# Shear Resistance Evolution of Geogrid Reinforced Expansive Soil Under Freeze-Thaw Cycles
## 3 Results and Discussion
### Stress-Strain Behavior  
Figure 5a-f indicates the stress-strain curves of expansive soil specimens subjected to under confining pressures (100 kPa, 200 kPa, 300 kPa), F-T cycles (0, 1, 3, 6, 10), and geogrid layers (0, 1, 3, 5). As shown in Figure 5a-f:  
Figure 4: (**a**) Reinforcement mechanism; (**b**) diagram of specimen reinforcement effect; (**c**) shear deformation of reinforced soil with different layers of geogrids.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test** & **Geogrid** & **Confining** & **Loading** \\
**Condition** & **Layers** & **Pressure/kPa** & **Rate/mm\_min\({}^{-1}\)** \\ \hline CU & 0/1/3/6/10 & 0/1/3/5 & 100/200/300 & 0.2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Triaxial test program.  
1. The effect of F-T cycles: F-T cycles destroyed the chain between soil particles, changed the internal structure of the soil, caused an increase in the number and size of pores, small pores linked to form large pores, and reduced the strength of the soil body [40; 41]. The strength of the specimens decreased with the increase in the number of freeze-thaw cycles, which was more obvious in unreinforced specimens than in reinforced specimens. Friction between the expansive soil and the geogrid and the geogrid's embedded locking effect indirectly provided additional pressure, limiting the increase and enlargement of pores. However, the decrease was not obvious after six F-T cycles, and the internal structure of the soil tended to stabilize and reach dynamic equilibrium. Previous studies have shown that the physical and mechanical properties tend to be stabilized after 5-7 F-T cycles [35; 36], which is basically consistent with the present study.
2. The effect of geogrid layers: With the increase in geogrid layers, the stress-strain curve changed from strain-softening to strain-hardening type. Geogrids create a restraining effect through a three-dimensional reinforcing network. Interfacial friction and me chanical occlusion between geogrid and soils redistribute the stress field, inhibit the localized development of the shear zone, and promote the transformation of the soil body from brittle damage to progressive plastic deformation. Through the synergistic reinforcing effect of multi-layer geogrids, the overall stiffness and ductility of the soil body can be significantly improved, and the freezing-expansion-contraction cyclic damage of expansive soils can be suppressed. Additionally, it was more obvious at low confining pressure, which indicates that the reinforcing effect was better at low confining pressure. The reason for this trend is that the lateral deformation of the specimen is greater at low perimeter pressures than at high perimeter pressures, the relative displacement between the tendons and soils is significant, and the geogrid restrains the soil particles more obviously.
3. The effect of confining pressure: When the confining pressure is 100 kPa, the stress-strain curve of the expansive soil specimens exhibited strain-softening. When the confining pressure is 200 kPa, the stress-strain curve of the expansive soil specimens exhibits strain hardening. At high confining pressures, soil particles reorganize, cracks close, and soil strength is increased [42].
4. Compared to previous studies: The correlation between elastic modulus and freeze-thaw cycle frequency in untreated expansive soil is quantitatively characterized in Figure 5f, compared to [PERSON] et al. [9] and [PERSON] et al. [39]. The elastic modulus is normalized for easy comparison. Figure 5f demonstrates strong congruence between the current experimental data and established findings from prior investigations, thereby validating the methodological reliability of this study.