this study.  
The specific test steps are as follows:  
1. Following the standard for geotechnical testing methods, samples are saturated via the vacuum saturation technique, which includes applying a vacuum for one hour and subsequently saturating for 10 h after the saturator is fully immersed in water;
2. Once the sample's vacuum saturation is finished, it is taken out of the saturator and placed on the permeameter for permeability testing under various overlying load conditions (i.e., 25 kPa, 50 kPa, 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa) using boiled, degassed tap water for the experiment. Four loading levels (50 kPa, 100 kPa, 200 kPa, 300 kPa) are chosen, and under these four load conditions, the permeation pressure is incrementally increased to investigate its impact on the permeability coefficient at different overlying loads. The initial hydraulic gradient (\(\mathrm{i}_{0}\)) is 1.25;
3. Correct operation of experimental apparatus, detailed recording of test results, and in-depth analysis of test results.  
Figure 3: Schematic diagram of dual variable head automatic osmometer.