## 3 Procedure for Making Samples
### Test Loading  
The biaxial compression test was conducted using a true triaxial electro-hydraulic servo rock fracture seepage testing machine from the Nanjing Hydraulic Research Institute, as shown in Figure 8. The electro-hydraulic servo true triaxial shear test testing machine for rock mass is mainly composed of a host machine, seepage water cylinder, oil source, and electric control system. The equipment is equipped with precision stress and displacement sensors, as well as three sets of mutually perpendicular loading actuators. The range of these loading actuators is 100 t, and their pressure source is provided by a special oil pipe through an oil pump. The axial deformation and load are synchronously recorded by the built-in stress displacement sensor, with a maximum error of \(\pm\)1% for the load sensor and \(\pm\)0.0005 mm for the displacement sensor. Accurate measurement and recording of displacement and deformation can be achieved, and the sensor transmits the test data to the control system.  
Figure 6: Stress–strain curve of Brazilian disc splitting test.  
Figure 7: The hole-crack specimen and its perspective structure diagram.  
1. Host system  
The test host system consists of five major components, as shown in Figure 8; namely, the X-axis loading system, Y-axis loading system, normal loading system, base platform, and pressure rail. Both the X and Y loading systems and the normal loading system can independently apply force to the sample, with a static load of 1000 kN and an actuator stroke of 140 mm. The servo valve is precisely controlled. The X (Y) direction loading system consists of a servo actuator, anti-rotation device, oil cylinder support plate, pull rod, load sensor, deformation measurement system, reaction support plate, and pressure head. It achieves free movement in the X (Y) direction through a guide rail installed on the bottom platform. The pressure head structure is composed of a universal head and a pressure plate, which can effectively compensate for the test force bias caused by sample shape and position errors. The normal loading system mainly consists of servo actuators, anti-rotation devices, oil cylinder support plates, columns, load sensors, etc.  
2. Testing machine control system  
The computer can display and record the load, displacement, deformation, and other parameters generated during the experiment in full digital real-time, and draw the results into a test curve, as shown in Figure 9. The system can collect and store test conditions, sample parameters, and test data. The test process has protection functions such as overload and setting conditions.  
Figure 8: The true triaxial electro-hydraulic servo rock fracture seepage testing machine (from the Nanjing Hydraulic Research Institute). (**a**) 1. X-axis loading system, 2. Y-axis loading system, 3. normal loading system, 4. base platform, 5. pressure rail. (**b**) Instrument image.  
Figure 9: The control system.  
3. Hydraulic source  
The hydraulic source system consists of an oil tank, a 7.5 kW electric motor, a 10 L/min axial piston pump, a cooling system, a valve group, a control cabinet, etc., as shown in Figure 10. The valve group includes a one-way valve and a manual overflow valve, which can adjust the required pressure according to different tests and be observed through a pressure gauge. A precision oil filter is used to ensure the cleanliness of the system oil.  
This test uses an electro-hydraulic servo rock triaxial testing machine. After applying lubricating oil to the contact surface between the sample and the loading head, the sample is loaded with a predetermined load in both the axial and lateral directions, maintaining a value of \(\sigma_{1}=\sigma_{2}\). The axial load is continuously applied to the specimen while keeping the lateral load stable. The loading mode is carried out by displacement control at a rate of 0.005 mm/s until the specimen is damaged, and the loading mode is schematically shown in Figure 11.