## 2 Soil-Bentonite Vertical Cut-Off Wall Service Performance and Improvement Measures
### Quality Control Measures for Soil-Bentonite Vertical Cut-Off Walls
Scholars have extensively investigated sodium-based bentonite waterproofing blankets (geosynthetic clay liners, GCL) [95; 96] and compacted clay bentonite [97; 98]. [PERSON] et al. [99] summarized five reasons for the changes in physical and engineering properties caused by the interaction of clay with contaminated liquids: (1) changes in cation concentration and dielectric constant in pore water affect the thickness of the clay particle bilayer; (2) alterations in the surface charge of clay particles modify inter-particle interactions and induce structural changes, as illustrated in Figure 8; (3) mineral solubilization due to strong acidic and alkaline solutions; (4) reactions between minerals and the solution that produce precipitates; and (5) the influence of the viscosity of the contaminated liquid and its polarity, along with other physicochemical properties. However, these conclusions have specific applicability conditions and do not apply universally to all clays. For instance, the double electric layer theory is primarily applicable to montmorillonite clay, such as bentonite, while the mechanism of surface charge changes is mainly relevant to kaolinite clay. The variation patterns of different clays in the same polluted environment may differ or even be opposite. [PERSON] [100] investigated the effect of ZnSO4-contaminated liquid on soil-cement-bentonite \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Degree of Impact** & **Marginal** & **Moderately** & **Big** \\ \hline Rate of change in engineering & & & \\ characteristic indicators (\%) 1  & \textless{}10 & 10–30 & \textgreater{}30 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The extent to which contamination affects the engineering properties of soils [94]. cut-off walls through experimental and instrumental observations, finding that the hydraulic conductivity of the cut-off wall initially decreased and then increased, accompanied by changes in pore structure and hydration products. Regarding the role of chemical compatibility in regulating the hydraulic conductivity, [PERSON] et al. [67] studied the chemical compatibility of soil-bentonite model backfill under composite pollutant conditions. They found that the addition of representative ions, such as Na\({}^{+}\) and Ca\({}^{2+}\), to the backfill resulted in changes in hydraulic conductivity, bound water content, and effective porosity, with a concentration threshold observed for Ca\({}^{2+}\) and chemical oxygen demand (COD). [PERSON] et al. [101] investigated the hydraulic conductivity of polyanionic cellulose (PAC)-modified bentonite (PB) in calcium chloride and found that the modified bentonite exhibited good chemical compatibility by reducing permeability compared to conventional soil-bentonite. [PERSON] et al. [102] modified bentonite with sodium polyacrylate and observed a reduction in the hydraulic conductivity, suggesting that polymer modification is one of the most effective methods to enhance bentonite's resistance to osmotic solutions. [PERSON] [101], [PERSON] [103], and [PERSON] [104] studied the chemical compatibility of modified bentonite with various pollutants, as shown in Figure 9, and found that the hydraulic conductivities of most modified bentonite did not exceed the lowest standard line, although hydraulic conductivities increased with rising ionic concentration. [PERSON] et al. [105] used sodium polyacrylate (PAA-Na) to modify domestically produced sodium-calcium-based bentonite and found that, within the studied polymer dosage range (2% to 10%), both impermeability and chemical compatibility of the bentonite improved with increasing polymer dosage. These studies show that chemical compatibility is vital for preserving the hydraulic conductivity and stability of bentonite-based materials in polluted settings. Polymer modification and proper polymer dosage can enhance bentonite's performance. Figure 8: Schematic diagram of cation exchange in bentonite internal structure [101].