Evaluation of the Service Performance of Soil–Bentonite Vertical Cut-Off Walls at Heavy Metal Contaminated Sites: A Review

Abstract

Soil–bentonite (SB) vertical cut-off walls are widely utilized to mitigate the transport of soil contaminants in groundwater. Evaluating their long-term service performance is crucial for ensuring environmental safety and effective pollution control. The evaluation model for the long-term service performance of contaminant cut-off walls considers key processes such as convection, diffusion, dispersion, and adsorption. These processes are closely linked to the physicochemical properties of the cut-off walls, which are influenced by the surrounding complex environment, ultimately impacting their long-term performance. This study delves into the long-term service performance of SB vertical cut-off walls. It focuses on the key factors that influence this performance and the measures that can enhance it. Moreover, it offers a detailed analysis of how the performance of seepage cut-off walls in soil–bentonite materials evolves under various environmental influences. These influences include chemical exposure, freeze–thaw cycles, and dry–wet cycles. Additionally, it outlines existing service performance evaluation methods and identifies their shortcomings. By leveraging the advantages of in situ testing methods, this paper proposes the establishment of a comprehensive evaluation system for the service performance of vertical cut-off walls based on in situ test parameters. The proposed evaluation system aims to provide a scientific assessment of the long-term service performance of SB vertical cut-off walls.

1. Introduction

Land resources are fundamental to human survival. Soil contamination represents a critical global environmental challenge. The 2014 National Soil Condition Survey Bulletin indicates that the overall soil environmental situation in China is concerning, with approximately 16.1% of soil classified as contaminated. Of this, heavy metal contamination accounts for about 3%, and the levels of heavy metal contamination have significantly exceeded acceptable standards [1]. According to a 2020 United Nations Food and Agriculture Organization (FAO) report, over 33% of the world’s soils are degraded by pollution, with heavy metals implicated in nearly 10% of agricultural land degradation [2]. In Europe, the European Environment Agency (EEA) highlights heavy metal contamination as a pressing concern, affecting 24% of industrial zones—particularly in post-mining regions of Poland and the Czech Republic [3]. Similarly, the U.S. Environmental Protection Agency (EPA) identifies heavy metals in 30% of Superfund sites, demanding durable containment strategies [4].

Heavy metal contaminated sites are defined as locations where the concentration of trace metal elements in the soil exceeds background levels due to human activities. These heavy metals, which include mercury, lead, cadmium, and arsenic [5], pose a risk of leaching into groundwater and farmland, thereby jeopardizing public health [6]. Given the challenges associated with the short-term elimination of contamination through direct treatment, cut-off and obstruction have become the most commonly employed remedial strategies [7].

The vertical cut-off wall, depicted in Figure 1 [8], is an effective anti-seepage and blockage technology widely utilized in developed countries, particularly in Europe and the United States, for the cut-off and containment of polluted sites. This technology is employed to block the diffusion of contaminated groundwater and to enclose leachate from landfills and mine tailings [9]. Among various technologies, vertical soil bentonite (SB) cut-off walls are often preferred for pollution prevention and control. They have low permeability, good chemical compatibility, and cost-effectiveness.

Table 1. Relative advantages and limitations of soil–bentonite walls.

Type	Cement–Bentonite Wall [10]	Slurry Trenche [11]
Relative advantages of soil–bentonite cut-off walls	Soil–bentonite walls have good plasticity and can handle settlement or deformation. They can also expand to fill minor cracks when wet.	Soil–bentonite walls are more impermeable than slurry trenches, with less waste and lower environmental impact during construction.
Relative limitations of soil–bentonite cut-off walls	Soil–bentonite walls are low-strength. They may deform or be damaged in complex geology or long-term water flow. In acidic or corrosive settings, they corrode easily, reducing seepage control.	Soil–bentonite wall construction is complex and slow, involving mixing and laying. It also performs poorly in complex geology.

presents the relative advantages or limitations of SB cut-off walls compared to other similar containment technologies like cement–bentonite walls and slurry trenches.

Pollutant cut-off walls must function effectively over the long term to block contaminants, making the study of their service performance essential for ensuring environmental safety and effective pollution control. Typically, breakdown time serves as a reference index for measuring the service life of cut-off walls [12]. The long-term service performance evaluation model, or seepage cut-off wall evaluation model, primarily considers key processes such as convection, diffusion, dispersion, and adsorption, as illustrated in Figure 1 [8]. The long-term performance of soil–bentonite cut-off walls hinges on parameters like permeability, microstructure, and chemical stability. To optimize impermeability, it is crucial to balance bentonite content, particle packing, and compaction. Challenges such as cations, pH changes, and contaminants can impact performance. Ensuring durability demands a combination of material engineering, sound construction practices, and site-specific assessments.

