### Specimens Contain Various Preseristing Fissues Used for Physical and Numerical Experiments  
To better comprehend the fracture behavior of rock masses, numerous researchers have performed experiments and numerical work, and different kinds of specimens (rock, rock-like materials, and numerical simulations) with various preexisting fissures have been used. Based on the number and shape of the preexisting fissures, these experiments can usually be grouped into three classifications: experiments on specimens with few preexisting fissures, specimens with multiple preexisting fissures, and specimens with fissures and other flaws (holes and inclusions). In the initial stages of investigations of rock masses with preexisting fissures, specimens with single, two, or three preexisting fissures are frequently adopted. Crack generation, expansion, and specimen coalescence are not intricate, making them convenient for results analyses. The peak strength, crack propagation process and coalescence mode are the main research objectives, and related research provides a foundation for further study. According to analyses of the experiments, the geometry of the preexisting fissures, including the preexisting fissure angle, ligament angle, and preexisting fissure length, has a significant impact on the peak strength and crack propagation [4]. Due to the impact of the arrangement of preexisting fissures and experimental setups, different kinds of cracks and coalescence modes can be observed. Generally, cracks are initiated because of two forms of stress: shear or tensile. Hence, the crack types can be classified into S mode (shear stress) and T mode (tensile stress) [3]. However, [PERSON] and [PERSON] (2002) [5] proposed that crack types should be classified into tensile cracks and quasi-coplanar shear cracks. This classification describes the crack type more specifically. In addition, the shear crack was further classified in the study by [PERSON] and [PERSON] (2002) [5]. It is possible to obtain a larger set of coalescence modes with crack propagation by linking one or more of these fundamental crack types. Due to the difference in the arrangements of preexisting crevices and experimental setups, the crack propagation process may also be different, and the coalescence mode can be classified into three categories [6], nine categories [7], or more. The classification results are correlated with the arrangement of preexisting fissures, crack generation, crack propagation, crack coalescence and the failure modes of specimens. Numerical simulations and many software applications, such as PFC2D [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18], PFC3D [19; 20; 21], RFPA2D [22], DDM [23], FLAC3D [4; 24; 25; 26], ANSYS [27], XFEM [28], ABAQUS [29; 30; 31], and UDEC [32], have been utilized to analyze the cracking processes of real rock materials. This is because numerical simulations can be repeated with a little economic cost, and the process of crack initiation and crack growth can be specifically observed. Numerical simulations have shown that the primary cause of wing cracks is tensile stress, while secondary cracks are caused by shear stress. [PERSON] (2016) [33], found that fissure length and location and stress interactions among nearby fissures have a great influence on crack generation, expansion, and coalescence behaviors, which are consistent with the physical experimental results. Although our specimens contained few preexisting fissures, they were easy to prepare, and our results were easy to analyze. However, in natural rock mass engineering, many sets of preexisting fissures exist. Therefore, experiments on specimens with multiple preexisting fissures may provide results which are closer to the reality of engineering practice, and the experimental results can provide a more useful basis for rock mass engineering. By using the artificial method, a specimen with a different alignment of preexisting flaws, such as parallel fissures [34; 35; 36], nonpersistent fissures [37; 38; 39; 40], and intersecting fissures [41], can be prepared.