## 2 Blasting Experiment Design  
To study the damage characteristics and fracture patterns of rock blasting under deep high in situ stress conditions, it is first necessary to achieve the coupled loading of blast-induced stress and in situ stress. The blast-induced stress is generated by detonating explosives within the borehole of the test specimen. To simulate the in situ stress state of deep rock masses, a loading device capable of applying different confining pressures to the specimen, as shown in Figure 1, was designed. The confining pressure loading device consists of an outer frame, four spacer blocks, and two sets of jacks. The outer frame measures 250 mm on the outer edge, 180 mm on the inner edge, and has a height of 100 mm. Each spacer block is derived by taking a rectangular solid measuring 132 mm in side length and 110 mm in height, removing a cylinder of 92 mm diameter and 110 mm height from the center, and then splitting it into four equal parts along the diagonal. Considering the pressure gradient required for loading, FPY-5 jacks with a range of 5 tons, a height of 42 mm, and a stroke of 8 mm were used, along with a hydraulic pump equipped with an electronic pressure gauge to apply pressure more accurately. The red sandstone used in the experiments was sourced from Ju County, Rizhao City, Shandong Province, China. The mineral composition of the sandstone consists of approximately 22.7% quartz (syn) and 77.3% albite low. The porosity of the sandstone is approximately 6%, and its uniaxial compressive strength is around 70 MPa. As shown in Figure 2, the red sandstone was processed into cylindrical specimens with a diameter of 90 mm and a height of 110 mm, with a borehole of a 4 mm diameter and 70 mm depth drilled in the center. As shown in Figure 3, nickel hydrazine nitrate was used as the explosive material. To ensure the uniform explosive density in the charge segment, 100 mgof explosive was first loaded into a transparent quartz tube with a length of 45 mm and an inner diameter of 2.5 mm, with a detonating cord inserted, and both ends of the quartz tube were sealed to prevent explosive leakage. The detonating cord consists of two copper wires with insulating coatings. First, one end of each copper wire was polished smooth and temporarily short-circuited. The other ends of the wires were then twisted together while ensuring that the insulating coatings remained intact. The ends of the twisted wires were then trimmed to ensure that the wire cores did not make contact, but the distance between the two ends is extremely small. This end of the copper wire was then inserted into the explosive material. Next, the quartz tube containing the explosive was placed in the borehole, and the borehole was sealed using a mixture of quartz sand and glue. Once the preparation was complete, as shown in Figure 4, the red sandstone specimen was wrapped in a film and placed in the confining pressure loading device. Afterward, the short circuit was removed, and the wires were connected to the positive and negative terminals of the detonator. Once the detonator was fully charged, it was activated, generating a voltage greater than 2500 V. The high voltage caused a spark at the ends of the wires, which, in turn, triggered the detonation of the explosive. The confining pressure loading device was then used to apply a preset pressure to the specimen to simulate the high in situ stress conditions of deep rock masses. And the explosive in the borehole was detonated using a high-voltage electric initiation method to apply blasting loading to the specimen. To investigate the effect of the in situ stress magnitude on the rock blasting performance, five sets of blasting model experiments with different in situ stress gradients were designed, with stress levels of 0 MPa, 0.5 MPa, 1 MPa, 2 MPa, and 3 MPa. As shown in Figure 5, the specimen can be divided along the borehole axis into the following three segments: the stemming segment, the charge segment, and the bottom segment.