## 2 Soil-Bentonite Vertical Cut-Off Wall Service Performance and Improvement Measures
### Main Factors Affecting the Long-Term Service Performance of Vertical Walls
#### 2.3.2 Long-Term Service Performance Influencing Factors  
The service life of soil-bentonite (SB) vertical cut-off walls is governed by both the intrinsic properties of the wall materials and external hydraulic head differences. For in situ SB walls, six critical factors influence their long-term performance:  
1. **Bentonite content**: Bentonite content impacts conductivity and effectiveness. Higher content improves impermeability and longevity, but usually \(\leq\)15%. Excessive amounts can raise porosity and cause particle grouping, potentially worsening perfor  
Figure 3: Contaminant transport model diagram [58].  
mance. Ions like Na\({}^{+}\) and Ca\({}^{2+}\) affect bentonite performance, but too much bentonite can hinder ion movement, impacting permeability. The ideal dosage must be found through experiments to balance cost and effectiveness. Bentonite content needs to exceed 2% of conventional levels to counter degradation [65; 66].
2. **Chemical compatibility**: Beyond material composition, chemical compatibility with contaminants is pivotal. High concentrations of Na\({}^{+}\) and Ca\({}^{2+}\) in landfill leachate compromise SB mixtures, increasing hydraulic conductivity and accelerating hazardous contaminant penetration. Na\({}^{+}\) and Ca\({}^{2+}\) ions enhance soil-bentonite hydraulic conductivity by exchanging with bentonite's cations, reducing particle charge and increasing porosity. Ca\({}^{2+}\) has a bigger impact due to its larger hydration radius. Other ions like K\({}^{+}\) and Mg\({}^{2+}\) can also influence conductivity depending on their properties and bentonite's characteristics. These ions mainly affect hydraulic conductivity by altering bentonite's electrical properties and pore structure. For example, chemical incompatibility can reduce a wall's service life from indefinite to just 75 years under contaminant exposure [67; 68].
3. **Environmental impact**: Environmental stressors further complicate performance. Temperature fluctuations, pH extremes, wet-dry cycles, and freeze-thaw cycles alter SB's physicochemical properties, degrading long-term functionality. Low temperatures inhibit chemical reactions but enhance PSB's Pb\({}^{2+}\) adsorption. High temperatures accelerate reactions, yet risk cracking. Acidic conditions reduce heavy metal adsorption and alter structure via mineral dissolution. Alkaline conditions boost adsorption but may modify bentonite's structure, impacting performance. Overall, these factors significantly influence soil-bentonite mixture stability and function [69; 70; 71; 72].
4. **Head difference**: The driving force behind pollutant transport--hydraulic head difference--directly impacts service life. Elevated head differences accelerate contaminant migration, necessitating strict control in field applications to maintain low differentials [73].
5. **Degree of solidification**: During consolidation, stress redistribution (e.g., lateral friction, consolidation loads, pore pressure) creates permeability heterogeneity, undermining the wall's pollutant retention capacity [74].
6. **Construction quality**: Despite the simplicity of SB wall construction, rigorous quality control is essential. Pre-hydration improves chemical resistance, while adequate curing ensures material uniformity. Precise slurry ratios and skilled personnel are critical to maintaining structural stability [75].  
Collectively, optimizing material properties (e.g., additive ratios, bentonite content) and enforcing stringent construction protocols are imperative to ensure SB vertical cut-off walls achieve reliable seepage control and long-term durability.