The researchers employed “soil–bentonite vertical cut-off walls” and “heavy metal contaminated sites” as key search terms, resulting in the identification of 154 relevant studies published through 2025. These articles were systematically retrieved from two authoritative databases: the Web of Science (WOS) and the China National Knowledge Infrastructure (CNKI). Through a rigorous manual screening process, each publication underwent critical evaluation based on its thematic relevance and scientific contribution to the field of soil–bentonite vertical cut-off wall performance in heavy metal-contaminated environments. This methodological approach ensured comprehensive coverage of current research advancements while maintaining rigorous selection criteria. The systematic analysis not only elucidates emerging trends in this domain but also provides critical insights into the functional interplay between soil–bentonite vertical cut-off wall systems and heavy metal contamination mitigation strategies.

Table 2. Comparison of the innovativeness of this study with the recent related literature.

Comparison Dimension	Characteristics of Existing Studies	Innovative Points of This Article	Contributions and Strengths
Environmental factors coverage	Focus more on single pollutants or short-term environmental impacts [13]	Long-term performance decay under chemical, dry–wet and freeze–thaw cycles is systematically studied	Provide lifecycle prediction models in complex environments to fill the long-term dynamic evaluation gap
Material improvement measures	Optimized mainly for traditional bentonite or single modified materials [14]	Comprehensive evaluation of a wide range of new materials	Propose material selection and proportioning guidelines for efficient cut-off wall design in heavy metal-polluted sites
In situ assessment techniques	Reliance on single-principle evaluation methods, not systematic [15]	ERT, CPTU, and dynamic stress modeling are integrated for multi-parameter detection	Improve leakage location accuracy and support in situ dynamic monitoring
Dynamic performance modeling	Prediction of service life based on static parameters or empirical formulas [16]	Combining multiple models to predict long-term performance	Transcend traditional model limitations, reduce prediction errors, and apply to complex pollution scenarios
Integrated pollution adaptation	The effects of combined heavy metal pollution and dynamic concentration shifts are often overlooked [17]	Analyze ion threshold effects and material compatibility for multi-heavy metal pollution	Offer theoretical basis for cut-off wall design in high-pollutant sites

clearly presents the breakthroughs of this study in environmental complexity, material diversity, technological innovation and practical applicability through multi-dimensional comparisons, highlighting its academic value and engineering significance in the field of contaminated site management.

Of the articles listed, 63 are from the period 2020 to 2025 and reflect the latest research on the performance of soil–bentonite vertical cut-off walls in heavy metal-contaminated environments. This ensures that the findings are based on the most recent methodological, technological, and theoretical frameworks. This timeframe captures the latest trends, including new materials, methods, and evaluation systems, thereby increasing the relevance and applicability of the research. Additionally, by analyzing recent literature, this study identifies new challenges and opportunities that arise in the assessment of soil–bentonite vertical cut-off walls. This ensures that the conclusions drawn in the research are consistent with the current state of knowledge and practice in the field. This approach not only strengthens the validity of the research but also positions it to make a meaningful contribution to ongoing discussions and future advancements in the discipline.

This study investigates the long-term service performance of soil–bentonite (SB) vertical cut-off walls, systematically summarizing key influencing factors and potential improvement strategies while presenting recent advancements in research on their impermeability and anti-clogging characteristics under diverse environmental conditions, including chemical exposure, freeze–thaw cycles, and wet–dry cycles. The paper critically examines current service evaluation theories for vertical containment cut-off walls and compiles existing in situ evaluation methodologies, aiming to establish a comprehensive reference framework for assessing SB vertical cut-off wall performance. Section 2 provides an analytical discussion on the service behavior of soil–bentonite vertical barriers and corresponding enhancement measures. Section 3 presents a detailed analysis of chemical compatibility effects on the operational performance of SB cutoff walls. Section 4 quantitatively explores the permeability degradation mechanisms in soil–bentonite vertical containment systems under environmental stressors. Section 5 evaluates state-of-the-art in situ assessment techniques for SB vertical cut-off walls. Section 6 describes potential future technological developments in soil–bentonite vertical cut-off wall monitoring. The concluding section synthesizes research findings and proposes future research directions. The flowchart is shown in Figure 2. This systematic investigation contributes to enhanced understanding of performance evolution in vertical cut-off wall systems and provides methodological support for their lifecycle management.

2. Soil–Bentonite Vertical Cut-Off Wall Service Performance and Improvement Measures

2.1. SB Vertical Separation Wall Construction Methods and Technical Characteristics

Vertical cut-off walls primarily utilize the low permeability of the wall material to impede the horizontal transport and diffusion of pollutants from contaminated sites [18,19]. Soil–bentonite (SB) vertical cut-off walls can be categorized into three types based on construction methods: slurry walls, grouting walls, and deep soil mixing [20]. The common construction methods and technical characteristics of soil–bentonite (SB) vertical cut-off walls are summarized in

Table 3. SB vertical separation wall construction methods and technical characteristics [21].

