## 3 Evaluation Indicators
### The Geological Body
#### 3.1.2 Rock and Soil Structure and Geological Structure  
The generalization of rock and soil structure and property parameters is typically crucial for GSEUUS. A comprehensive description of the thickness, properties, and interrelations among the various geotechnical layers is essential, with a particular focus on the physical and mechanical properties of each layer. It is important to consider the impact of structural features, such as faults and folds, on the development of UUS and to identify potentially unstable areas. Geological drilling, in-situ testing, and laboratory testing can aid in constructing rock and soil structures and in quantifying stratigraphic property parameters. Additionally, geophysical detection technology is commonly employed in UUS exploration and in the detection of field geological resources [76,77]. (1) Quaternary geological structure;  
UUS development is typically more active in shallow regions, particularly within the Quaternary strata. Consequently, understanding the Quaternary geological structure is essential for studying the geological origins of UUS layers and addressing origin-related geological issues. It is imperative to analyze the types and causes of Quaternary sediments, including alluvial, diluvial, and aeolian deposits, through drilling, geophysical prospecting, and experimental test analysis, while also evaluating their impact on the stability of UUS. Alluvial and lacustrine strata are relatively stable and suitable for large-scale development; however, diluvial layers, are characterized by larger particles and poor sorting, present challenges for the creation of UUS, with groundwater infiltration being a significant concern. Conversely, marine and swamp-phase sedimentary strata typically contain soft soil, which can be relatively corrosive [78,79]. Therefore, delineating Quaternary geological divisions based on their structural characteristics is crucial for informed evaluation. (2) Engineering geological structure;  
Engineering geology is essential for GSEUUS. It involves analyzing the engineering geological properties of each sub-layer within the geological layer group, including stratum depth, thickness, and extension, as well as addressing engineering geological issues, such as soft soil, sand soil liquefaction index, freeze-thaw properties, and expansion characteristics. Additionally, the relationship between the distance to active fault zones and rock compressive strength is crucial. For instance, bedrock typically demonstrates low weathering and high natural compressive strength, rendering it stable and safe for development, even in the face of challenging excavation conditions. The engineering properties of rock masses vary significantly by lithology [80]. For instance, granite exhibits a high uniaxial compressive strength (UCS) and low porosity (Figure 3a), making it ideal for deep underground facilities; however, joint spacing must be evaluated to avoid block instability. Limestone, on the other hand, is susceptible to dissolution, which necessitates geophysical surveys to detect potential cavities (Figure 3b). Sandstone demonstrates a moderate UCS but may exhibit high permeability, thereby requiring waterproofing measures (Figure 3c). It is important to evaluate the bearing capacity of various rock and soil layers and consider appropriate foundation treatment methods. The geotechnical properties, such as bearing capacity and compressibility, can vary significantly across different locations within the same construction area. Therefore, identifying critical strata is paramount. Engineering geological survey methods encompass engineering geological mapping, exploration, laboratory or field tests,and long-term observations. In conducting suitability evaluations, values can be assigned based on engineering geological layer groups (2D) or evaluation units (3D), allowing for the assessment of carrying capacity and unique engineering geomechanical characteristics. During 3D modeling, both spatial and attribute structural parameters can be constructed.