# Shear Strength Analysis and Slope Stability Study of Straight Root Herbaceous Root Soil Composite
## 2 Methods and Materials
### Experimental Design  
_Tagetes eercta_, an herbaceous plant with a shallow root system, was selected to determine the shear strength indexes of undisturbed soil with a root system (USRS) and rootless undisturbed soil (RUS). USRS samples were acquired at distinct depths (20, 40, and 60 cm) while preserving their natural structure and moisture content (16.7%, 23% and 28.6%, respectively). Each depth yielded 20 soil samples. Undrained triaxial shear tests were performed under three levels of confining pressure (50, 100, and 200 kPa) using the consolidated undrained method. The testing of the sample adopted the suction saturation method. The saturator containing the sample was placed in an anhydrous extraction cylinder for suction. When the vacuum approached 1 local atmospheric pressure, suction was maintained for at least 0.5 h, and water was slowly injected to maintain a stable vacuum. Pumping was stopped when the saturator was completely submerged and the vacuum  
Figure 3: Sample preparation and plant samples: (**a**) Preparation of soil samples containing roots (USRS); (**b**) _Tagetes eercta_ sample.  
was subsequently released from the cylinder. The sample was left standing underwater for over 10 h and then taken out and weighed.  
The water surface of the measuring tube was placed at the center height of the sample, the measuring tube valve was opened, and the sensor was read. When applying confining pressure, the body transformer sensor or body transformer pipe and pore pressure valves were closed, and the surrounding pressure valves were opened. The confining pressure was applied at 50, 100, and 200 kPa. The pore water pressure valve was then opened in order to measure the pore water pressure. Following this, the drain valve was opened to allow for drainage process measurements, whereby the drainage and pore pressure gauge readings were recorded until the pore water pressure dissipated by more than 95%. After consolidation was completed, the drain valve was closed, and the drainpipe and pore water pressure readings were recorded. The cutting rate of the sample was 0.08 mm/min. After completion, the motor was turned off, the lifting platform was lowered, and the air vent was opened. The water in the pressure chamber was drained, and the pressure chamber cover and rubber film outside the sample were removed to weigh the sample and measure the moisture content.  
The selected _Tagetes erecta_ variant had relatively long primary roots, with lengths within 10-20 cm, while the lateral roots were shorter, generally 8-10 cm. The mass percentage method was used to determine the root content of each sample, namely the ratio of root mass to soil mass. Table 2 reports the clayy silt sample parameters. The non-uniformity coefficient of the soil was 2.88, and the curvature coefficient was 1.58, indicating that the soil particles were relatively uniform. The maximum dry density was 1.58 g/cm\({}^{3}\), the specific gravity of the soil particles was 2.72, the optimal moisture content was 16.7%, the plastic limit was 18.8%, and the liquid limit was 44.8%. Four representative samples were used per group in the experiment. Each root content type retained three similar root content data for subsequent correlation analysis. Upon completing all tests, the triaxial shear results of the undisturbed root-containing soil were compared and analyzed under equivalent confining pressures and root content conditions. For the shear test, the axial force (constant axial strain rate) was gradually increased, and the confining pressure was maintained to shear the specimen until it failed (failure typically occurs within 20% of the axial strain). If the strain-stress curve exhibited a peak, the failure deviation stress \((\sigma_{1}-\sigma_{3})_{f}\) was denoted as the deviation stress corresponding to the peak. However, if the testing curve exhibited continuous hardening without a peak, the deviatoric stress corresponding to 15% of the axial strain value was denoted as \((\sigma_{1}-\sigma_{3})_{f}\).