Type	Slurry Walls	Grouting Walls	Deep Soil Mixing
Depth of construction	Less than or equal to 60 m [22,23]	Depth 45–60 m, influenced by soil type [24,25]	Larger than 15 m [26,27]
Construction methods	Excavate and backfill the slurry mixing area [28,29]	Jet grouting; cementitious grouting; chemical grouting, etc. [30,31]	Mechanical in situ mixing of water, soil and cement or other mineral additives [32]
Construction defects	Improper construction or substandard slurry ratios may create high permeability zones [33]	Differences in ground permeability affect the grouting effect, which in turn affects consolidation and waterproofing [34]	Damage to the soil, construction easily deflected [35]
Major limitations	The main limitation is in the materials, which are relatively costly [36]	Unsuitable for substrates with high water content, ground disturbance due to grouting pressure and uplifting [37,38]	Strength may be adversely affected by factors such as organic matter, high soil moisture content and pH [39,40]
Technical Benefits	Proven construction techniques, early research, effective cut-off of contaminants [41]	Grouting slurry is injected into the soil and rock in a special way to maintain cut-off [42]	Enhanced soil strength and stiffness, reduced settlement, low cost, high efficiency, thin and deep walls, structural flexibility [43,44]

[21].

2.2. Case Study of Soil–Bentonite Vertical Cut-Off Walls

Soil bentonite-based cut-off walls have been widely used in practical engineering applications for controlling contaminant migration and preventing groundwater contamination due to their good impermeability properties and relatively low construction costs. This literature reviews a number of studies and practical cases to demonstrate the performance and application effects of different types of bentonite-based cut-off walls under complex environmental conditions, including their long-term performance and dynamic behavior under different soil layers and varying pollutant concentrations, which provide technical references and directions for improvement for future application in heavy metal contaminated sites and other complex environments. Relevant cases are shown in

Table 4. Practical case studies.

Case Studies	Practical Application Performance
Karkheh Dam, south-west Iran [45]	The impermeable walls reduced total seepage and permeability by 25% at the right dam shoulder.
Landfills in the Tropics [46]	Tropical soils amended with bentonite exhibit reduced hydraulic conductivity, suitable for waste disposal facility liners.
Taihu Meiliangwan Whitetail Storage Yard [47]	Bentonite reduces permeability; higher pressure minimizes permeability changes with bentonite. Dredged material meets landfill requirements; 1.5–3% bentonite aids long-term self-healing.
Changzhou City Sanitary Landfill [48]	PBFC slurry cut-off wall has high load-bearing capacity and ductility, matching landfill conditions.
Landfill in the Lagouat region, Algeria [49]	Waste leachate cuts bentonite mixtures’ compressibility and permeability but raises their strength.

.

2.3. Main Factors Affecting the Long-Term Service Performance of Vertical Walls

The service life of vertical cut-off walls is influenced by various factors, including material selection, structural design, construction quality, environmental conditions, and maintenance management. Breakdown time is commonly used as a reference index for assessing the service life of these walls in both domestic and international standards. This study summarizes the research progress related to service life evaluation methods and identifies the key factors affecting the long-term performance of vertical cut-off walls. The findings aim to provide valuable insights for the design and construction of soil–bentonite (SB) vertical cut-off walls.

2.3.1. Service Time Evaluation Methodology

When a pollutant penetrates the cut-off walls, causing the outer concentration to reach a specified threshold, the cut-off walls are considered penetrated, and the corresponding duration is referred to as the penetration time [17]. Rowe [50] suggested that the service life is influenced by the physical and chemical aging of the material. Xu et al. [51] emphasized that the assessment of service life should consider three distinct zones of the cut-off walls: the erosion zone, the central effective zone, and the outer stretching zone. Additionally, Yu et al. [52] identified deformation, stress, and damage distribution as critical determinants of service life, based on simulations employing a coupled finite element method (FEM)-Peridynamics (PD). Sun et al. [53] further confirmed these findings, noting that the primary factors affecting service life include the deformation modulus of the soil matrix and the creep effect, as determined through a combination of FEM, PD, and centrifuge model testing.

The transport of pollutants must account for diffusion, adsorption, and degradation processes, leading to extensive research on analytical models. Zhang [54] and Gan et al. [55] developed a coupled analytical model addressing seepage and dissolution. Ding et al. [56] created a transient two-dimensional transport model that incorporates advection, dispersion, adsorption, and biodegradation within a multilayered system, overcoming the limitations of traditional wall models that primarily address non-point source pollution at constant concentrations. Furthermore, Peng et al. [57] proposed a two-dimensional analytical model that considers transient diffusion, adsorption, and degradation processes of organic pollutants within a composite geomembrane containment wall (CGCW) and a continuous aquifer. For evaluating the service life of composite cut-off walls consisting of a geomembrane and a soil–bentonite mixture for organic pollutants, Lin et al. [58] developed a geometric model (Figure 3). In this model, R_d is the delay factor of the soil–bentonite cut-off wall, C₁ is the pollutant concentration at any location x in the soil–bentonite cut-off wall, D₁ is the effective diffusion coefficient of the soil–bentonite cut-off wall, v₁ is the pollutant’s average linear velocity in the soil–bentonite cut-off wall, ρ_d is the dry density of the soil–bentonite cut-off wall; n₁ is the porosity; K_d is the distribution coefficient. The impacts of soil–bentonite cut-off wall defects and their locations were quantitatively analyzed through the modeling of control Equations (1) and (2).

