## 2 The Basic Frame of the Geological Suitability Evaluation of Urban Underground Space (GSEUUS)
### Evaluation Elements  
This comprehensive evaluation encompasses five fundamental elements: the subject of evaluation, evaluation indicators (Figure 1 green rectangle), weight coefficients (Figure 1 blue rectangle), a comprehensive evaluation model (Figure 1 yellow rectangle), and evaluator and explanation (Figure 1 purple rectangle) [62].  
In GSEUUS, the subject of the evaluation is the USR, which incorporate physical aspects found on the Earth's exterior. This includes natural elements, such as groundwater, soil, and rocks, as well as artificial constructions, like diverse engineering buildings situated both above and beneath the Earth's surface. Additionally, it is essential to consider conceptual elements, which include the various concepts, principles, and knowledge necessary to comprehend the system's elements, structures, and functions. Furthermore, this evaluation must take into account the various regulations, planning documents, technical standards, and emergency plans established to govern and manage the interactions between humans and the Earth [59].  
Choosing the appropriate evaluation indicators must adhere to the principles of independence, comprehensiveness, quantifiability, and measurability. It is essential to incorporate all relevant indicators for assessment while minimizing any redundancies among them. Furthermore, the indicators should be appropriately refined. When a single indicator can be articulated through multiple evaluation metrics, it facilitates the differentiation of indicators and enhances the analysis of the issues at hand. Importantly, the information required for these indicators should be readily accessible.  
The weight coefficients play a vital role in quantifying the evaluation index, as it indicates the importance of that index. There are several techniques for determining the weight coefficient, such as the method of expert survey, the analytic hierarchy process (AHP), and the entropy weighting method (EWM). Depending on a solitary evaluation method might yield fixed results that do not accurately reflect real conditions or could overstate specific factors. To address these challenges, using a strategy that combines multiple methods can improve the scientific credibility of the findings. For example, applying GIS spatial analysis along with the AHP and multi-criteria analysis (MCA) [63] or utilizing the AHP guided by expert insights [64] can produce more reliable assessments.  
The development of a thorough evaluation model generally depends on the measurement matrices of each system, as well as a weighted average derived from the evaluation index metrics across these systems.  
It is essential for the evaluator and explanation to identify the key thresholds of the evaluation outcomes, categorize the results, and establish a suitability score. To select the critical value, various methods can be employed, including analogy, analysis of frequency curves, probability assessments, or graphical illustrations. The application status of the output products will prompt adjustments in case strategies, thereby achieving a closed-loop evaluation.