## 3 Results
#### 3.2.3 Quantification of Excavation-Induced Fracture Zone  
The ZF1212 excavation roadway adopted drilling-blasting excavation. This method generates blast-induced transient energy shock during detonation, significantly expanding the surrounding rock damage scope [27].  
Under instantaneous intense dynamic impact loading from blasting, surrounding rock primarily experiences compressive and tensile failure. The damage zones radiate outward sequentially, forming a loosened fracture zone (\(j_{1}\)), a secondary fracture zone (\(j_{2}\)), and a crack initiation zone (\(j_{3}\)). blasting excavation charge configurations are categorized as either coupled charges or decoupled charges. Different charging methods induce distinct loading patterns on borehole walls, resulting in varying degrees of rock damage.  
Within the loosened fracture zone, coal-rock masses exhibit severe fragmentation with extensively interconnected fractures. The dominant failure mechanism under blasting is tensile failure under plane stress conditions [28]. The influence of excavation-induced hoop stresses is negligible here; only radial effects are considered. This failure mode satisfies the Drucker-Prager criterion under pure tensile conditions. Based on the radial stress expression derived from tensile failure under the Drucker-Prager criterion during blasting shockwave propagation, the radius R\(j_{1}\) of zone \(j_{1}\) is expressed as  
\[R_{j_{1}}=r_{b}\left[\frac{P(1-\sin\varphi)}{2c\cos\varphi}\right]^{\frac{1}{d}} \tag{51}\]  
In the formula, \(P\) represents the shockwave pressure, \(C\) represents the cohesion of the roof surrounding rock, \(\varphi\) represents the internal friction angle of the roof surrounding rock, \(a\) represents the stress wave attenuation index, and \(r_{b}\) represents the borehole radius.  
The secondary fracture zone contains numerous relatively wide fractures with limited propagation and connectivity [29]. Coal-rock masses in this zone experience combined tensile and compressive stresses. The boundary between zones \(j_{2}\) and \(j_{3}\) corresponds to the critical position where the rock mass loses its hoop-bearing capacity. By establishing the hoop stress equilibrium equation at this interface, the radius R\(j_{2}\) of zone \(j_{2}\) is derived as  
\[R_{j_{2}}=r_{b}\left(\frac{\lambda P}{\sigma_{b}}\right)^{\frac{1}{2}} \tag{52}\]  
In the formula, \(\lambda\) represents the lateral pressure coefficient, and \(\sigma_{b}\) represents the tensile strength of the coal-rock mass.  
When fractures in zone \(j_{2}\) propagate into the crack initiation zone (\(j_{3}\)), the radius R\(j_{3}\) of zone \(j_{3}\) is determined through the simultaneous solution and simplification of the critical stress intensity factor at crack tips [30], and the impact forces generated by blasting gases filling zone \(j_{1}\). This yields the expression  
\[R_{j_{2}}=R_{j_{2}}\left[\frac{2(1+\gamma){R_{j_{1}}}^{2}}{r_{b}(r_{b}+r_{c}) \rho_{\text{B}}D^{2}}\cdot\frac{P}{F}\right]^{2}\cdot\left(\frac{R_{j_{2}}}{r_ {b}}\right)^{-2\alpha} \tag{53}\]  
In the formula, \(\gamma\) represents the adiabatic exponent of detonation product expansion, \(r_{c}\) represents the charge radius (where \(r_{b}/r_{c}\) constitutes the radial decoupling coefficient of the charge), \(F\) represents the fracture length coefficient, \(\rho_{B}\) represents the explosive density, and \(D\) represents the detonation velocity of the explosive.  
The radii of blast-induced fracture zones were calculated under varying decoupling coefficients: \(R_{j1}\) (loosened fracture zone), \(R_{j2}\) (secondary fracture zone), and \(R_{j3}\) (crack initiation zone), with total radius \(\sum R_{ji}\) (combined excavation-induced fracture zone). The results are compiled in Table 3. At a decoupling coefficient of 1.25, the final calculated extents are \(y_{j}\) =7.052 m and \(z_{j}\) = 7.074 m. Notably, the crack initiation zone radius \(R_{j3}\) constitutes over 57% of the total blast-induced fracture radius \(\sum R_{ji}\).