## 1 Introduction  
Natural rock is damaged material to a varying degree. With the increasing scale and size of rock mass engineering, rock masses transform from an intact state to a heavily fractured state, and the analysis of deformation and prediction of collapse for the rock mass also become more difficult. Among all kinds of flaws, en echelon flaws are seen as a unique set of subparallel flaws, and they have been commonly found in rock masses of high slopes and yield pillars [1, 2]. For example, Figure 1 shows a natural rock mass containing en echelon flaws in an open-pit mine. The initiation, propagation and coalescence of cracks between en echelon flaws may occur and result in the instability failure of a rock mass [3, 4, 5] because a rock mass containing en echelon flaws is subjected to compressive loading resulting from earthquakes, rock mass gravity, blasting, traffic and so on. Therefore, understanding the fracture behavior of rock materials containing en echelon flaws under compression conditions is very important to predict the collapse and instability of rock mass structures. To avoid the use of expensive large cutting equipment to make real rock samples, it is easy to fabricate pre-existing fissures in rock-like materials by inserting thin steel discs or paper to simulate open or closed fissures, and many scholars have used rock-like materials with pre-existing fissures to study the fracture behavior of rock materials containing en echelon flaws. [PERSON] and [PERSON] [6] carried out a series of uniaxial compression tests on rock-like specimens containing en echelon flaws. They found that the crack types and coalescence patterns produced in specimens with two flaws can be extrapolated to specimens with multiple flaws and that crack coalescence was produced by the linkage of two flaws through wing and/or secondary cracks. [PERSON] et al. [7] investigated the crack initiation, propagation, coalescence and interaction mechanisms of multicrack in brittle rock, and identified seven types of coalescence based on the nature of the cracks involved. [PERSON] and [PERSON] [8] performed biaxial compression tests on rock-like materials containing en echelon flaws. In their study, the failure mode and maximum strength of the specimen depended on the ratio between the intermediate stress and intact material compressive strength, the orientation of the principal stresses, the geometry of the pre-existing fissures and the degree of fracture persistence. To further understand the fracture behavior of rock materials containing en echelon flaws under compression conditions, some scholars obtained precracked rock materials using mechanical or hydraulic cutting equipment, and studied the specimens by various auxiliary testing methods. [PERSON] et al. [9] used a loading system, an optical microscope and an image recording system to analyze the fracture characteristics of Carrara marble containing en echelon flaws. They mainly reported the linking behaviors of flaws, correlations between stress and flaw configuration, fracture process zone and structures in fault tips. [PERSON] et al. [10] established the relationship between the axial stress-strain curve of precracked sandstone specimens and the real-time crack coalescence process and used this relationship to evaluate the macroscopic deformation characteristics of precracked sandstone specimens. Among the various auxiliary testing methods, acoustic emission (AE) monitoring technology can record the fracture process of various materials in real time, so acoustic emission test data (count, cumulative count, energy, frequency, etc.) were often used to characterize the fracture properties of rock materials in various experiments [11; 12; 13; 14; 15]. In addition, [PERSON] et al. [16; 17; 18; 19] conducted a statistical analysis of acoustic emission test data obtained from the experiment.