## 3 Evaluation Indicators
### Geological Resources  
The urban underground encompasses a diverse array of valuable resources, including space, water, energy, and geological materials [82] (Figure 2). Conflicts may arise in the development of various types of urban underground resources. Therefore, it is essential to understand and scientifically evaluate the diversity of these resources, as well as to implement comprehensive planning and management strategies for their development. (1) Groundwater resources;  
Groundwater resources, as a valuable asset, necessitate the implementation of appropriate protection measures during the planning of UUS [86]. While surface water serves as the primary source of drinking water for some cities, it is essential to conduct thorough investigations and ensure the protection of groundwater [87]. Surface water is inherently fragile and groundwater can act as an emergency recharge source when it becomes contaminated or degraded [82]. Based on the findings from water quality assessments and hydrogeological drilling, groundwater protection zones are delineated according to water quality grades and abundance. In areas of significant water supply importance within the city, which serve as urban emergency water supply sources, the development of UUS is subject to restrictions. (2) Underground space resources;  
The author posits that not all UUS qualify as effective resources. The development of UUS must preserve the geological structure and its functions to a certain extent, while also protecting the scale and quality of the overlying geological resources. Only under these conditions can we consider the developed resources as effective USR. Prior to development, it is essential to determine whether the primary layer for UUS development will be situated within an aquifer, aquifuge, or across multiple geological layers. Based on the characteristics of the aquifers, suitable \"skylights\" should be selected as channels for UUS development. It is inadvisable to excavate aquifers directly or to construct extensive underground structures that span multiple aquifers. Such actions would inevitably transform the area housing the underground structures into a pathline for groundwater flow, leading to issues, like soil erosion and groundwater mixing. This would compromise the quality of high-quality groundwater and elevate the risk of pollutant infiltration. Furthermore, development should not excessively occupy aquifer space, disrupt the groundwater flow field, create localized groundwater mounds, increase the buoyancy of nearby underground structures, or heighten the risk of secondary disasters [88]. Therefore, it is crucial to strategically arrange the thickness of UUS development and utilization with the aim of protecting the aquifer during evaluations, using the developable thickness and range as key factors in GSEUUS. (3) Natural building materials;  
The natural building materials discussed in this article pertain to geotechnical materials, such as sand, gravel, and granite. Historically, these materials have been essential for human settlements, from ancient adobe structures to Roman concrete (volcanic ash-based) [89]. In modern UUS development, natural materials serve dual roles. Crushed rock and sand form the backbone of concrete and backfill materials. Low-permeability clays are used for landfill liners and waterproofing layers. On the one hand, there is a significant demand for construction materials; on the other hand, much of the excavated geotechnical materials are wasted, contributing to a substantial environmental burden in many cities [82]. Current advancements concentrate on optimizing material performance and sustainability. While natural building materials can serve as a secondary factor limiting the development of UUS, they can also be advantageous if produced during the development process. Bio-mineralization techniques enhance the strength of sand-based materials. Tunnel-excavated soils can be reused as backfill, thereby reducing waste.