# Experimental Study on Mechanical Properties of Highway
## 3 Experiment Results and Analysis
### The Influence of Water Content on Mechanical Properties  
Figure 12 shows the stress-strain curves of the thawed subgrade soil under different water contents (6%, 8%, 10%, and 12%) after one freeze-thaw cycle, with a peripheral pressure of 100 kPa and a freezing temperature of \(-10\)\({}^{\circ}\)C. It can be seen from the figure that, as the water content increases, the curve gradually changes from a softening type to a hardening type, and the strength of the roadbed soil decreases significantly with increasing water content. When the water content is 6%, the stress-strain curve of the subgrade soil exhibits the characteristics of a softening curve, which indicates brittle damage. When the water content reaches 8% and above, the stress-strain curve shows the characteristics of a hardening curve.  
The influence of water content on the destructive stress of the roadbed soil is shown in Figure 13. As can be seen from the Figure, the destructive stress of the roadbed soil decreases with an increase in water content from 405.43 kPa to 288.4 kPa, which is 28.9% lower. The most significant decrease in destructive stress occurs when the water content increases from 6% to 8%, with a decrease of 19.1%. When the water content increases from 8% to 12%, the destructive stress decreases approximately linearly, with a decrease of 2.4% for every 1% increase in water content. The modulus of elasticity of the subgrade soil also decreases approximately linearly with increasing water content. The modulus of elasticity decreases by an average of 8.4% for every 1% increase in water content. The attenuation of the modulus of elasticity reaches 50.4% when the water content increases from 6% to 12%. Based on the analysis of the test results, it can be concluded that water content has a strong influence on the destructive stress and elastic modulus of the subgrade soil.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Freezing Temperature (\({}^{\circ}\)C)** & \(R_{\text{f}}\) \\ \hline \(-5\) & 0.90803 \\ \(-10\) & 0.89539 \\ \(-15\) & 0.90869 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Damage ratio of soil under different loading rate.  
Figure 11: Ultimate deviation stress—loading rate relation diagram.  
Calculating 1/a and 1/b, the initial tangent modulus and ultimate deviatoric stress versus moisture content were obtained, as shown in Figure 15. As can be seen from the Figure, the water content increases from 6 to 8 percent, the parameter 1/a decreases by 11.9 percent, and the parameter 1/b decreases by 24.9 percent. The increase in water content from 8 to 10 percent decreased parameter 1/a by 6.1 percent and parameter 1/b by 5.4 percent. The increase in water content from 10 to 12 percent decreased parameter 1/a by 8.5 percent and parameter 1/b by 4.7 percent. The above results show that the initial tangent modulus of the soil gradually decreases with the increase in water content, and its rate of decrease also gradually decreases. Additionally, the ultimate deviatoric stress of the soil gradually decreases, and its rate of decrease slows down gradually.  
The damage ratio \(\mathrm{R_{f}}\) of the soil at different water contents is calculated, as shown in Table 9. From the Table, it can be seen that with the increase in water content, the damage ratio \(\mathrm{R_{f}}\) of the soil gradually increases, and the growth rate is relatively slow. The soil damage ratio \(\mathrm{R_{f}}\) increases by 9.3% when the water content increases from 6% to 12%.  
Figure 14: Curves of \(\epsilon_{1}/(\sigma_{1}-\sigma_{3})-\epsilon_{1}\) of different water content.  
Figure 15: Ultimate deviation stress—water content relation diagram.  
This indicates that the increase in water content brings the strength of the soil closer to its strength limit, and the utilization of strength increases.