## 2 Experiments
### Experimental Protocol  
In underground engineering projects, such as tunnels or mining operations, the excavation process often leads to uneven stress distribution around exposed rock surfaces. Stress levels near these exposed surfaces, particularly in the direction of excavation, drop significantly [22]. This localized stress concentration and steep stress gradient are primary external factors that can lead to strain-mode rockbursts. In the study of rockbursts, true triaxial loading has emerged as a precise and widely used experimental method. It is particularly advantageous for studying the mechanical mechanisms and failure characteristics of strain-mode rockbursts in deep engineering projects. This technique allows researchers to apply independent stresses along three perpendicular directions, enabling accurate simulation of the non-uniform stress states in deep underground rock masses. For strain-mode rockbursts, rock failure near exposed surfaces often manifests as directional splitting and slabbing. Moreover, true triaxial loading makes it possible to simulate these conditions by reducing the stress on the exposed surface to nearly zero while maintaining high stress levels in other directions. This ability to replicate the directional failure patterns associated with strain-mode rockbursts provides a robust experimental platform for uncovering their underlying mechanisms. The experiment was performed using a high-stress servo true triaxial loading and unloading test system, as illustrated in Figure 2. This true triaxial machine, developed independently by Guangxi University, has a vertical stiffness of 9000 kN/mm and a horizontal stiffness of 5000 kN/mm. The maximum vertical loading capacity of the machine is 5000 kN, while the maximum horizontal loading capacity is 3000 kN. During the testing process, a PCI-II acoustic emission monitoring system with multiple channels (manufactured by the U.S. Physical Acoustics Corporation, West Windsor Township, NJ, USA) and a GigaView high-speed camera (manufactured by U.K. Southern Vision Systems, Rochester, England) were employed simultaneously. The acoustic emission monitoring system has a maximum sampling frequency of 40 MHz and an 18-bit digital-to-analog conversion capability. It can capture the complete waveform of acoustic emission and record various characteristic values such as count and energy. The high-speed camera was used to observe and record the experimental phenomena, while the AE monitoring instrument provided information on the development of microcracks and the release of energy. Figure 1: The granite specimens AG2 and AG3. In the calibration and parameter settings of the acoustic emission monitoring system, the threshold has been set at 40 dB to accommodate environmental noise; the frequency range has been defined as 20 kHz to 1 MHz with a frequency resolution of 1 kHz; sensitivity has been adjusted to \(-50\) dB/V; and gain has been established at 20 dB. Signal processing is conducted using a bandpass filter with a low cutoff frequency of 20 kHz and a high cutoff frequency of 1 MHz. The data sampling frequency has been established at 1 MHz, the dynamic range has been defined as 80 dB, and the recording interval has been set to every 100 ms, with a storage capacity of no less than 1 GB. These parameter settings are designed to ensure the efficiency and accuracy of the system during the testing process. To simulate the evolution process of strain-mode rockbursts under complex excavating stress conditions, the researchers used the true triaxial experimental system to apply loading on five surfaces of the rectangular granite sample measuring 100 mm \(\times\) 100 mm \(\times\) 200 mm, leaving one surface free (as depicted in Figure 3). The stress on the rock specimen can be categorized into three types: vertical stress \(\sigma_{z}\), transverse stress \(\sigma_{x}\), and longitudinal stress \(\sigma_{y}\). Before the test, load blocks were placed between the specimen and the triaxial testing machine.