## 3 Geological Conditions  
Geomorphologically, the RIs potential sites (CSNS, DBNL, JUNO, HIAF and ADS, HEPS, and LHAASO) can be categorized as consisting of plains, hills, mountains, or plateaus, with elevations a.s.l ranging from tens of meters to more than 4000 m. The geology of these sites is typically composed of sandy gravel soil, volcanic rocks, granite,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Phase** & **Name** & **Evaluation** & **Objectives** & **Main Problems** \\ \hline
1 & Site selection & Suitability & Geological model & Discontinuity \\
2 & Plan and design & Feasibility & Parameters & Discrete layers \\
3 & Construction & Disturbance & Materials & Unfavorable geological body \\
4 & Operation & Reliability & Rebalance & Complex site evolution \\ \hline \hline \end{tabular}
\end{table}
Table 2: Timeline for engineering site selection of the RIs.  
gneiss, and marble. The host rocks were identified with problematic geological bodies, such as saprolites (i.e., residual soils and completely decomposed granites), fractured rocks, and mud layers. Additionally, intermediate acidic dykes are susceptible to fracturing under weathering and geostress.  
The DBNL site is composed of Yanshanian (Cretaceous) medium-coarse grained granite and Devonian sandstone. The former occurs as one shallow intrusion at the north wing of one anticline, whereas the sandstone appeared as a capping layer sporadically exposed on the top of granites. The DBNL site is found at the southern slope of the mountain and anticline in the east-west. Several faults of small scale occur in the DBNL site trending in the east-west and south-north [29]. The CSNS site is composed of Yanshanian (Cretaceous) coarse-grained granite (northern part) and Sinian gneiss (Southern part). The intrusive contact between these rock bodies is located in the east-west, where diorite porphyrite intrusion is found along the contact zone. Small scale faults in the direction of east-west are seldom seen [30].  
For a better understanding of the host rocks, we conducted microscopic, textural [29; 30], physical, mechanical, and hydrogeological sample analysis from the two RIs sites. Moreover, we drilled some boreholes for granites (from DBNL and CSNS sites) and gneiss (CSNS site) to acquire the local geological knowledge. Long borehole cores, up to 1.7 m in length, ensured that we were reliably obtaining the essential subsurface characteristics for each RI site (Figure 2). The examples of encountering the underlying complex geological bodies during the RIs development (Table 1 and Figure 1a) include karst caves (LHAASO site) moving from the Shikashanin Yun'nan Province to Daocheng in Sichuan Province [31], large uneven subsidence (HEPS site) moving northward by nearly 5 km to the plain area at Huairou [18], and water-rich fracture zones (ADS site) moving with tens of meters towards the north [19]. As the host rocks of the two sites (CSNS and DBNL) are mainly granite and gneiss, the subsurface geology is more complicated. The case studies of these two sites were undertaken in an effort to better understand the complex geological settings in order to facilitate the RI siting process. We used integrated geophysical techniques at the DBNL site because boreholes are unsuitable for assessing the deep geologic structures in a steep slope area. Similarly, we employed the EGSI index to analyze the shallow subsurface at the CSNS site. Our interpreted model results show a strong correlation with the local geological conditions, and our recommendations were implemented in the construction of these two sites.  
Figure 2: Drilled cores from borehole ZZK23 used to locate the ring accelerator at the CSNS site.