## 1 Introduction  
Due to rapid urbanization, many cities are facing saturation in the utilization of above-ground land, leading to problems such as traffic congestion and land resource shortage [1, 2]. To accommodate urban sustainable development, the utilization of underground space has emerged as an efficient means to expand urban space [3, 4, 5]. Therefore, integrated planning that includes both aboveground and underground space is considered an optimal approach [6, 7]. Unlike aboveground space, underground space is a non-renewable resource that is difficult to transform once built into some kind of infrastructure. As a result, conducting suitability assessment of underground space before urban planning becomes particularly significant. Existing research shows that, as the carrier of underground space, the geological environment plays a crucial role in the utilization of underground space [8]. Understandinghow the geological environment constrains the use of underground space and identifying the geological factors that affect it can help in avoiding or transforming unfavorable geological conditions. In particular, in newly building cities, the utilization of underground space is primarily influenced by geological factors rather than existing surface buildings and underground infrastructure, unlike in already built-up cities [9]. Assessing suitability can provide valuable insights for optimizing planning and utilization strategies of underground space. Therefore, studying the geological suitability of underground space holds significant theoretical and practical importance. Existing studies on the suitability of underground space primarily rely on geological analysis, which is an essential and valid method [10; 11]. The assessment system for the suitability of urban underground space (UUS) incorporates various geological factors, which differ depending on the region's geological characteristics [12; 13; 14]. These factors typically include active faults, land subsidence, groundwater level, bearing capacity of soils, compression of soils, ground elevation, sands liquefaction, soft soils [15; 16; 17; 18]. However, while researchers generally agree on the geological factors that affect the suitability of underground space in the same study area, they may differ in the weights assigned to these factors. Therefore, the critical issue lies in selecting an assessment method that incorporates reasonable indicator weights. Various methods have been developed to analyze the suitability of underground space, including AHP (analytic hierarchy process) [19; 20; 21], entropy [22; 23; 24], neural network [25; 26], fuzzy mathematics [27; 28; 29], TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) [16; 30], etc. The combined application of different methods has been shown to be more reasonable [31; 32; 33], considering that each method has its own advantages and disadvantages [34; 35]. The critical issues in the selection of an assessment method are the objectivity of weights, correlation, discreteness, and comparative intensity of affecting factors. In this paper, an entropy and CRITIC (Criteria Importance Through Intercriteria Correlation) combined weighting method was applied to obtain reasonable weights. The entropy method, derived from information theory [36], was used to derive objective weights but focused on discreteness while ignoring correlation and comparative intensity of data [11; 37]. However, the CRITIC method can effectively compensate for this deficiency [38; 39; 40]. Therefore, the combination of these two methods ensures the rationality of the objective weight of the selected indicators. Consequently, taking Xiong'an New Area, a newly planned city, as a case study, this research established a multilayer assessment indicator system for the underground space. The study also identified the suitability and critical affecting factors of underground space.