$$R_{\mathrm{d}}·\partial C_1(\mathrm{x}, \mathrm{t})/{\partial }_{\mathrm{t}}=D_1·{\partial }^2{C}_1(\mathrm{x}, \mathrm{t})/\partial {\mathrm{x}}^2 - v_1·\partial C_1\left(\mathrm{x}, \mathrm{t}\right)/\partial \mathrm{x}$$

(1)

$$R_{\mathrm{d}}=1+{\rho }_{\mathrm{d}}{K}_{\mathrm{d}}/n_1$$

(2)

In the context of pollutant transport within cut-off walls, Guo [59], He [60], Li [61], and Geng et al. [62] investigated the mechanisms of pollutant transport in vertical cut-off walls constructed from various materials. Zhan et al. [63] examined the migration and adsorption of lead ions in soil–bentonite cut-off walls through centrifuge tests and numerical simulations, modeling the pollutant migration process over a period of 50 years. Xie et al. [64] employed Monte Carlo simulations and Sobol global sensitivity analysis to study the migration of organic pollutants in both the cut-off wall and the aquifer, focusing on breakthrough time, decay time, and cumulative concentration.

In summary, while the current model for evaluating the service life of cut-off walls is well developed, it primarily relies on pre-design methods that consider influencing factors. However, actual conditions are often more complex than those simulated, encompassing factors such as dry and wet cycles, freeze–thaw actions, heavy metal pollution, fluctuations in water levels, and construction quality. These factors can significantly diminish the impermeability performance of the cut-off walls, and a dynamic evaluation method for service life has yet to be proposed.

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:

• Bentonite content: Bentonite content impacts conductivity and effectiveness. Higher content improves impermeability and longevity, but usually ≤15%. Excessive amounts can raise porosity and cause particle grouping, potentially worsening performance. Ions like Na⁺ and Ca²⁺ 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].
• Chemical compatibility: Beyond material composition, chemical compatibility with contaminants is pivotal. High concentrations of Na⁺ and Ca²⁺ in landfill leachate compromise SB mixtures, increasing hydraulic conductivity and accelerating hazardous contaminant penetration. Na⁺ and Ca²⁺ ions enhance soil–bentonite hydraulic conductivity by exchanging with bentonite’s cations, reducing particle charge and increasing porosity. Ca²⁺ has a bigger impact due to its larger hydration radius. Other ions like K⁺ and Mg²⁺ 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].
• 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 Pb²⁺ 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].
• 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].
• 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].
• 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.

2.4. Soil–Bentonite Vertical Cut-Off Wall Material Improvement Measures

The montmorillonite content in bentonite typically exceeds 85%, granting it abundant adsorption sites within interlayer spaces, outer surfaces, and edges. This structural feature underpins bentonite’s strong adsorption capacity, making it a critical component in containment systems for waste disposal or contaminated sites. However, pure bentonite’s low mechanical strength limits its standalone use in vertical cut-off walls. To address this, bentonite is often blended with soil mixtures. The fine particles of bentonite disperse within soil pores, where hydration-induced swelling fills voids and reduces permeability. Concurrently, its high specific surface area enhances pollutant adsorption, particularly for heavy metals [76].

Despite these advantages, soil–bentonite (SB) vertical cut-off walls face limitations under diverse pollutant types and environmental conditions. To overcome these challenges, researchers have explored additives to optimize SB material performance. For instance, Keramatikerman [77] and He et al. [78] demonstrated that incorporating sawdust (SD), controlled amounts of zeolite, or activated carbon significantly reduces hydraulic conductivity and improves impermeability (Figure 4a). Notably, He et al. [78] cautioned that excessive activated carbon increases hydraulic conductivity, as shown in Figure 4b. Beyond permeability enhancements, compressibility adjustments are also critical. Chegenizadeh et al. [79] reported that adding powdered or crushed recycled tires (PRT/CRT) elevates vertical strain in SB walls, thereby enhancing compressibility, as demonstrated in Figure 5a,b.

Internationally, modified bentonite products have emerged to address seepage control limitations. Innovations such as organic clay bentonite (OB) [80], trisoplast bentonite (TSP) [81], multi-swollen bentonite (MSB) [82], and sodium carboxymethylcellulose-modified bentonite [83] exhibit superior performance compared to conventional bentonite. Further advancing this field, Geng et al. [62] developed an alkali-activated slag-bentonite-soil composite, which outperformed unmodified bentonite in compressive strength, permeability, Zn²⁺ adsorption, and chloride diffusion resistance. This paper synthesizes optimal additive ratios for minimizing hydraulic conductivity in SB walls, as illustrated in Figure 6 [62,78,84,85,86].

2.5. 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 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].

3. 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. The extent to which contamination affects the engineering properties of soils [94].

Degree of Impact	Marginal	Moderately	Big
Rate of change in engineering characteristic indicators (%) 1	<10	10–30	>30

¹ Ratio of the difference in engineering characteristic indexes before and after contamination to the indexes before contamination.

