# Experimental Study on Mechanical Properties of Highway
## 3 Experiment Results and Analysis
### The Effect of Freeze-Thaw Cycles on Mechanical Properties  
Figure 4 shows the stress-strain curves of the thawed roadbed soil after different numbers of freeze-thaw cycles (0, 1, 3, 5, and 7 times) with a water content of 10%, a peripheral pressure of 100 kPa, and a freezing temperature of \(-\)10 \({}^{\circ}\)C. The stress-strain curves of the thawed roadbed soil are shown in Figure 4. From the figure, it can be seen that with the increase in the number of freeze-thaw cycles, the stress-strain relationship curves of the roadbed soil have basically the same morphology, which are hardened curves, and the strength of the roadbed soil gradually decreases with the increase in the number of freeze-thaw cycles. In the early stage of loading, when the axial strain is less than 0.3%, the shape of the curve is nearly a straight line, which indicates that the axial bias stress of the specimen at this time increases sharply and linearly, and the specimen is undergoing elastic deformation. For the strain-hardening type curve, the destructive stress corresponding to 15% of the axial strain is taken at the axial bias stress. The change in the destructive stress and elastic modulus of the subgrade soil under freeze-thaw cycles is shown in Figure 5. From the figure, it can be concluded that the destructive stress of the subgrade soil shows an approximate exponentially decreasing trend with an increase in the number of freeze-thaw cycles. After seven freeze-thaw cycles, the destructive stress of the subgrade soil decreased from 321.7 kPa to 289.9 kPa, representing a total decrease of 9.9%. Among them, the destructive stress decreased the most after the first freeze-thaw cycle, up to 5.5%, indicating that the effect of the first freeze-thaw cycle is the most significant. Overall, the effects of the first three freeze-thaw cycles on the destructive stress were more pronounced, while the effects of the third to seventh freeze-thaw cycles were smaller. Especially after the  
Figure 3: Appearance of soil samples after loading. seventh freeze-thaw cycle, the destructive stress only decreased by 0.3%, which was already essentially stable. According to the characteristics of the stress-strain relationship curve in Figure 4, the ratio of the axial deviatoric stress increment, corresponding to 0.3% axial strain to the axial strain increment, is used as the elastic modulus of subgrade soil, that is:  
\[E=\frac{\Delta\sigma_{0.3\%}}{\Delta\epsilon_{0.3\%}} \tag{11}\]  
As shown in Figure 5, the elastic modulus of the roadbed soil also shows an approximate exponential decrease as the number of freeze-thaw cycles increases, but the decay rate gradually slows down. After seven freeze-thaw cycles, the elastic modulus of the  
Figure 4: Influence of freeze–thaw cycles on stress–strain relations. Figure 5: The relationship curves of failure stress, elastic modulus, and freezing–thawing times. roadbed soil decreased from 0.832 MPa to 0.666 MPa, resulting in a decay of 20.0%. Among them, the first freeze-thaw cycle had the greatest impact on the specimen, with the elastic modulus of the subgrade soil decreasing the most, reaching 8.0%. After the 7 th freeze-thaw cycle, the modulus of elasticity only decreased by 1.9%. Soil strength decreases due to freezing and thawing. The main reason is that the pore volume in the soil increases as water freezes. When the ice in the pores melts, the increased pore space cannot return to its original state, causing the soil to become loose and less dense. This reduction in density weakens the linkage between soil particles. After multiple cycles of freezing and thawing, the pore volume reaches its limit, and the soil strength eventually stabilizes. The data from the triaxial shear test were compiled as a function of \(\varepsilon_{1}/(\sigma_{1}-\sigma_{3})\) and \(\varepsilon_{1}\). It was observed that there was a deviation in the test points in the initial stage. Therefore, some test points with larger deviations were adjusted using the fitness line method.