## 3 Experimental Results  
### Deformation Characteristics  
Salt rock is a kind of \"soft\" rock, which does not show \"yield\" characteristics under triaxial stress conditions. The higher the confining pressure level, the more obvious this feature becomes [32]. A typical axial stress-strain relationship of a \"hard\" rock under a triaxial compression test is presented in Figure 2a [33]. The stages of crack closure, elastic behavior, stabilized-growth of cracks, and  
Figure 1: Triaxial compression loading test of rock salt specimens: (**a**) diagram of an integrated testing device with ultrasonic wave and acoustic emission (AE) activity; (**b**) installation of strain gauges, ultrasonic sensors, and AE sensors; and, (**c**) layout of six AE sensors. accelerated-growth of cracks can be clearly distinguished. However, these clear boundaries between the different stages are not present for rock salt, as shown in Figure 2b. Not only is the crack closure stage unclear, but also the peak stress cannot be easily detected. Even though its axial strain had exceeded 5%, rock salt still had a higher load carrying capacity. Based on the axial stress-strain curve and volume strain information, the deformation process of rock salt can be divided into four stages (Figure 2b): the elastic compression stage, the brittle-ductile transition with crack initiation, the brittle-ductile transition with damage initiation, and the plastic deformation and strain hardening stage [29; 34]. The details of these stages are, as follows. Stage I, the elastic compression stage (OB). When the rock was drilled from the deep underground, its original structures were disturbed due to the reduction of the surrounding stress [35]. In this stage, the salt crystal became more compact under compressive stress from three directions, which could be regarded as compensation for artificial structural disturbance that is caused by excavation. The elastic compression stage was a short-term and completely reversible process. The first part of this stage was primary crack closure. Subsequently, axial stress-strain curves and volumetric deformation were dominated by a linear change responding to the increasing of loading stress. The modulus of elasticity can be obtained by this linear slope of the axial stress-strain curve. This stage ended at the elastic linearity boundary, which is also called the crack initiation threshold (point B in Figure 2b). Figure 2: Deformation characteristics of rock under a triaxial loading test: (**a**) stress–strain curve for typical hard rock; and, (**b**) four deformation stages of rock salt under the confining pressure of 5 MPa. Stage II: brittle-ductile transition with crack initiation (BC). The salt samples experienced a transition from elastic deformation to plastic deformation in this region and the elastic deformation played a leading role. A few micro-cracks were generated with a controllable development, and the slippage among the salt crystals began to occur. The compressive volume strain of rock salt first significantly increased and then reached its peak at the end of this phase. This region began at point B, which was recorded as the crack initiation threshold, and it ended at point C, which was defined as the dilatancy boundary. During this process, AE activity began, while S-waves' and P-waves' velocities remained steady. Stage III: brittle-ductile transition with damage initiation. When the loading stress exceeded the dilatancy boundary, the lateral strain became much larger than the axial strain, which brought about a dilatancy to rock salt. In previous studies [29], rock permeability would sharply increase when dilatancy occurred, which posed a threat to the sealing ability for the gas storage. During this stage, the volumetric strain curve turned from the peak to 0, and the axial stress at the volume strain of 0 (point D in Figure 2b) was regarded as the short-term strength of the rock salt. This was a transitional phase from elastic deformation to plastic deformation, and the latter had a dominant position.