## 4 Discussion  
From the assessment result, it is evident that the relatively high-quality space, characterized by higher \(Ql\) values, is situated at depths ranging from 10 to 30 m in most areas, as shown in Figure 7. This space is composed of soil layers including silty sand (3), silty clay (4), and silty sand (5). Conversely, the low-quality space, exhibiting lower \(Ql\) values, is found at depths of 5 to 10 m and below 60 m in the area with soil layers consisting of soft clay (2) and gravel with coarse sands (2), (3), which are the main negative soil layers for tunnel construction in the study area. Particularly in some areas, the soft soil (2) and its contact zone with silty sand (5) at the depth of 20 to 30 m contributes the lowest \(Ql\) values. In our previous study [36], we conducted the 3D assessment of UUSR quality based on the traditional regional GEF indicators and voxel-based geological model. The assessment results showed that QI distribution ranges from 0.28 to 0.85, which is similar to this study. However, the QI distribution in the previous study exhibited a dispersed bimodal pattern, contrasting with the more concentrated unimodal pattern observed in this study. This variance may stem from the previous study's inclusion of additional indicators, such as water content, pore ratio, and characteristic value of bearing capacity.  
In order to apply the assessment result to support the design of tunnel construction, a planned subway section in the study area (Figure 9) was analyzed. The quality profile along the planned route was extracted from the 3D assessment result, as shown in Figure 10. There is a low-quality area with the \(Ql\) value of 0.28-0.37, situated within a depth range of 20-30 m in the middle of the section. The strata at this depth are mainly composed of soft soil (2) and its contact zone with silt sand (3), according to the geological profile. Particularly, the upper thick, soft soil and the lower silt sand form a hard-soft heterogeneous formation, which is unfavorable for shield tunneling construction.  
Figure 10: The \(Ql\) value profile along the planned subway section.  
Figure 9: Layout of the planned subway section in the study area based on a Google Earth image.  
In this study, the assessment indicators selected focused on the geotechnical parameters (\(G\)), hydrogeological parameters (\(H\)), and 3D spatial position parameters (\(S\)) for shield tunnel construction in the area covered by thick soil layers. Considering the geological condition of the study area, the geotechnical parameters of \(c\), \(\varphi\), and \(k\) were selected. Thus, the framework proposed is only suitable for regions characterized by thick loose layers and may not be applicable to bedrock mountainous areas. Furthermore, the existing surface and subsurface facilities in urban areas that may influence the difficulty degree of UUSR development were not taken into consideration. Some cities with rich historical heritage often have numerous protected historical or archaeological sites. Development underneath such areas may be prohibited in order to preserve their integrity. Furthermore, the assessment result is the \(QI\) value of each independent node in the 3D model, which represents the relative quality of UUSR without any classification. It is more accurate to describe the 3D quality of UUSR, but less user-friendly for underground space planning applications. In future endeavors, it is crucial to incorporate the influence of existing surface and subsurface infrastructures, as well as the historical or archaeological significance, as pivotal indicators in forthcoming 3D assessments. Moreover, quality classification should be carried out based on the assessment results to be better applied in the urban planning area.