## 2 Soil-Bentonite Vertical Cut-Off Wall Service Performance and Improvement Measures
### Quality Control Measures for Soil-Bentonite Vertical Cut-Off Walls  
The performance of vertical cut-off walls necessitates careful consideration of pollutant propagation pathways, the required depth of the cut-off wall, structural integrity, environmental compatibility, and factors influencing pollutant migration. Collectively, these elements affect the ability of a vertical cut-off wall to effectively isolate pollutants and manage risks under site-specific conditions [86; 87]. A flowchart synthesizing existing quality control measures for the cut-off wall has been developed and is presented in Figure 7.  
Before designing the vertical cut-off wall, a comprehensive site investigation is conducted to accurately assess the characteristics of the contaminated medium, the degree of contamination, and its geographic distribution, which aids in establishing a suitable vertical cut-off wall model [88; 89]. The nature of pollutants significantly impacts vertical cut-off wall design. For heavy metals, materials with high adsorption and chemical stability, such as bentonite clay mixed with additives like lime and phosphate, are chosen to form insoluble compounds and reduce metal mobility [77; 78]. For hydrocarbons, high-density polyethylene (HDPE) geomembranes, which have low permeability and good chemical stability, are effective. Modified bentonite or composite materials can also enhance hydrocarbon adsorption and barrier properties [57]. The design process requires precise specifications for wall dimensions and material standards, the establishment of a comprehensive treatment system, construction  
Figure 6: Optimal ratio of multiple additives for SB cut-off walls [78; 84; 85; 86]. Figure 7: Roadmap for quality control of the vertical cut-off walls. guidance, and strict monitoring and maintenance protocols [90; 91]. Additionally, the strength, durability, corrosion resistance, and chemical stability of the wall must be considered in detail. During construction, rigorous quality control and assurance measures should be implemented to ensure precision at each operational step [81]. Regarding construction quality control methods for vertical cut-off walls, previous specifications should be referenced, and relevant experience should be utilized [92; 93]. Effect of Chemical Compatibility on the Service Performance of Soil-Bentonite Vertical Cut-Off Walls  
In the field of environmental geotechnical engineering, chemical compatibility refers to the ability of various types of engineering cut-off wall materials to withstand the adverse effects of pollution on their engineering properties [68]. The rate of change in engineering property indices, which evaluates the degree of influence of pollution on the engineering properties of soil, is presented in Table 5[94]. The strength, hydraulic conductivity, and other properties of the material may vary before and after exposure to pollutants. If the material's properties exhibit minimal change following pollution exposure and continue to meet engineering requirements, this indicates good chemical compatibility. The chemical compatibility of bentonite is a critical performance indicator for its application as a vertical cut-off wall material in environmental engineering, particularly in contaminated site remediation and risk management. The advantages and disadvantages of chemical compatibility directly influence the effectiveness and durability of bentonite as an impermeable cut-off wall. Current research focuses on the factors affecting chemical compatibility, the regulation of hydraulic conductivities, optimization strategies, the impact on the long-term stability of the material, and the latest developments in the field. These advances not only deepen the understanding of bentonite's chemical compatibility but also provide a scientific basis for enhancing its performance in environmental engineering applications. The study of bentonite's chemical compatibility primarily examines the effects of its interaction with contaminated fluids on compressibility and permeability.