## 4 Distinctions Between Failure Mechanisms of Dried and Saturated Rocks
### Microscopic Failure Mechanism  
Through the AE test, [PERSON] [31] proposed the theory that micro-shear failure generates a high frequency AE signal and micro-tensile failure releases a low frequency one. Based on that theory, the result of this paper shows that micro-shear and micro-tensile failure account for approximately 68% and 29%, respectively, of the total failure events during uniaxial compression for dried rocks. This observation indicates that micro-shear failure plays a dominant role for dried rock's failure under compression. This observation coincides with past findings for rock micro failure modes ([PERSON] et al. [32]; [PERSON] et al. [29]).  
Compared with dried rocks, the phenomenon of increasing micro-tensile failure events (from approximately 29% to 75% on average) and decreasing micro-shear failure events (from approximately 78% to 21% on average) can be found in saturated rocks. This observation shows that, for saturated rocks, the dominant category of micro failure under compression is varied from micro-shear to micro-tensile.  
The ranges of low and high frequency bands reflect the complexity of micro-tensile and micro-shear failures, respectively. Under compression, stress distribution around pores is changed due to the existence of pore water, which results in relatively complicated micro failure in saturated rocks.  
The roles of micro-tensile and micro-shear failures during the compression of rock can be reflected by the characteristics of cumulative low frequency and high frequency AE signals with stress level [33]. For dried rocks, micro-shear failures mainly contribute to the final macro-failure. In contrast, the existence of pore water in saturated rocks results in a more complicated accumulation of micro-shear and micro-tensile failure during compression. The relative relationship in magnitude between the percentages of micro-tensile and micro-shear failures is not stable during the whole progress of compression. Compared with dried rock, a significant increase in the number of micro failure events can be found in some saturated rocks at low stress level. This observation indicates fractures in the saturated rock could be initiated earlier than in the dried rock.  
In the view of energy release, it can be observed clearly that in dried specimen, most of the energy was released during the crack of micro-shear failure. On the other hand, micro-tensile failures contribute more energy release in saturated specimens. Considered that rock's tensile strength is lower than its shear strength, the increasing of ratio and energy of micro-tensile failures in saturated rock can be considered as a main cause of moisture-induced softening.  
Based on the research of [PERSON] et al. [21], AE waveforms with relatively high and low AF represents micro-tensile and -shear failure, respectively. The test results indicate that in the last stages  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Specimen No.** & \
\begin{tabular}{c} **Percentage of Energy of Low** \\ **Frequency Waveforms (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Percentage of Energy of High** \\ **Frequency Waveforms (\%)** \\ \end{tabular} \\ \hline DM1 & 13.99 & 79.28 \\ DM2 & 15.86 & 80.20 \\ DM4 & 27.03 & 71.21 \\ SM1 & 69.35 & 29.29 \\ SM2 & 84.71 & 15.02 \\ SM3 & 39.04 & 56.97 \\ SM4 & 64.62 & 35.17 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Percentage of energy of high and low dominant frequency waveforms in dried and saturated rock specimens.  
prior to the sample failure for normalized applied stress higher than 80%, the cracks in dried rocks are mainly micro-shear failures, whereas the ones in saturated rocks are a combination of tensile and shear failures.