## 1 Introduction  
Original fractured structures (including flaws, joints, cracks, and faults) distributed in rock mass commonly led to severe damage in mining engineering, civil engineering, the construction of hydropower stations, and other underground artificial structures. It is thus important to study the failure characteristics and the corresponding mechanisms of fractured rock to ensure the safety of underground constructions. Many studies have focused on various types of fracture conditions of rock mass, which simulate the conditions including single flaw (hole) [1; 2; 3; 4; 5; 6; 7], double parallel/nonparallel flaws (elliptical holes) [8; 9; 10; 11], and non-persistent flaws [12; 13; 14]. [PERSON] and [PERSON] [15] carried out the shear tests on a series of gypsum specimens with intermittent joints. The experimental results have been simulated and reduced by the hypothesis of averaged cohesion and the theory of fracture mechanics, and the latter is more suitable for the situation of crack interaction. Taking the joint's mechanical and geometrical parameters into account, [PERSON] et al. [16] proposed a method for estimating the spatial anisotropy deformation of a jointed rock mass based on the joint deformation tensor method combined with the elastic anisotropy index. The effectiveness of the proposed method has been tested and verified by numerical modelling and in situ triaxial compressive experiments. [PERSON] et al. [17] examined the peak uniaxial compressive strength (UCS) and failure patterns of ubiquitous joint rock-like specimens by similar material testing and PFC 2D numerical simulation. The results showed that the failure patterns of specimens can be classified into four types: stepped path failure, planar failure, shear-I failure, and shear-II failure. [PERSON] et al. [18] conducted the uniaxial compression test to investigate the interaction of parallel joints and its influence on the mechanical behavior of jointed rock mass models; subsequently, four basic failure modes have been identified. [PERSON] et al. [19] have investigated the fatigue mechanical properties of synthetic jointed rock models under various cyclic conditions, namely, four loading frequencies, four maximum stresses, and four amplitudes. The experimental results have revealed that three different crack coalescence patterns are identified between two adjacent joints. [PERSON] et al. [20,21] tested and analyzed the effect of joint inclination and joint spacing on the strength behavior of the rock mass with double holes specimen under compression-shear loading based on DIC and DEM modelling. Four typical failure modes and five types of crack coalescence surrounding the holes and adjacent joints have been found. Digital image correlation (DIC) technology is widely applied to monitor the development of the deformation field and visible secondary crack shown on the sample surface. [PERSON] et al. [22,23] have conducted a series of dynamic tests on sandstones using a split Hopkinson pressure bar (SHPB), and the high-speed three-dimensional digital image correlation (3D-DIC) method is used to characterize the evolution of strain field of rock materials. [PERSON] et al. [24] have examined the failure mechanism of filling samples by the split Hopkinson pressure bar tests combined with the high-speed DIC method. [PERSON] et al. [25] have conducted physical experiments on specimens and used DIC to obtain the displacement patterns as the fracture evolved, which aimed at investigating the rock fracture and the induced displacement field. In addition, application of the both DIC and acoustic emission (AE) technology helps to detect the full scales of secondary failures during the rock mechanism tests. [PERSON] et al. [26] have proposed a novel DIC-based method for the quantitative identification of the crack path and fracture mechanism of the initiated cracks in rock. The proposed method has revealed that the crack evolution of flawed marble specimens is closely related to infilling conditions.