[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. Scholars have extensively investigated sodium-based bentonite waterproofing blankets (geosynthetic clay liners, GCL) [95,96] and compacted clay bentonite [97,98]. Fan 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. Guo [100] investigated the effect of ZnSO₄-contaminated liquid on soil–cement–bentonite 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, Fu 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 Ca²⁺, to the backfill resulted in changes in hydraulic conductivity, bound water content, and effective porosity, with a concentration threshold observed for Ca²⁺ and chemical oxygen demand (COD). Du 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. Yu 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. Du [101], Jia [103], and Fan [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 bentonites did not exceed the lowest standard line, although hydraulic conductivities increased with rising ionic concentration. Kang 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.

The effect of chemical compatibility on the long-term stability of materials is crucial and warrants attention to the latest research developments. Katsumi et al. [106] found that multi-swelled bentonite (MSB) and dense prehydrated geosynthetic clay liners (DPH-GCL) exhibit chemical compatibility under electrolytic chemical solution permeation, making them suitable for use as cut-off wall materials in waste containment facilities. Yu et al. [102] and Razakamanantsoa [107] found that the chemical compatibility and impermeability of bentonite can be significantly enhanced by polymer modification, sodicization, and microstructure optimization. He et al. [108] demonstrated that chemical solutions significantly influence the hydrodynamic behavior of laterite/bentonite mixtures used as engineered cut-off walls. Yang [109] shows that sodium hexametaphosphate-modified calcium bentonite has a stronger heavy metal adsorption capacity (like lead and chromium) than high-quality sodium bentonite. Also, it works well in acidic, alkaline, and high-salt solutions, and its permeability coefficient does not rise much in polluted liquids.

4. Study on the Attenuation of Seepage Control Performance of SB Cut-Off Wall Under Environmental Influence

4.1. Dry–Wet Cycles

Seasonal rainfall and temperature fluctuations cause soil–bentonite cut-off walls to repeatedly undergo dry–wet cycles during long-term operation. Dry–wet cycles refer to the process of testing materials under alternating dry and wet conditions. Dry–wet cycles make soil structure more uniform and create cracks, increasing permeability. They also reduce water-holding capacity in the low-suction range and lower cohesion and friction angle of bentonite-modified clay [110]. These effects impair the long-term stability and barrier effectiveness of soil–bentonite cut-off walls. With divalent cations (e.g., Ca²⁺) in the permeate, cation exchange occurs in bentonite. They replace monovalent cations, compressing the bentonite bilayer and reducing hydration and swelling. This creates more connected pores or cracks, increasing fluid flow and permeability. Cation exchange also adjusts osmotic pressure to aid fluid passage through bentonite clay [67]. When cation exchange interacts with dry–wet cycles, bentonite may develop severe cracks, as shown in Figure 10. This interaction enhances hydraulic conductivity, thereby undermining the effectiveness of the cut-off wall [111].

Lin and Benson [112] found that the hydraulic conductivity of natural sodium-based bentonite increased by three orders of magnitude when exposed to a 0.0125 mol/L CaCl₂ solution after undergoing six dry and wet cycles indoors, indicating a significant decrease in impermeability performance. A study by Michela et al. [83] demonstrated HYPER Clay, especially HC + 8%, outperforms natural sodium bentonite in swelling ability, self-healing capacity, and maintaining low hydraulic conductivity in seawater conditions. After three wet–dry cycles in seawater, the performance of natural sodium bentonite and HYPER Clay can be summarized as

Table 6. Comparison of properties of natural sodium bentonite and modified bentonite (HYPER Clay) after three freeze–thaw cycles.

Performance	Natural Sodium Bentonite	HYPER Clay (HC + 8%)
Swelling ability	Initially high but significantly decreases after three cycles	Increases with polymer content, remains higher even after multiple cycles
Self-healing ability	Cracks formed during drying do not heal well when rewetted	Stronger crack healing ability due to polymer’s role
Hydraulic conductivity	Sharp increase, reaching 2.93 × 10−7 m/s by the fourth cycle	Remains low, with only a slight increase to 9.11 × 10−11 m/s in the third cycle

follows:

In terms of modifications for wet and dry cycling, Guo et al. [113] found that their self-developed polymer-modified bentonite clay exhibited greater resistance to the effects of wet and dry cycling and swelling compared to unmodified bentonite, as shown in Figure 11. The polymer was incorporated into the interlayer structure of montmorillonite and grafted onto its surface, allowing the modified bentonite clay to more effectively seal inter-particle voids, significantly reducing its permeability.

4.2. 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.

Foose 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. Kraus 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. Liu 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. Wang 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. Wang 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.

5. In Situ Evaluation Methods

5.1. Leakage Evaluation Based on Resistive Chromatography Imaging Technique ERT

Traditional detection methods for vertical cut-off wall leakage, such as excavation and drilling, are time-consuming and costly. As an alternative, the non-destructive testing technique known as electrical resistivity tomography (ERT) is often employed. ERT provides 2D and 3D images of resistivity changes (which are inversely proportional to conductivity) based on electrodes placed on the ground, allowing for the assessment of the resistance or conductivity distribution within the cut-off wall [121]. However, while ERT can detect wall seepage and predict necessary repairs, it may not accurately locate seepage points on its own and often needs to be used in conjunction with other geophysical methods.

