## 6 Conclusions  
In this study, an SRM model combining FDEM and DFN models was used to systematically explore the size effect of the mechanical properties of basalt containing hidden cracks and the fracture evolution laws of the surrounding rock mass during the excavation of underground caverns. The failure mechanisms of hard brittle rocks represented by basalt under high stresses were comprehensively revealed. The main conclusions are as follows:  
1. The SRM model was established by combining an FDEM model representing an intact matrix and a DFN model generated based on the statistical characteristics of the geometric parameters of hidden cracks. It represents an important method for efficiently and accurately studying the mechanical properties of hard brittle rocks with complex structures.
2. The obtained REV size of basalt blocks containing hidden cracks was 0.5 m. With the increase in the sample size, the mechanical properties as reflected in the stress-strain curve changed from elastic-brittle to elastic-plastic or ductile, the strength failure criterion changed from linear to nonlinear, and the failure modes changed from fragmentation failure to local structure-controlled failure and then to splitting failure. The essence of the rock size effect is its own structural effect, influenced by external conditions, such as the confining pressure, temperature, and loading rate, indicating that it is crucial to consider the real structural features of rocks to understand their mechanical behaviors.
3. The surrounding rock mass during excavation in the Baihetan Hydropower Station mainly exhibited a spalling failure mode, closely related to the complex structural characteristics of basalt and high in situ stresses; that is, a tensile fracture mechanism characterized by stress-structure coupling. The nucleation process of typical failure behaviors (e.g., spalling, rock-bursts) in deep underground engineering has a certain relationship with the fracture evolution of surrounding rocks at different depths, and they have a certain degree of synergy. Therefore, it is important to understand the fracture evolution laws of surrounding rocks during excavation to reveal the failure mechanism and formulate control measures.
4. This study has some limitations. On the one hand, the main focus was on basalt blocks with small-scale hidden cracks. There was less focus on basalt rock mass with engineering-scale hard structural planes, such as random joints and faults. On the other hand, this study mainly focused on the size effect analysis of the rock mechanical properties with a given distribution characteristic of the structural planes, without considering the effect of the change in the structural plane under different geometric parameters on the REV and corresponding mechanical properties. Related research will be performed in future work.  
[PERSON] contributed to the methodology, investigation, funding acquisition, and formal analysis. [PERSON] contributed to the conceptualization, resources, and project administration. [PERSON] contributed to the software and validation. [PERSON] contributed to the data curation. All authors contributed to the writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (52309123), the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonal Research Institutes (CKSF2023310/YT, CKSF2023319/YT), and Open Research Fund of State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences (SKLGME023013).  
Not applicable.  
The datasets generated within the scope of the current study are available from the corresponding author on reasonable request.