## 4 Results  
### Assessment Indicator System  
Based on the actual geological conditions of the study area, an assessment indicator system was established, comprising four kinds of criteria indicators: topography, geotechnical characteristics, hydrogeological conditions, and adverse geological phenomena (Table 1). Additionally, the assessment of four application layers of UUS (L\({}_{1}\): 0\(\leftarrow\)\(-\)15 m, L\({}_{2}\): \(-\)15\(\leftarrow\)\(-\)30 m, L\({}_{3}\): \(-\)30\(\leftarrow\)\(-\)50 m, L\({}_{4}\): \(-\)50\(\leftarrow\)\(-\)100 m) involved the evaluation of different combinations of sub-criteria indicators. These sub-criteria indicators included ground elevation, bearing capacity of soils, compression modulus of soils, groundwater buried depth, aquifer thickness, land subsidence rate, sands liquefaction index, chemical corrosion of groundwater, and chemical corrosion of soils (Table 1).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Criteria** & **Sub-Criteria** & \multicolumn{5}{c}{**Grading Criteria of Suitability**} & **Application** \\ \cline{3-6}
**Indicators** & **Indicators (Unit)** & **Grade I** & **Grade II** & **Grade III** & **Grade IV** & **Layer** \\ \hline Topography & Ground elevation (m) & \(<\)8 & 8–8.5 & 8.5–9 & \(>\)9 & L\({}_{1}\)\(\sim\)L\({}_{4}\) \\ \hline  & Bearing capacity of soils & \(<\)80 & 80–100 & 100–130 & \(>\)130 & L\({}_{1}\) \\  & (0\(\leftarrow\)\(-\)5 m) (kPa) & \(<\)100 & 100–130 & 130–160 & \(>\)160 & L\({}_{1}\) \\  & Bearing capacity of soils & \(<\)110 & 110–130 & 130–170 & \(>\)170 & L\({}_{1}\) \\  & (\(-\)10\(\leftarrow\)15 m) (kPa) & \(<\)130 & 130–160 & 160–200 & \(>\)200 & L\({}_{2}\) \\  & Bearing capacity of soils & \(<\)130 & 130–160 & 160–200 & \(>\)200 & L\({}_{3}\) \\ Geotechnical characteristics & (\(-\)30\(\leftarrow\)50 m) (kPa) & \(<\)130 & 130–160 & 160–200 & \(>\)200 & L\({}_{4}\) \\  & (\(-\)50\(\leftarrow\)–100 m) (kPa) & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{1}\) \\  & & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{1}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{1}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{1}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{1}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{2}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{3}\) \\  & Compression modulus of soils & \(<\)4 & 4–11 & 11–15 & \(>\)15 & L\({}_{4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Assessment indicators of suitability to multilayer UUS in the study area.  
The suitability of underground space in the study area was categorized into four grades: grade I (Worse suitability), grade II (Inferior suitability), grade III (Moderate suitability), and grade IV (Good suitability) [15,18]. The classification of ground elevation, chemical corrosion of groundwater, and chemical corrosion of soils was based on the grading criteria outlined in the 'Code for geo-engineering site investigation and evaluation of urban and rural planning (CJ]57-2012)'. The classification of bearing capacity of soils and compression modulus of soils was referenced from [PERSON]'s grading criteria [47]. The land subsidence rate was classified according to the grading criteria specified in the 'Specifications for risk assessment of geological hazard (GB/T 40112-2021)', while the sands liquefaction index was classified based on the grading criteria provided in the 'Code for seismic design of buildings (GB 50011-2010)'. The classification of groundwater buried depth and aquifer thickness was determined in accordance with the recommendations of local geological experts.