## 3 Study of the Physical and Mechanical Parameters of Backfill Soil
### Indoor Triaxial Test
#### 3.2.2 Analysis of the Experimental Results  
As shown in Figure 7, it was found that the moisture content has a significant impact on the stress-strain curve due to the change of confining pressure, which mainly manifests in the peak value and the morphological features of the curve. Under the same confining pressure, the stress-strain curve of different moisture contents will accompany an increase in axial strain; the deviatoric stress value will rise to its peak first and then gradually decrease, resulting in a flatter curve with continued axial strain. Additionally, differences exist between the curves of varying moisture contents; the overall height decreases as moisture content increases, and peak deviatoric stress diminishes accordingly.  
Figure 6: Three axis sample and three axis test.  
Figure 7: Stress and strain curves under different water content and surrounding pressure.  
This phenomenon indicates that as moisture content rises, maximum deviatoric stress withstandable by specimens during shearing decreases while failure mode shifts from 'brittle' to 'plastic'. Considering also confining pressure's role on the stress-strain curves, we found the following: at 50 kPa confining pressure, the morphology changes from'steep' to 'gentle', with increasing moisture content showing relatively large differences; at 300 kPa confining pressure, morphological differences are smaller and curves tend towards gentleness. Thus, it is evident that the influence of moisture content on the stress-strain curves is constrained by confining pressure.  
According to the standard \"Test Methods of Soils for Highway Engineering (Number: JTG 3430-2020)\" [14] unconsolidated undrained shear tests, the data corresponding to the internal friction angle and cohesion at four different water content levels of 18%, 19%, 20%, and 21% are shown in Figure 8. Under the influence of water content, both the internal friction angle and cohesion exhibit a decreasing trend as the water content increases. When the water content increases from 17% to 19%, the internal friction angle decreases from 36.05\({}^{\circ}\) to 32.88\({}^{\circ}\), representing a decrease of 9%. The cohesion also decreases from 89.66 kPa.  
The results indicate that cohesion is more significantly affected by water content than the internal friction angle. This is attributed to an increase in free water within unsaturated clay with increasing water content, leading to a reduction in the shear strength of the soil mass. According to the parameter values of the saturation state of other soil masses in the on-site investigation report and the results of the comprehensive test, it was found that when the moisture content of the sample soil mass is 21%, the soil mass has reached the saturation state.  
The shear strength values corresponding to different confining pressures of 50 kPa, 100 kPa, 200 kPa, and 300 kPa are depicted in Figure 9. With varying confining pressures, the soil's shear strength diminishes as water content increases. Higher confining pressure leads to greater shear strength values and accentuates the declining trend. At low water content levels, non-saturated soil particles possess a thin surface film of bound water that promotes adhesion and cohesion, resulting in robust shear resistance. As the water content rises, the bound water film thickens and the free water content increases, leading to reduced friction and cohesion between soil particles and consequently decreasing shear strength. According to the unconsolidated, undrained shear test data and the field survey results, the shear strength index of the high fill slope soil sample is 35 kPa, and the internal friction angle is 19\({}^{\circ}\).  
Figure 8: Internal friction angles and cohesion at different water contents.