# Shear Resistance Evolution of Geogrid Reinforced Expansive Soil Under Freeze-Thaw Cycles
## 3 Results and Discussion
### Shear Strength  
The shear strength determination protocol followed these criteria: (a) For specimens exhibiting strain-softening behavior in their stress-strain response, the peak deviatoric stress was designated as the shear strength parameter; (b) In cases of strain-hardening characteristics, the maximum deviatoric stress recorded at 15% axial strain was adopted as the representative shear strength value. Figure 6 illustrates the normalized shear strength evolution under F-T cycles, where the dimensionless parameter represents the strength ratio between F-T-exposed specimens and their untreated counterparts sharing identical geosynthetic reinforcement configurations. As shown in Figure 6: (1) The normalized shear strength exhibited a decay trend with F-T cycles, the decrease was obvious in 1-3 F-T cycles, and smaller and stabilized in 6-10 F-T cycles, which indicated that the internal structural reorganization of the specimen reached a dynamic equilibrium after 6 F-T cycles. These are in good agreement with the findings of [35; 36]. (2) The higher the confining pressure, the more obvious the effect of reinforcement. Take six F-T cycle and three-layer geogrid reinforced specimens as an example, with 100 kPa confining pressure, the shear strength of the three-layer geogrid reinforced specimen decreased by 15.1%, with 300 kPa confining pressure, it decreased by 10%. (3) Compared with the plain specimens, the geogrid-reinforced specimens were more stable under the influence of F-T cycles. When the confining pressure was 200 kPa, the shear strength of plain specimens decreased linearly with the number of F-T cycles, while the shear strength of geogrid-reinforced specimens slowed down after three F-T cycles and increased slightly after six freeze-thaw cycles. Figure 7 shows the normalized shear strength (\(q_{uN}/q_{u0}\)) in the present paper compared to the previous studies [9; 34; 39; 43]. \(q_{uN}\) is the shear strength after F-T cycles. \(q_{u0}\) is the shear strength before F-T cycles. It gives the variation of normalized shear strength of reinforced soil for different reinforcing materials under the F-T cycles, including geogrids, geotextiles, glass fibers, and basalt fibers. Overall, the normalized shear strength of the reinforced soils decreased with the increasing F-T cycles, but there were significant differences in the magnitude of the decrease. The geogrid-reinforced soil showed a stable behavior with the smallest decrease, a larger decrease after the one F-T cycle, a significant slowdown after the three and six F-T cycles, and a slight upward trend after the ten F-T cycles. Geotextile also shows a better reinforcing effect; it has many small holes; the tendon soil contact area is large and resulting in high friction. In addition, small holes are more obvious to embed a locking effect in the fine-grained soil. Our future works will continue to investigate the reinforced effect of grid aperture (bi-directional geogrids) and cross rib spacing (unidirectional geogrids) under F-T conditions. The shear strength of the geogrid-reinforced specimen was reduced by 14.6% after the six F-T cycles and 15.5% after the ten F-T cycles. However, the normalized shear strength of fiber-reinforced soils decreased significantly with increasing number of F-T cycles. Also, the shear strength of the glass-fiber-reinforced specimen was reduced by 30.75% after the five F-T cycles; this decreasing trend was 33.8% for the basalt-fiber-reinforced specimen, 46.9%, and 53.8% after the 10 F-T cycles. It is obvious that the reinforcement effect of geogrids or geotextiles is significantly better than that of fibers (glass fibers, basalt fibers) in F-T conditions. They have different mechanisms of reinforcement influence: for geogrids or geotextiles, friction and embedded locking effect indirectly provide additional pressure, and limit shear deformation to achieve the reinforcement effect; for fibers, mainly due to friction and local space constraints [44].