## 2 Soil-Bentonite Vertical Cut-Off Wall Service Performance and Improvement Measures
### Freeze-Thaw Cycles  
Freeze and thaw cycles, as a form of severe weathering, significantly alter the structural properties of soil, affecting its expansion and contraction behavior. Freeze-thaw cycles impact soil bentonite cut-off walls. They cause micro-fractures, alter pore-size distribution, and reduce water-holding capacity in low-suction ranges. Permeability coefficients of soil-bentonite first drop, then may rise, with post-thaw values exceeding pre-freeze ones. Bentonite's free expansion initially rises, then falls, weakening soil-bentonite cut-off wall strength and stiffness. Chemically, freeze-thaw cycles modify bentonite mineral structures, reducing pollutant adsorption and accelerating contaminant breakthrough [114; 115]. This shrinkage occurs leading to a reduction in volume because water is expelled during the freezing process, allowing for closer contact between soil particles, as illustrated in Figure 12.  
\begin{table}
\begin{tabular}{c c c} \hline
**Performance** & **Natural Sodium Bentonite** & **HYPER Clay (HC + 8\%)** \\ \hline Swelling ability & Initially high but significantly decreases after three cycles & Increases with polymer content, remains higher even after multiple-cycles \\ \hline Self-healing ability & Cracks formed during drying do not heal well when rewetted & Stronger crack healing ability due to polymer’s role \\ \hline Hydraulic conductivity & Sharp increase, reaching \(2.93\times 10^{-7}\) m/s by the fourth cycle & Remains low, with only a slight increase to \(9.11\times 10^{-11}\) m/s in the third cycle \\ \hline \end{tabular}
\end{table}
Table 6: Comparison of properties of natural sodium bentonite and modified bentonite (HYPER Clay) after three freeze–thaw cycles.  
Figure 11: Schematic diagram of the microstructure of polymer-modified bentonite before and after modification [115].  
Figure 12: Schematic diagram of the freeze–thaw cycle.  
[PERSON] et al. [116] evaluated the effect of freeze-thaw cycles on the permeability of soil-bentonite mixtures through permeability testing and found that these cycles significantly increased permeability while decreasing impermeability performance. [PERSON] et al. [117] conducted permeability tests on geosynthetic clay liners (GCL) and sand-bentonite mixtures, discovering that one or two winter freeze-thaw cycles had little effect on permeability. [PERSON] et al. [118] studied the impermeability performance of geosynthetic composite bentonite mats (GCL) under long-term freeze-thaw cycles, finding that the permeability of bentonite material initially increased and then decreased with fewer freeze-thaw cycles, while free expansion also increased initially before stabilizing. [PERSON] et al. [119] studied cured siltstone soil with 1%, 3%, and 5% bentonite. A 3% dosage optimized freeze-thaw resistance, mechanical strength, and structural stability. However, a 5% dosage improved initial properties but caused pore blockage and agglomeration, reducing long-term durability. The wet expansion and dry contraction partially offset the effects of freezing expansion and thawing contraction, thereby weakening the overall impact of the freeze-thaw cycle. [PERSON] et al. [120] studied the influence of different environmental parameters on the expansion of bentonite in GCL materials after freeze-thaw cycles, as shown in Figure 13, and found that factors such as temperature, salt concentration, and pH significantly affect the particle size distribution of bentonite, which in turn influences the expansion index and permeability characteristics. However, the effects of dry, wet, and freeze-thaw cycles on the impermeability properties of vertical cut-off wall materials in heavy metal environments have not been sufficiently studied.