Effect of Fissure Penetration Degrees on Spalling Process and Feature of Surrounding Rock in Rectangular Tunnel Under True Triaxial Loading  
[PERSON]  
Correspondence: [EMAIL_ADDRESS] School of Resources and Safety Engineering, Central South University, Changsha 410083, China;  
[EMAIL_ADDRESS]  
[PERSON] Luo  
mechanisms of structural planes [28]: fault-slip bursts, shear rupture bursts, and tensile buckling bursts. In fact, numerous scholars have investigated the mechanisms and characteristics of spalling in deep mine tunnels using in situ tests [28; 29], laboratory experiments, and computational simulations. In terms of laboratory experiments, [PERSON] et al. [30] conducted uniaxial compression tests on rectangular prismatic granite specimens to explore how height-to-width ratios influence slab cracking. They found that in shorter specimens, cracks propagated parallel to the loading direction, forming slab cracks, whereas in longer specimens, failure occurred as shear cracks. Similarly, [PERSON] et al. [31] conducted biaxial compression tests on horseshoe-shaped tunnel models, proposed formulas for predicting cracking stress and critical strain and identified two main stages of spalling: instantaneous cracking and layer-by-layer spalling. In another set of experiments, [PERSON] et al. [32; 33; 34] used a true triaxial test system on cubic red sandstone and granite specimens with prefabricated round holes to categorize the rockburst process in tunnel sidewalls. They identified four stages: the calm period, pellet ejection, rock fragment exfoliation, and rockburst. These findings, including the formation of a symmetrical V-shaped failure zone, were consistent with those of [PERSON] et al. [35; 36; 37; 38], who observed similar rockburst patterns in cubic sandstone and granite samples. Further, [PERSON] et al. [16] used a unilateral void-true triaxial environment and studied how different loading rates affected slab failure in rectangular prismatic granite specimens. They found that as loading rates increased, failure shifted from static to dynamic, accompanied by an increase in the kinetic energy of ejected fragments. [PERSON] et al. [36] also examined the effect of loading rates on spalling in cubic red sandstone, concluding that lower rates increased failure likelihood, while higher rates intensified rockburst severity. [PERSON] et al. [12; 13] used true triaxial compression tests to explore the failure mechanisms of hard rock tunnel slabs with varying arch structures. They found that higher vertical stress occurred when the arch height/roadway width ratio was \(1/3\). As axial stress increased, the surrounding rock showed greater failure strength and more violent fragment ejection. Finally, [PERSON] et al. [39] applied numerical simulations used numerical simulation software at the Mine-by Tunnel in Manitoba, Canada, compared acoustic emission events with on-site microseismic signals, and studied the role of the intermediate principal stress in rock damage development. Figure 1: \"11.28\" rockburst encountered in Jinping II Hydropower Station [26]. The previous analysis highlights that, while significant research has been conducted on deep, hard-rock rockbursts, several issues remain unaddressed:  
1. Based on previous studies, most research has used circular holes and cubic specimens to simulate the failure of circular tunnels, but there is limited research on rectangular tunnels. As the use of rectangular tunnels and shafts increases in mining and deep engineering projects, it is essential to investigate spalling in the surrounding rock of rectangular tunnels. 2. The spalling failure process in deep mine tunnels is inevitably influenced by complex geological conditions, including bedding, discontinuous joints, and faults. Many rock mass engineering disasters result from the expansion and interconnection of fractures in the rock mass [40; 41; 42; 43].