# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 2 Specim and Testing Procedure
### Test Setup and Procedures  
A uniaxial compression test, triaxial compression test, and hydraulic coupled triaxial test were conducted in this paper. The hydraulic coupled triaxial test was a triaxial compression test with water injected inside the specimen through a water injection channel simultaneously inside the pressure chamber, so that the flaw surface was filled with water pressure (this can be seen as the loading condition in Figure 2). The entire experiment was conducted in the Key Laboratory of Geotechnical and Underground Engineering at the Ministry of Education, Tongji University, Shanghai 200092, China. The uniaxial compression test adopted a 300 kN rock mechanics servo pressure testing machine, which was made by Shanghai Hualong Test Instruments Co., LTD. The loading carried out with 0.02 mm/s until specimen failure. The conventional triaxial test and hydraulic coupled triaxial test were conducted using a GCTS RTX-3000 triaxial rock testing system (Figure 3). The RTX-3000 consists of a hydraulic station, a loading rack, a pressure chamber and a confining pressure and pore pressure supercharger, high-precision sensors, a microprocessor, and an analysis system. During the test, the axial stress was applied by a piston mounted, the confining pressure was applied by the confining fluid (silicone oil) in the pressure chamber, and the water pressure was applied by the pore pressure supercharger through the water injection channel. The hydraulic coupled triaxial test procedure consists of five steps: First, adjust the pressure cushion to contact the specimen boundary. Second, apply the confining pressure to the target level at a rate of 0.4 MPa/min. Third, inject water at a rate of 0.1 MPa/s into the specimen through the water injection channel until the water pressure reaches the target level, then keep the stress condition for a moment to make sure the water pressure fulfills the pre-existing flaws. After that, apply the vertical loading at a rate of 0.1 mm/min (0.1% per minute) until the specimen is fractured. Finally, unload the water pressure, the confining pressure, and the axial pressure alternately. In the triaxial test, the loading procedure skips step 3 (applying water pressure). During the procedure, data including the deviator stress, axial strain, and radial strain were recorded every 0.1 s.  
In order to prevent equipment damage caused by the fractured debris blocking the oil return hole, the specimens need to be wrapped in heat-shrinkable film so that the water pressure in the pre-existing  
Figure 1: (a) Schematic diagram of the specimen; (b) stress–strain curves for consistency evaluation. flaw should smaller than the confining pressure, or the water pressure will burst the film. Table 1 below illustrates the experimental scheme. In the triaxial test, the confining pressure was 2 MPa, 4 MPa, and 6 MPa. In the hydraulic coupled triaxial test, the confining pressure was 2 MPa, 4 MPa, and 6 MPa, while the water pressure was 1 MPa, 3 MPa, and 5 MPa; the water pressure lower than the confining pressure. Throughout this paper, specimens are described by specimen number shown in Table 1. For example, \"30-UCT\" denotes a 30\({}^{\circ}\) specimen under uniaxial compression test; \"45-4\" denotes a 45\({}^{\circ}\) specimen under conventional triaxial compression test and the confining pressure (CP) is 4 MPa; \"60-6-5\" denotes a 60\({}^{\circ}\) specimen under hydraulic coupled triaxial test with 6 MPa confining pressure (CP) and 5 MPa water pressure (WP). Figure 3: GCTS RTX-3000 triaxial rock testing system. Figure 2: The loading condition of the hydraulic coupled triaxial test. After the test, X-ray computed tomography (CT) was employed to investigate the internal cracks. The CT observation was carried out on Phoenix V|tome x m in Yinghua Inspection and testing (shanghai) Co., Ltd. The Phoenix V|tome x m is a high precision 16-bit micro-focus X-ray CT scanning device that enables high definition images and observes the internal fracture details.