# Mechanical and Acoustic Emission (AE) Characteristics of Rocks under Biaxial Confinements
## 1 Introduction
Because the testing conditions are different in kinds of literature, so the change regular of the \(\sigma_{\text{bcs}}\) is not clear for different rocks. It is necessary to carry out in-depth studies on the \(\sigma_{\text{bcs}}\) of rocks. The end friction between the loading platen and rock specimen has a great effect on the failure properties of rocks in biaxial compression tests [23]. It is necessary to take anti-friction measures when the biaxial compression tests are conducted. Figure 1c shows two typical groups of biaxial compressive strength of rocks, and there are clear differences between the strength results without anti-friction measures and these with anti  
Figure 1: Stress state and the associated strength characteristics, including (**a**) biaxial stress state in rock around underground opening, (**b**) \(\sigma_{\text{bcs}}\) envelope curves, and (**c**) difference between rock strengths with anti-friction measures (AFM) and these without AFM. friction measures. In general, brush platen and solid steel platen pasted with molybdenum disulfide (MoS2) powder were the most used measures to reduce end friction in biaxial compression tests and it was observed that the end friction could increase the \(\sigma_{\text{bcs}}\), and the use of molybdenum disulfide to eliminate the end friction is more effective than brush platen [24]. It was also found that the 1:1 mixture of stearic acid and vaseline produces the lowest coefficient of friction compared to Teflon, graphite, and MoS2 [22]. In the biaxial compression tests, the failure surfaces of specimens without anti-friction measures are coarse and fine, and most of them are curved, indicating that the friction effect at the end limits the free propagation of cracks, while the failure of most specimens is spalling along the direction of \(\sigma_{1}\), and the failure surface is large and straight when \"1 mm thick PTFE gasket pasted with vaseline\" is used to anti-friction [25]. Additionally, the end friction effects markedly weakened with increasing confining pressure under the same anti-friction measures [8; 26; 27; 28]. Acoustic emission (AE) is a nondestructive inspection technique utilizing the transient elastic energy which results mainly from crack propagation events inside a material [29; 30; 31]. AE has been broadly applied to rock to obtain the information about crack initiation and propagation, failure mode, abrupt failure of the structure, and correspondingly some of the phenomena in time sequence for a detailed and systematic analysis [32; 33; 34; 35]. [PERSON] proposed that the distinct signature of the cracking modes is reflected on AE waveform parameters like the amplitude, RA-value (where RA is the ratio of rising time to the amplitude of AE wave), and average frequency, and achieved the conclusion that AE waveform signals generated in shear tests have a greater RA-value and a smaller average frequency than that of AE waveform signals generated in tensile tests [36; 37]. [PERSON] investigated the generation time of the initial crack in terms of the information of the AE events generated in the failure process of rock specimens and AE characteristics of rocks in lab experiments and accordingly established the generalized AE initiation and damage thresholds [38]. [PERSON] et al. studied AE activity during a catastrophic fracture of fault in rock and obtained the detailed time-space distribution of AE events from lab tests [39]. [PERSON] et al. discovered that the granite and marble specimens in tensile tests have different fracture surfaces and AE characteristics, and rock lithology played a vital role [40]. AE was applied to investigate the fracture and damage mechanisms of rock induced by the propagation of microcracks under triaxial compression test, and it was found that shear failure was a major microscopic failure mechanism of rock under triaxial compression [41]. [PERSON] et al. demonstrated that the precision and sensitivity of AE technology are high for detecting crack propagation in the rock under a direct shear test [42; 43].