## 3 Experimental Results
### Tension vs. Shear Failure Mode  
In the study of rockburst phenomena, analyzing crack propagation patterns is essential to understanding the mechanisms and evolution of rockbursts [23]. Through laboratory experiments that simulate strain-mode rockbursts, researchers can explore these propagation patterns to uncover the underlying mechanisms driving rock failure at both the micro and macro levels. This research not only deepens theoretical insights but also provides crucial guidance for hazard prediction and protective measures in engineering projects.  
The results of strain-mode rockburst simulation tests on granite specimens AG2 and AG3 are shown in Figure 5. The tests reveal that under splitting forces, multiple vertical cracks develop and propagate through the specimens, causing rock slabs to detach. Under shear forces, diagonal shear cracks form, with multiple shear cracks penetrating the specimens and breaking them into distinct rock fragments. When energy accumulates to a critical level, it is abruptly released, ejecting pieces of rock and leaving behind characteristic V-shaped rockburst craters. Meanwhile, the surface of the rock displays splitting cracks parallel to the free surface, with through-going shear cracks along the diagonal of the specimens. Finally, the free surface becomes the primary rockburst failure plane, while the opposite side, which is supported and constrained, remains mostly intact.  
The overall failure characteristics of the rock specimens in the triaxial compression test exhibited a tension-shear coexistence mechanism. The primary failure mode was characterized by a dominant shear crack cutting diagonally through the entire granite specimen at a 45\({}^{\circ}\) angle. Several vertical splitting cracks, aligned parallel to the principal stress, formed and expanded in the vicinity of the shear fracture. If these tensile cracks fully penetrate the rock, large slabs of granite may spall off. When the shear and tensile cracks intersect, the detached slabs are further split into multiple rock fragments. Additionally, near the free surface, a shear-tension composite failure mechanism becomes more pronounced and intense. When vertical tensile cracks reach a certain length, the stress distribution in the surrounding material is reconfigured. The free surface effect leads to an increase in shear stress near the free surface, which causes the cracks to progressively extend and propagate in a diagonal direction.  
This process manifests as large rock slab detachment accompanied by shear sliding, breaking into thinner rock fragments. These fragments, which stored excessive elastic potential energy, were violently ejected from the free surface. Rockburst craters formed V-shaped depressions or wedge-like indents on the free surface and displayed clear directional patterns and localized characteristics. Their formation is closely linked to the shear-tension composite crack propagation mechanism, the brittle characteristics of rock masses, and the influence of high-stress conditions. The V-shaped rockburst craters present an upside-down triangular wedge, characterized by sharp, tapered edges that vividly illustrate the progression of cracks from the surface into the interior. Its rough surface highlights the dynamic characteristics of rock failure under high strain rates. Inside the crater, the stepped structure reveals a gradual spalling pattern during the failure process. Generally, higher stress concentrations and more intense energy releases result in deeper craters with sharper edges.