## 5 Discussion  
Firstly, in the suitability assessment of UUS, faults are typically an important geological factor to consider [53]. However, in this case study, faults were not included in the assessment indicator system due to their weak activity and minimal impact on the planning and construction of Xiong'an New Area [43; 51]. The study area did not show other adverse geological phenomena, such as collapsible loess and gravel. However, it is important to note that the river flow in the study area is generally small and seasonal. Since these rivers have limited impact, they have not been included in the assessment indicator system. Additionally, based on the evaluation results of the four layers, the areas that were deemed good and moderate in suitability were mainly located in the north, south, and southwest of the study area. On the other hand, the inferior and worse suitable regions were mostly found around Baiyangdian Lake (Figure 18). This finding aligns well with the characteristics of stratigraphic sedimentary facies.  
Secondly, when the study area is divided into smaller evaluation units, such as smaller grids and more cells, the calculated weights are theoretically more accurate. However, in this paper, the grid size of 500 m by 500 m was temporarily set without further subdivision, in order to focus on the research method and reduce the amount of calculation. Subsequent researchers can refine the segmentation based on the acreage of the selected study area.  
Thirdly, the planning and construction period of a city, especially for newly building cities such as Xiong'an New Area with a'millennium plan' [54], is generally long. The utilization of urban underground space is a gradual process that may be also influenced by various socioeconomic conditions. In this study, we focused on examining the geological factors that affect the suitability of multilayer underground urban spaces (UUS), without considering socioeconomic factors and excavation types of underground facilities. However, it is important to note that future research should also investigate these aspects. This is also the limitation in this paper. If data on socioeconomic factors and excavation types could be obtained, they would provide more valuable insights for planning managers in optimizing planning and utilization strategies of underground space.  
Finally, up to now, there have been limited studies on the suitability of underground space in Xiong'an New Area, as revealed by the literature review. Only [PERSON] [47] has conducted a study on this topic. In contrast to this paper, [PERSON] classified the underground space of Xiong'an New Area into three layers: shallow layer (0\(--\)30 m), sub-deep layer (\(-\)30\(\--\)50 m), and deep layer (\(-\)50\(\--\)70 m). The evaluation indicators used in [PERSON]'s study included compression modulus of soils, groundwater depth, land subsidence rate, and aquifer thickness. However, the bearing capacity of soils was not considered. The  
Figure 18: The integrated suitability assessment results of multilayer urban underground space.  
evaluation method employed was the entropy-cloud model. The evaluation results differed from those of this paper. The areas with the worst grades were primarily located in the northern part of Xiongxian County. Similar to the findings of this paper, the Baiyangdian area generally exhibited a lower grade compared to its surrounding areas. After consulting the opinions of local geological experts, it is believed that the weights of indicators determined in this paper are more reasonable. This is because the evaluation takes into account the correlation, discreteness, and comparative intensity of the affecting indicators, making the results more valuable for reference.