## 5 In Situ Evaluation Methods
### 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 provides a critical synthesis of the strengths and limitations inherent in current in situ evaluation methodologies. Figure 16: Estimation of permeability coefficients based on effective stress SB vertical cut-off walls [143; 145]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Evaluation Methods** & **Advantages** & **Limitations** \\ \hline \multirow{2}{*}{ERT} & Non-destructive, cost-effective, efficient, generates 2D/3D & Hard to locate leaks alone, easily disturbed by site \\  & resistivity images for leakage localization and enables rapid & conductivity, e.g., metal ion contamination, requires \\  & large-scale site screening. & merging other geophysical methods for better accuracy. \\ \hline \multirow{3}{*}{CPT/CTPU} & Measures shear strength, permeability, stress state; products & Lacking universal applicability, prone to corrosion in \\  & permeability via pore pressure curves, high lab correlation; & high metal contamination, and limited to \\  & enables depth-specific cut-off wall assessment. & non-homogeneous walls. \\ \hline \multirow{2}{*}{In situ permeability assessment methods} & Directly determines permeability coefficients from & Long and costly sampling and analysis; block samples \\  & post-long-term service samples to verify material performance & may weaken wall structure; inability to dynamically \\  & and adaptability to complex geological conditions. & monitor permeability changes. \\ \hline \multirow{3}{*}{In situ modeling based on stress states} & Integrate stress distribution and permeability coefficients for & Relies on accurate in situ stress data, neglects heavy \\  & high-precision predictions, evaluate depth-dependent & metal contamination effects on material aging, and lacks \\  & permeability decline, and support dynamic stress-porosity & complex environment validation. \\ \hline \multirow{3}{*}{Integrated geophysical methods} & Enhances leakage positioning accuracy, adapts to heavy & Equipment is complex and needs high operational skills, \\  & metal-contaminated fluid leakage, and synergies with & data interpretation requires interdisciplinary expertise, \\  & traditional hydrothermal methods. & and long-term monitoring is costly. \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison of the advantages and limitations of in situ evaluation methods. 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.