Cardarelli et al. [11] utilized ERT alongside surface geophysical surveys (SRT) to compare subsoil stratigraphic imagery with an initial geological model, thereby enhancing the effectiveness of seepage repairs in a dam’s wall. Similarly, Martínez et al. [122] innovatively combined ERT with SRT to improve seepage repair effectiveness in dams. Additionally, Martínez et al. [123] integrated ERT with induced polarization (IP) methods to detect cut-off wall leakage and internal erosion. Loperte’s team [124] employed ground-penetrating radar (GPR), ERT, thermal infrared (TIR) imaging, and geotechnical in situ measurements to diagnose dams, finding that this combined approach was effective in identifying seepage issues.

Despite the widespread application of geophysical detection techniques for landfill structures and leakage problems, accurately determining the location of leakage in vertical containment walls remains challenging [124]. Furman et al. [125] investigated specific subsurface zones to locate seepage using ERT. Mao et al. [126] combined geophysical methods with traditional water chemistry and hydrological techniques, utilizing surface and water ERT to scan reservoirs and identify seepage areas [127,128]. Additionally, drilling process monitoring systems, borehole resistivity tomography, borehole sonic logging, and water injection testing are suitable for detecting potential leakage locations in grouted cut-off wall walls in karst areas. While ERT is commonly used to detect general seepage issues, a combination of ERT, time-domain induced polarization (TDIP), very low frequency (VLF), frequency-domain electromagnetic (FDEM), time-domain electromagnetic (TDEM), seismic refraction (SR), and seismic tomography (ST) methods are required for detecting and locating seepage of heavy metal-contaminated fluids [129,130]. The resistive tomography imaging technique ERT is primarily applied to detect localized leakage points, and further investigation is needed to understand the correlation between conductivity and the service life of the cut-off wall.

5.2. CPT/CPTU-Based Evaluation Methods and Models

After the construction of the slurry wall is completed, in situ evaluations of the strength, permeability, and stress state of the wall are typically conducted using cone penetration testing (CPT) or cone penetration testing with pore pressure measurement (CPTU). Ruffing et al. [131,132] proposed combining CPT with the vibratory shear test (VST) technique to assess the continuity and quality of soil–bentonite used as backfill material for vertical cut-off walls. Their study indicated that the ratio of the shear strength of the backfill to the typical strength-consolidation stress ratio could predict permeability changes based on the stress values obtained from in situ tests.

Li et al. [133] estimated the permeability change in soil–bentonite vertical cut-off walls by analyzing pore pressure dissipation during pore pressure tests, finding that penetration yielded permeability values greater than those obtained from laboratory experiments. For calculating the shear strength of soil–bentonite, Ruffing et al. [131] recommended using CPTU data in conjunction with Equations (3) and (4) to determine the shear strength and horizontal effective stress of the backfill. In these equations, q_c represents the original tip resistance, a is the area ratio, u₂ is the pore pressure measured at the shoulder of the cone, and the tip coefficient N_ke is set at 11.5.

$${\mathrm{s}}_{\mathrm{u}}=(\left(q_{\mathrm{c}}+\left(1-\mathrm{a}\right)·u_2\right)-u_2)/N_{\mathrm{k}\mathrm{e}}$$

(3)

$${\mathsf{\sigma }}_{\mathrm{h}}^{\prime }=(\left(q_{\mathrm{c}}+\left(1-\mathrm{a}\right)·u_2\right)-u_2)/0.3·N_{\mathrm{k}\mathrm{e}}$$

(4)

Ke et al. [134] analyzed the Hydraulic conductivity of soil–bentonite vertical cut-off walls as influenced by the stress state and established a stress model based on force balance using Coulomb’s theory. The model demonstrated good agreement with static touch test results from actual cases, allowing for general estimations of horizontal stress.

Li et al. [133] synthesized a variety of empirical equations for the hydraulic conductivity during the pore water pressure dissipation process, as shown in

Table 7. Empirical equations related to hydraulic conductivities based on in situ tests [133].

Author	Empirical Equation (Math.)
Baligh and Levadoux [135]	$k_{\mathrm{h}}={ (\mathsf{\gamma }}_{\mathrm{w}}/2.3{\mathsf{\sigma }}_{\mathrm{v}0}^{\prime })R_{\mathrm{R}}{c}_{\mathrm{h}}$
Parez and Fauriel [136]; Cai et al. [137]	$k_{\mathrm{h}}={\left(251t_{50}\right)}^{-1.25}$
Elsworth and Lee [138]	$k_{\mathrm{h}}=K_{\mathrm{D}}Ur{\mathsf{\gamma }}_{\mathrm{w}}/4{\mathsf{\sigma }}_{\mathrm{v}0}^{\prime }$
Shen et al. [139]	$k_{\mathrm{h}}= (1/2.976\mathsf{\beta }{\mathrm{e}}^{0.076\mathsf{\beta }})·{(K}_{\mathrm{D}}Ur{\mathsf{\gamma }}_{\mathrm{w}}/{\mathsf{\sigma }}_{\mathrm{v}0}^{\prime })$

. They analyzed the non-monotonic characteristics of the dissipation curves, illustrated in Figure 14. By examining these non-monotonic pore water pressure dissipation curves using various interpretation methods, they found that the hydraulic conductivity estimated based on consolidation theory closely matched the results from flexible wall permeability tests. In contrast, the hydraulic conductivities obtained from empirical formulas were lower, while those derived from penetration-based methods were significantly higher than the laboratory test results.

