## 1 Introduction  
Due to the high-pressure mechanical environment of the surrounding rock in deep-buried tunnels, the original rock will produce discontinuous structures such as joints, fault fissures, and weak surfaces [1; 2; 3]. When joints and cracks containing primary rock were externally disturbed, secondary cracks were created, and these secondary cracks continued to grow until they became connected [4; 5], ultimately altering the mechanical properties of the rock mass [6; 7]. The existence of rock fissures directly affects the stability and safety of engineering. Understanding and studying the characteristics and behavioral laws of rock fissures can avoid the occurrence of engineering-instability landslides and other disaster accidents. Prefabricating internal cracks in natural rock is difficult [8; 9]. In the past, scholars have conducted extensive experimental studies on pre-cracked specimens of alternative materials: including glass [10; 11], Columbia Resin [12], clay [13], and cement mortar [14]. [PERSON] investigated crack initiation stress and coalescence stresses in polymethyl methacrylate (PMMA), gypsum, and Guangdong granite under uniaxial compression [15]. Some scholars have identified two basic types of micro-cracks through cyclic uniaxial compression experiments: tensile wing cracks and shear cracks [16]. [PERSON] experimentally measured the length of nascent cracks accurately and proposed two new types of crack branching: full branching (FB) and attempted branching (AB) [17; 18]. [PERSON] obtained three merging modes of secondary cracks through many uniaxial experiments: wing stretching mode, shear mode, and mixed mode [19]. Numerical simulation can predict the crack extension process of specimens under loading conditions and verify the accuracy of experimental results. The mechanical numerical calculation methods of rock mass with defects are mainly divided into three kinds: finite element method (FEM) [20; 21], boundary element method (BEM) [22; 23], and discrete element method (DEM) [9; 24; 25]. Previous studies have shown that parallel bonding modeled by the particle flow code (PFC2D) under uniaxial compression conditions is consistent with laboratory testing phenomena [14] in simulating rock cracking results [26; 27; 28]. Although PFC2D can only be performed on a 2D planar scale, it can be used to simulate many problems in rock engineering: crack propagation and coalescence of brittle rock materials [29; 30; 31]. The discrete element method (DEM) can affect the macroscopic properties by calibrating the contact parameters between the particles, avoiding the complex constitutive relationships on the macroscopic side [32]. As such, it is well-suited for studying aspects of fractured rock masses. Digital image correlation (DIC) is a non-contact material deformation measurement technology [28; 33]. During specimen loading, the whole process of micro-crack extension is obtained by identifying and recording digital images of unique corresponding physical points on the specimen surface in different states and calculating the full-field displacement in pixels [4]. Based on the direct analysis of the captured photographs, two-dimensional digital image correlation (DIC) analysis of a single stationary camera is used to measure the strain of the specimen in all directions under external loading, which directly reveals the mechanics and deformation mechanisms of the tested materials. In previous studies, many scholars had selected specimens with a single type of crack to carry out single-crack or conjugate crack studies [29; 34; 35; 36]. Still, there are few studies on combining single cracks and conjugate cracks under uniaxial compression. This study used the DIC technique and the PFC2D discrete element numerical simulation to comprehensively analyze and obtain rock samples' characteristics and mechanical properties after damage.