## 5 Problems and Suggestions for Improvement
### Solve the Problem of Heterogeneity  
Evaluations conducted in 2D fall short in effectively merging UUS geological data with the intrinsic 3D characteristics or complicated spatial configurations, which frequently leads to a decline in depth information. By partitioning the UUS into multiple vertical underground spatial layers [151; 152], a quasi-3D assessment can be achieved. Various technologies, such as 3D geological modeling, 3D-GIS, and Building Information Modeling (BIM), are extensively utilized during the phases of urban planning, construction, maintenance, and management [153], thus promoting the progression of 3D geological assessments. Additionally, 3D techniques, such as the 3D spatial data model and sagittal integration modeling technology, have been employed to evaluate the geological appropriateness of UUS planning [67; 133; 154]. The intricacy of 3D geological modeling largely arises from the necessity to align strata with geological principles, especially in areas with extensive sedimentary cover. The challenge of simulating the 3D spatial arrangement of sedimentary layers utilizing drilling and geophysical data becomes pronounced, particularly at smaller scales (less than 1:50,000). Given the non-homogeneous and anisotropic characteristics of formations, relying solely on borehole data proves inadequate for precisely identifying the dimensions of minor geological formations. Moreover, geophysical interpretation outcomes can appear somewhat unrealistic without thorough verification through detailed drilling data, which can result in poor accuracy regarding the inferred vertical stratigraphic arrangement. Additionally, the difficulty in assessing the suitability of 3D geological models stems from the need to effectively capture the planar and layered variability of each parameter, which is crucial for accurately representing the true geological, engineering geological, hydrogeological, and environmental geological contexts [69]. The spatial variable weight allocation technique is notably beneficial for performing 3D-GSE [74]. The diversity of underground infrastructure necessitates a shift from generic models to customized frameworks in geological suitability assessments, thereby completely breaking the constraints of stratum homogeneity. For instance, energy facilities require enhanced deep geological exploration, while municipal facilities necessitate high-density shallow geotechnical testing. To meet this demand, future research should establish a facility-geology matching database to support the automatic recommendation of assessment indicators. Concurrently, a modular weighting system should be developed that allows users to load predefined parameters based on facility types. Furthermore, exploring digital twin technology could simulate the long-term performance of different facilities under dynamic geological conditions, such as conducting century-scale settlement predictions. The applications of 3D geological modeling technology in the development and utilization of UUS are numerous. For example, the Kunming GSEUUS proposed an innovative 3D-CWC framework that integrates the weighted cloud model with 3D geological modeling to address vertical complexity and uncertainty in geological assessments [155]. In Xuzhou's 3D geological modeling, a regional modeling method based on overlay integration technology was utilized [156].