5.3. In Situ Permeability Evaluation Methods

Abbaslou et al. [140] developed an empirical interpretation scheme for estimating the permeability of slag–cement–bentonite cut-off walls based on the method. The test results, presented in Figure 15, demonstrate that the minimum impermeability requirements are satisfied, despite the irregular hydraulic conductivities observed in the block samples after 11 years of service.

5.4. In Situ Evaluation Model Based on Stress State

Laboratory test data indicate that the soil pore ratio and permeability decrease with increasing consolidation pressure [141,142]. Given that consolidation pressure is closely related to the in situ stress state of the soil, studying the in situ stress state of soil–bentonite backfill is particularly important. An accurate assessment of the hydraulic conductivity of in situ backfill necessitates proper calculation of effective stresses [143,144].

Li et al. [145] developed an advanced model for predicting steady-state horizontal and vertical effective stresses in consolidated backfill. This model integrates the composite effects of lateral stress and lateral compression, demonstrating significant improvements in predicting the stresses in soil–bentonite slurry tank walls compared to conventional models [131,146]. By utilizing the effective stresses calculated by the model, the variation in the hydraulic conductivity of the backfill with depth can be estimated. The results indicate that the hydraulic conductivity decreases with depth, as shown in Figure 16 [143,145], providing a new perspective for understanding the hydraulic conductivity of soil–bentonite backfill. Additionally, the model accounts for changes in pore ratio, which is crucial for accurately predicting the permeability properties of backfill, thereby offering a scientific basis for the design and construction of soil–bentonite cut-off walls.

5.5. Insufficient and Prospective In Situ Evaluation Methods

Existing investigations predominantly focus on non-contaminated environments, with limited attention to in situ testing under heavy metal-laden conditions. Conducting such evaluations necessitates careful consideration of equipment corrosion resistance and potential interference from metal ions in acquired datasets. Notably, laboratory-derived parameters often fail to capture field realities, as evidenced by discrepancies between laboratory and field-derived permeability coefficients [133,140]. Consequently, in situ assessments of soil–bentonite vertical barriers within contaminated settings are imperative for generating reliable design benchmarks.

Table 8. Comparison of the advantages and limitations of in situ evaluation methods.

Evaluation Methods	Advantages	Limitations
ERT	Non-destructive, cost-effective, efficient, generates 2D/3D resistivity images for leakage localization and enables rapid large-scale site screening.	Hard to locate leaks alone, easily disturbed by site conductivity, e.g., metal ion contamination, requires merging other geophysical methods for better accuracy.
CPT/CPTU	Measures shear strength, permeability, stress state; predicts permeability via pore pressure curves, high lab correlation; enables depth-specific cut-off wall assessment.	Lacking universal applicability, prone to corrosion in high metal contamination, and limited to non-homogeneous walls.
In situ permeability assessment methods	Directly determines permeability coefficients from post-long-term service samples to verify material performance and adaptability to complex geological conditions.	Long and costly sampling and analysis; block samples may weaken wall structure; inability to dynamically monitor permeability changes.
In situ modeling based on stress states	Integrate stress distribution and permeability coefficients for high-precision predictions, evaluate depth-dependent permeability decline, and support dynamic stress-porosity coupling analyses.	Relies on accurate in situ stress data, neglects heavy metal contamination effects on material aging, and lacks complex environment validation.
Integrated geophysical methods	Enhances leakage positioning accuracy, adapts to heavy metal-contaminated fluid leakage, and synergizes with traditional hydrochemical methods.	Equipment is complex and needs high operational skills, data interpretation requires interdisciplinary expertise, and long-term monitoring is costly.

provides a critical synthesis of the strengths and limitations inherent in current in situ evaluation methodologies.

Current models for evaluating vertical cut-off walls predominantly rely on in situ testing and performance assessments conducted in non-contaminated environments. However, these frameworks exhibit critical limitations when applied to heavy metal-contaminated scenarios, as they fail to account for factors such as ion-induced material degradation, electrochemical corrosion of monitoring equipment, and dynamic interactions between pollutants and barrier materials [147]. To address these gaps, future research must prioritize the development of advanced models that integrate in situ dynamic evaluation software. Such software should enable real-time monitoring of stress-permeability coupling, contaminant transport kinetics, and long-term chemical compatibility under field conditions, thereby bridging the disparity between laboratory simulations and practical performance [68,148].

