## 1 Introduction
[31] identified a sharp permeability rise in coal when deviatoric stress exceeded its compressive strength during triaxial loading/unloading cycles. Studies by [PERSON] and [PERSON] et al. [32; 33] linked permeability evolution to adsorption-induced matrix swelling and pressure-driven pore deformation, highlighting synergistic effects of gas adsorption and mechanical deformation. Further, [PERSON] et al. [34] and [PERSON] et al. [35] emphasized the role of anisotropy, showing that permeability along the strike direction is higher than other orientations due to preferential fracture alignment. [PERSON] and [PERSON] et al. [36; 37] attributed nonmonotonic permeability changes to temperature effects, where gas-saturated coal exhibited matrix swelling, shrinkage, and microfracture generation under increased temperature. They pointed out that excessive thermal stresses are the essential causes of the coal damage and gas permeability enhancement. And permeability is further modulated by geological factors such as vitrinite reflectance and structural discontinuities (faults, folds) [38; 39]. In addition, plentiful permeability theoretical models were established, including the empirical permeability models (EPM), volumetric strain permeability models (VSPM), plastic strain permeability models (PSPM) and damage variable permeability models (DVPM) [40]. Furthermore, significant advancements have been made in clarifying the effects of gas seepage on coal's physico-mechanical properties and coal fragmentation. Under unloading conditions, [PERSON] et al. [41] demonstrated that elevated gas pressure accelerated coal deformation and seepage velocity while reducing strength, establishing a power-law relationship between confining pressure and Poisson's ratio during destabilization. [PERSON] et al. [42] pioneered the triaxial seepage tests, revealing that gas seepage weakened coal strength, accelerated failure, and generated high-pressure gas zones capable of storing critical outburst energy. In addition, [PERSON] [43] and [PERSON] et al. [44] attributed mechanical deterioration to adsorption-induced structural softening, where gas adsorption disrupted coal's rigidity. [PERSON] et al. [24] believed that adsorbate-matrix interactions significantly reduced surface energy, leading to the deterioration of coal properties, consistent with the \"Rehbinder effect\" [45]. [PERSON] and [PERSON] et al. [46,47] validated gas-weakening effects through triaxial tests, revealing that increased gas pressure reduced compressive strength, elastic modulus, peak strain, and bearing capacity, while elevating Poisson's ratio. Moreover, [PERSON] and [PERSON] et al. [48] utilized shock tube experiments to demonstrate that gas-driven coal continuous failure exhibited multi-layered tensile cracking, predominantly governed by free gas seepage. [PERSON] et al. [49] modeled gas-saturated coal failure under sudden pressure relief using a rupture wave framework, emphasizing transient stress wave propagation. The study of [PERSON] et al. [50] indicated that free gas energy was primarily consumed during coal fragmentation, highlighting energy partitioning in outburst mechanisms. Finally, [PERSON] [51] and [PERSON] [52] simulated abrupt gas desorption in shock tubes, showing that rapid pressure drops triggered instantaneous coal fragmentation and ejection, with high ejection velocities in high-gas-pressure scenarios. Despite significant advancements in characterizing gas permeability affected by multifactorial conditions, the gas seepage regularity in structural bituminous coals is still inadequately characterized. Current research predominantly focuses on permeability dependencies (e.g., stress state, gas type) and mechanical deformation effects, while investigations into gas seepage characteristics of heterogeneous bituminous coals and their role in promoting coal fragmentation and outburst initiation remain rarely reported.