## 1 Introduction  
Different discontinuous structural planes exist in rock masses, including joints, beds, faults, and fissures. In most cases, the existence of discontinuities weakens a rock mass, which significantly affects its mechanical behavior and influences the safety of geotechnical structures like underground works and excavations in slopes [1; 2; 3; 4; 5]. At present, lots of previous researchers [6; 7; 8; 9; 10] have investigated the shear behavior of joints through experiments and numerical simulations in terms of shear strength, shear failure characteristics, and changes in fracture permeability. Studies by many scholars [11; 12] show that the degradation of joint surface roughness is one of the main reasons for the change in joint shear characteristics. Therefore, it is important to accurately describe the degradation of joint surface roughness during shearing. Acoustic emission (AE) technology can dynamically and non-destructively monitor micro-failure signals of rock and has been widely used in the study of joint surface roughness degradation law. For example, [PERSON] et al. [13] applying the AE technique, characterized the location of an asperity failure zone and the failure intensity in a joint surface during shearing. Their results reveal that the AE method provides sufficient precision to capture the asperity damage process. [PERSON] et al. (2014) performed a direct shear to look at the effect of normal stress on the AE indicators of joints. It indicated that the AE energy rate of joints was more active under a high normal stress condition. [PERSON] et al. (2015) identified and predicted fatigue crack growth (FCG) of 316 LN stainless steel under various tension proportions by drawing out several AE specifications from AE waves and associating AE indicates with FCG under various tons proportions. The study of [PERSON] et al. (2016) found that there were encoded features in the frequency domain of waveform which linked acoustic emission with damage mode. [PERSON] et al. (2017), by embracing the electronic picture correlation, showed a collection of heterogeneous CJB designs were developed. The constant fracture procedure and acoustic emissions are caught numerically under differing side pressures. The load contours under different joint range proportions and design limits are evaluated. [PERSON] et al. (2018) have studied the influence of rock heterogeneity and cylindrical irregularity on the AE energy evolution of CJB faults under lateral pressure. [PERSON] et al. (2019) studied the non-linear mechanical response and failure mechanism of articulated cylindrical basalt with transversal joints using a meso-mechanical model, statistical damage theory, and continuous mechanics. [PERSON] et al. (2019) used AE to analyze the damages development system of joints under various roughness as well as normal stress. [PERSON] et al. (2019) used acoustic emission signal data to put forward shear failure and tensile failure judgment criteria and systematically analyzed the time and type of joint failure in the shear process. However, this aforementioned research has targeted the joints sheared beneath constant normal loading (CNL) boundary conditions. This boundary condition is only applicable to surface or shallowly buried works for deep underground engineering conditions. However, the normal stress of the joints may be changed due to the stiffness of the surrounding rock. Under this scenario, the constant normal stiffness (CNS) status is more suitable than the CNL condition ([PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2019). To date, few studies have studied the AE qualities of the sheared joints beneath CNS circumstances. [PERSON] et al. (2019) investigated the AE characteristics of artificial joints during shear under CNS conditions. However, the AE characteristics of joints under variations of normal stiffness were not presented. [PERSON] et al.