## 3 Experimental Results  
### Deformation Features  
Salt rock has a very good creep ability and produces large deformation during loading. According to the test results, the axial strain of salt rock was greater than 0.5% before losing carrying capacity. Therefore, we adopted the modified section area to calculate the mechanical strength of salt rock in this study. In the tests, the axial and lateral strains were measured directly, whereas volume strain and crack volume were calculated from these values. Crack volume strain, \(\varepsilon_{\
u}^{c}\), was determined by Equation (1) [29]:  
\[\varepsilon_{\
u}^{c}=\varepsilon_{1}^{c}+\varepsilon_{2}^{c}+\varepsilon_{3} ^{c}=\varepsilon_{\
u}-\frac{(1-2\mu)}{E}(\sigma_{1}+\sigma_{2}+\sigma_{3}), \tag{1}\]  
where \(\sigma_{1}\), \(\sigma_{2}\), and \(\sigma_{3}\) are vertical space stresses, \(\sigma_{1}\) is in the axial direction, \(\sigma_{2}\), and \(\sigma_{3}\) are in the horizontal direction; \(\varepsilon_{1}^{c}\) is the crack strain in the axial direction, \(\varepsilon_{2}^{c}\) and \(\varepsilon_{3}^{c}\) are crack strains in the horizontal direction; \(\mu\) is the Poisson ratio and \(E\) is the elastic modulus. Based on the stress-strain curves, the rock deformation process can be divided into five stages: the compression fissures stage, the linear deformation stage, the stabilized-growth cracks stage, the accelerated-growth cracks stage, and the after peak stress stage, as given in Figure 4 for sample #2 (SA-2). **Stage 1, the compression fissures stage (OA):** OA was a short-term process and the stress range was approximately 0-20% of the uniaxial compression strength. In this stage, original cracks closed under compression, which led to a decrease in the crack volume of the specimens, but the compression volume strain increased slightly. Both P-wave and S-wave velocities rose slightly, and the P-wave velocity was approximately twice that of the S-wave velocity. The number of AE events in this stage was approximately 0.04% of the total number of AE events. This stage is completely reversible. _Stage 2, the linear deformation stage (AB):_ After the compression fissure stage, the compression volume strain increased, while the crack volume remained constant. The compression stress was approximately 21-45% of the uniaxial compression strength, and the volume strain had an obvious linear trend. The ultrasonic wave velocity was stable during this stage, and AE activity was weak, representing approximately 2.66% of the total AE events. _Stage 3, the stabilized-growth cracks stage (BC):_ When the stress was approximately 46-65% of the uniaxial compression strength, the linear stress-strain curve altered, cracks in rock samples began to generate and expand, and the compressed volume strain increased to a peak. Owing to  
Figure 3: (**a**) Diagram of ultrasonic velocity and AE testing for salt rock under uniaxial stress; (**b**) AE sensor’s layout, #1, #2, #3, and #4 = the sensor’s number. irreparable damage produced in rock samples, the beginning stress at point B was representative of rock damage stress, about 42-50% of peak strength (Table 1). At the end of this stage, the volume strain of rock samples reached their peak, indicating that inflection and rock dilatancy would happen next. Thus, the stress at point C was termed as dilatancy stress, and was 62-67% of peak strength (Table 1). The P-wave velocity fluctuated slightly, while the S-wave velocity began a gradual decline. AE activities became active and accounted for approximately 13.0% of the total AE activity. _Stage 4,the accelerated-growth cracks stage (CD):_ As the stress increased to a peak in the accelerated-growth cracks stage, the samples exhibited visco-plastic deformation and the axial stress-strain curve showed nonlinear characteristics. While this was short-term behavior, is caused serious damage. At this stage, the ultrasonic wave velocity declined sharply, and the S-wave decelerated more than that of P-waves.