## 1 Introduction  
As a geological formation that has undergone prolonged natural processes, rock masses preserve distinct traces of geological tectonic activities. Within various geological regions, the rock masses manifest diverse quantities and types of structural surfaces. For broken rock masses that are characterized by a high-dispersion structure due to extensive jointing, fissuring, and faulting, the discontinuity and anisotropy of the structure become more pronounced [1, 2]. In engineering practices involving broken rock masses, such as tunnels at near-fault regions and side slopes in highly weathered rock formations, the randomness of discontinuities, the variability in rock mass parameters, and the uncertainty associated with surrounding rock loads pose potential risks to the safety of the construction projects [3, 4]. Therefore, to enhance the construction safety of such engineering practices, it is essential to conduct in-depth investigations on the mechanical properties and behavior of broken rock masses. Due to the haphazard structural surfaces within the broken rock masses, their mechanical properties are particularly complex and challenging to be predicted quantitatively. Even with the implementation of large-scale rock mechanics tests on-site, it is only possible to obtain local mechanical parameters of limited-sized rock blocks or joint surfaces. Directly measuring the macroscopic or statistically meaningful mechanical properties and parameters of larger-scale rock masses is unattainable through experiments [5; 6]. Therefore, building upon experiments and theoretical analyses, numerous researchers have employed numerical methods to construct computational models of fractured rock masses and carry out relevant numerical mechanical tests. For example, the equivalent rock mass technology, which combines a three-dimensional fracture network simulation technique and discrete element platform, has been extensively employed in the construction of fractured rock mass models at both indoor and engineering scales [7; 8; 9; 10; 11; 12]. Meanwhile, the parameters of the equivalent rock masses, such as stiffness, deformation modulus, Poisson's ratio, internal friction angle, trace length, and fracture aperture, have also been adopted to estimate the REV size of fractured rock masses [13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. However, for broken rock masses with a more intricate distribution of fractures, limited research has been conducted on the determinations of their digital characterization and REV. Moreover, such studies have not fully captured the structural characteristics of the real rock masses. The random fracture network based on the probabilistic model description is a local macro-fracture, which only reflects some of the main structural features of the broken rock mass, and the structural surface simulation technology, which is closer to the real rock mass structure, still needs to be developed. With the continuous development of pumped storage power station projects in the North and Southeast coastal regions of China, broken surrounding rock masses of large underground cavern groups, which serve as the main structural component of pumped storage power stations, have become increasingly prevalent in practical engineering projects. In this paper, based on the relevant theories of discrete fracture network (DFN) and discrete element method (DEM), the distribution characteristics of structural surfaces in the broken surrounding rock masses of a specific hydropower station in Southeast China, i.e., the Liyang pumped storage power station, is statistically analyzed. By employing the Monte Carlo method, a three-dimensional fracture network automatic generation program is developed based on the DFN-DEM coupling approach, which can construct the equivalent rock mass models for broken rock masses at a certain site. Using the proposed program, cylindrical rock mass models with different scales are generated to analyze the size of the RVE of the broken rock mass at the Liyang pumped storage power station.