The role of in situ testing and evaluation methodologies is pivotal across the lifecycle of vertical cut-off walls. During the initial design phase, these methods provide essential geotechnical parameters (e.g., hydraulic conductivity, shear strength) to optimize material selection and structural configurations. For mid-term predictions, they facilitate the calibration of aging models to forecast permeability degradation under cyclic environmental stressors (e.g., freeze–thaw, wet–dry cycles). In post-construction safety assessments, in situ evaluations serve as a diagnostic tool to identify localized leakage or structural defects, ensuring compliance with environmental safety standards [149,150].

Building on field-collected data (e.g., CPTU-derived pore pressure dissipation curves, ERT resistivity profiles) and laboratory validations (e.g., accelerated aging tests, adsorption isotherms), this study proposes a comprehensive evaluation system for vertical cut-off wall service performance. The system hierarchically integrates as illustrated in Figure 17. This framework not only enhances the accuracy of assessments but also provides actionable insights for remediation strategies in contaminated sites.

6. Potential Future Technological Developments

6.1. Deepening the Application of Intelligent Monitoring Technology

The future is likely to see further refinement of smart sensor technology, which will play an integral role in enhancing the precision of monitoring key performance indicators of vertical cut-off walls. Of particular note is the anticipated development of micro-sensors designed for real-time, in situ monitoring of the permeability coefficient of bentonite vertical cut-off walls. It is expected that these advanced sensors will feature significantly improved sensitivity and stability [151]. They can be strategically embedded at various depths and locations within the cut-off wall structure. This strategic placement will facilitate continuous data collection, with the data being transmitted to a cloud-based platform for storage and preliminary processing. Subsequently, through the application of sophisticated big data analytics and machine learning algorithms, researchers and engineers will be able to conduct in-depth mining and analysis of the vast dataset. This data-driven approach will not only enable a more accurate assessment of the current barrier efficacy of the cut-off wall but also allow for the prediction of potential performance decline trends. Such insights will provide a robust scientific basis for the timely implementation of maintenance measures, ensuring the long-term service performance and effectiveness of the cut-off wall in containing contamination.

6.2. Development of New Materials and Composite Structures

On the one hand, there has been a continuous development of modified bentonite materials that are more resistant to pollution. By incorporating specific nano-additives or organic polymers, the adsorption capacity of bentonite for heavy metal ions and its ability to immobilize them have been enhanced. Simultaneously, the chemical stability and durability of the material have been improved. On the other hand, efforts have been made to explore the composite structural design of cut-off walls. For instance, based on bentonite vertical cut-off walls, new combinations with biochar material layers or photocatalytic material layers have been proposed [152]. Biochar can adsorb some organic pollutants and create a synergistic effect with bentonite. Under light conditions, photocatalytic materials are able to decompose part of the toxic and hazardous substances. These advancements further enhance the cut-off wall’s environmental adaptability and pollution prevention capabilities, thereby broadening its application in complex pollution sites.

6.3. Virtual Reality and Augmented Reality Based Visualization Assessment Techniques

Through the integration of virtual reality (VR) and augmented reality (AR) technologies, a digital three-dimensional model of the cut-off wall and its surrounding soil environment is constructed [153]. Real-time monitoring data, geological information, and pollution diffusion simulation results are seamlessly integrated into the model, creating an intuitive and dynamic visual interface for performance evaluation. Engineers and decision-makers, equipped with VR/AR devices, can view the internal structure, performance status, and pollution migration pathways of the cut-off wall in real time. This capability facilitates more efficient performance assessments and program optimization, thereby enhancing the scientific basis and accuracy of decision-making processes. Ultimately, this technological advancement promotes the remediation of heavy metal-contaminated sites towards intelligent and visualized approaches.

7. Conclusions and Outlook

This study analyzes the performance of soil–bentonite vertical cut-off walls used as underground pollutant barriers. Key conclusions and recommendations are as follows:

• Construction methods and applicable conditions for soil–bentonite vertical cut-off walls are summarized, with bentonite-based materials being most common. New materials could enhance research on heavy metal-contaminated site remediation.
• Current research on single metal pollution’s impact on chemical compatibility is no longer representative. More attention should be given to composite pollution and long-term exposure effects.
• Research on vertical cut-off wall materials in heavy metal pollution and dry–wet cycle environments is insufficient. Modified materials show potential but need more field verification. Environmental factors can affect material structure and impermeability, so further research is needed for long-term effectiveness.
• Longevity evaluation of vertical cut-off walls typically relies on contaminant transport calculations. However, real-world conditions are more complex, requiring in-depth studies on wall performance. In situ testing is essential but lacks long-term data. Future research should focus on long-term in situ testing to better inform design and construction.
• Future technological advances to improve soil bentonite vertical containment walls were outlined, such as smart monitoring systems, contamination-resistant materials and VR/AR assessment tools.

The failure of vertical cut-off walls has significant environmental impacts, making the identification of the most effective life evaluation program crucial for pollutant containment.