## 3 Quality Grade Survey of Surrounding Rock Mass
### Design of Roadway Support Parameters in Broken Rock Mass Areas
#### 3.4.2 Parameter Design of Anchored Mesh Supports  
The bolt diameter, length, spacing, and arrangement are optimized according to the geological conditions and on-site deformation measurement of the surrounding rock of the roadways in Shilu Iron Deposit. Surrounding rocks can remain stable under the anchored mesh support. Resin bolts consolidate the anchoring end of the bolt with the broken rock mass of the hole wall, which creates a cohesive unit through cementation. They exhibit superior shear and tensile strengths compared to other bolts. Therefore, resin bolts are employed. A rectangular metal mesh with dimensions of 80 mm by 80 mm is used. The parameter design of anchored mesh support involves bolt length, diameter, spacing, and anchoring depth.  
1. Calculation and Determination of the Bolt Length The bolt length is calculated as in Equation (9): \[L\geq L_{a}+L_{b}+L_{c}\lim_{x\to\infty},\] (9) where \(L\) is the bolt diameter, m; \(L_{a}\) is the exposed part of the bolt (\(L_{a}=0.10\) m); \(L_{b}\) is the effective length of the bolt (Pressure-free arch height h is taken for the roof bolt), m; and \(L_{c}\) is the total length of the anchor (\(L_{c}=0.6\) m).  
The effective length of the bolt is consistent with the radius of the crushing zone of the surrounding rocks, which ensures optimal support effect. The radius of the crushing zone of the roadway roof can be considered equivalent to the height of the cavity zone (\(L_{b}=\) h). The height of the cavity zone is calculated to be 1.45 m using Equation (10) for goals with spans less than 6 m according to the parabolic pressure arch theory [18]. The bolt length is calculated to be 2.15 m and subsequently rounded up to 2.2 m.  
\[h=\frac{a+b[ctg\alpha+ctg(45^{\circ}+\frac{\varphi}{2})]}{f}, \tag{10}\]  
where a is the length of the roadway (\(a=\) 4.5 m); b is the width of the roadway (\(b=\) 3.8 m); \(\alpha\) is the angle between the two sides and the ground (\(\alpha=\) 85\({}^{\circ}\)); \(\varphi\) is the internal friction angle of the rock (\(\varphi=\) 24.1\({}^{\circ}\)); and f is the stability coefficient of the surrounding rock (\(f=\) 3.16).  
Row and Column Spacing Design  
Bolts are arranged in an even pattern with uniform spacing both in rows and columns. The bolt spacing is calculated to be 1.08 m and rounded to 1.1 m using Equation (11), considering the weight of rocks suspended by each bolt.  
\[a=\sqrt{\frac{Q}{K\gamma L_{2}}}, \tag{11}\]  
where \(a\) is the spacing of the bolt, m; \(Q\) is the anchoring force determined by the tensile test, kN; \(\gamma\) is the average bulk weight of the surrounding rock (\(\gamma=\) 28.0 kN/m\({}^{3}\)); and \(K\) is the safety coefficient of the bolt (\(K=\) 2.0).  
Design of Bolt Diameters  
Bolt diameter d is expressed as Equation (11) based on the strength principles of bearing and anchoring force. It is calculated to be 19.6 mm and rounded to 20 mm.  
\[d=35.52\sqrt{\frac{Q}{\sigma_{t}}}, \tag{12}\]  
where \(d\) is the bolt diameter, mm; \(Q\) is the anchoring force determined by the tensile test, kN; and \(\sigma_{t}\) is the tensile strength of the bolt material, MPa.  
The surrounding rocks of the transportation roadways in the middle section of Baoxin-120 is re-supported according to the above support structure and parameters (Figure 5). The roadway exhibits an optimal support effect without issues such as roof caving, rib spalling, and damage to the bottom corners.  
Figure 5: Support design and effect: (**a**) support design; (**b**) supported roadway.