## 1 Introduction  
In geotechnical engineering, moisture-induced softening of saturated rocks is an important issue that has been studied at length. To date, many researchers tried to study its mechanism through various experimental tests. [PERSON] [1] in 1960 compared the different compressive strength in saturated, air-dried and 'completely' dry sandstone specimens, and the result showed that sandstone's strength decreased due to the water effect. After summing up the test results of 35 different categories of non-expansive clay sandstones, [PERSON] and [PERSON] [2] argued that, for moisture-induced softening, the main factor is the content of quartz and clay minerals, and the secondary factor is the microstructure of rock. The influence of water content on strength and elastic modulus of rocks was investigated by [PERSON] et al. [3]. Through laboratory tests and data analyses, [PERSON] et al. [4] deduced an empirical formula of rock's permeability coefficient and effective stress. [PERSON] et al. [5] reported that the limestone obtained from Fars Province, southern Iran suffered from a 70% reduction in UCS (uniaxial compressive strength) due to saturation. [PERSON] and [PERSON] [6] found out the water effect can result in the reduction of UCS in siltstone, mud stone, and marl. To igneous rocks, the reduction of UCS due to water effect can also be observed in diabase [7], tuffs [6; 8; 9] and granite [10; 11; 12]. [PERSON], [PERSON] and [PERSON] [12] investigated the effects of temperature and water saturation on Lac du Bonnet pink granite based on not only traditional mechanical properties, such as peak strength, Young's modulus and Poisson's ratio, but also the stress at the onset of stable crack growth and the stress at the onset of unstable crack growth. [PERSON], [PERSON] and [PERSON] [13] considered stress corrosion as a very important mechanism of crack growth thorough data analysis. Although the above examples of research were based on massive laboratory tests, the experimental data used were only macroscopical mechanical parameters such as uniaxial compressive strength and Young's modulus. Therefore, a microcosmic mechanism to describe the rock's moisture-induced softening is still required.  
In recent years, acoustic emission (AE) technology has been introduced to the analysis of mechanical properties of rocks and rock-like materials under compression [14; 15; 16; 17; 18], impact load [19], seismic load [20] and four-point bending tests [21; 22; 23; 24]. Many researchers studied micro-failure mechanism of rock by employing AE technology [17; 18; 21; 22; 23; 24; 25; 26; 27; 28]. AE signal contains a series of parameters such as ib-value [18; 21], AF(average frequency) and RA(Rise Time/Amplitude) [18; 21; 22; 23; 24], energy and RMS (Root Mean Square) [17], dominant frequency [29] and so on. In addition, among them, the domain frequency AF and RA have been proved to be associated with the mode of microscopic failure [22; 23; 24; 29; 30; 31].  
On account of the existing drawbacks of the moisture-induced softening research, this article focuses on investigating the mechanism of rock's softening from both macroscopic and microscopic perspectives through newly introduced waveform-based AE analysis, which involves a statistical analysis of the double dominant frequency bands of AE waveforms. First, macroscopic failure patterns of dried and saturated rocks under compression are compared and analyzed. Subsequently, differences between the failure mechanisms of dried and saturated rocks will be investigated. The findings in this article will facilitate the understanding of the intrinsic failure mechanisms of rock's moisture-induced softening.