## 5 Conclusions  
In this study, controlled laboratory experiments on salt rock samples under triaxial stress were performed to analyze the ultrasonic wave characteristics and AE activities during the rock damage process. The conclusions of the study are summarized, as follows:  
1. The deformation of rock salt under triaxial loading stress was divided into four stages with individual characteristics. The elastic compression stage was a short-term and reversible process, with a slight increase of ultrasonic wave velocity and few AE events. This stage ended at the crack initiation threshold, after which the brittle-ductile transition stage with crack initiation began with a controllable crack development. At this stage, the deformation was mainly due to the slippage of salt grains. AE activity began to increase, while the S-waves' and P-waves' velocities remained constant. When the loading stress exceeded the dilatancy boundary, the rock salt entered the brittle-ductile transition of damage initiation stage, with intense AE activity and fluctuation of ultrasonic wave velocities. The last \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Critical Strength/MPa**} & \multicolumn{4}{c}{**Confining Pressure**} \\ \cline{2-5}  & **5 MPa** & **10 MPa** & **15 MPa** & **20 MPa** \\ \hline Crack initiation threshold stress & 10.1 & 12.2 & 13.3 & 14.0 \\ Dilatancy boundary stress & 13.6 & 14.5 & 15.3 & 16.2 \\ Short-term strength & 16.9 & 18.1 & 19.7 & 22.2 \\ Damage ultimate stress & 27.1 & 29.9 & 41.2 & 52.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Four critical strengths of rock salt under triaxial compression test. stage was plastic deformation with strain hardening, and the sample exhibited a typical drum-shaped failure, however, it did not \"yield\" at the end of the test. The AE activity was especially pronounced, and the P-waves' and S-waves' velocities began to significantly decrease. 2. Ultrasonic wave velocity, crack density, ultrasonic wave amplitude, and attenuation coefficient have consistent changes with the damage process of the rock salt under the triaxial compression tests. However, the ultrasonic wave amplitude is recommended as the damage prediction index, not the ultrasonic wave velocity or crack density. The wave velocity attenuation is not significant before the plastic deformation phase, and the interpretation of the waveform information, especially S-waves, was confounded by the subjective influence of the tester. Additionally, the higher the confining pressure level, the smaller the ultrasonic wave fluctuation. 3. The confining pressure inhibited the crack expansion and AE activity. The higher the confining pressure, the lower the effective shear stress on the fracture surface and the less pronounced the AE events. Based on the AE activity and energy release characteristics, the damage ultimate stress was defined and then determined. It could be regarded as the triaxial compression strength and it could play a key role in the evaluation of the stability of underground engineering structures. 4. The ultrasonic wave test is an active testing method, and AE activity monitoring is a passive testing method. Both technologies can reveal the rock damage process in different ways. A mathematical model relating the two types of parameters should be obtained to precisely and accurately predict the rock damage evolution. This goal is the next research direction of the authors. Data curation, H.L., Z.D., W.Y. and H.Y. ; Formal analysis, H.L. ; Investigation, H.L. and Z.D. ; Methodology, Z.O. ; Resources, H.Y. ; Writing--original draft, W.Y. ; Writing--review & editing, B.L. The authors acknowledge the financial support of the National Natural Science Foundation for Young Scientists of China (Grant No. 41602328), the Natural Science Foundation of Hebei Province (Grant No. D2017210182 and No. E2018210046), and the collaborative innovation center of disaster prevention and mitigation for large infrastructure of Hebei province.