## 4 Determination of Parameters for Slit Pipe Anchor Net Spray Support  
In the combined support system of anchor, mesh, and sprayed concrete, the addition of a metal mesh enhances the crack resistance of the sprayed concrete layer while improving its overall flexural, tensile, and shear strength. This reinforcement helps to effectively seal the surrounding rock, preventing weathering and protecting anchor rods from water erosion. Additionally, it optimizes the joint support function of the anchor rods and metal mesh, ensuring a more stable roadway structure. The sprayed concrete also acts as an isolating layer, further safeguarding the anchor rods and metal mesh from environmental factors. For economic efficiency, a metal mesh size of 2000 mm \(\times\) 1000 mm with a 100 mm \(\times\) 100 mm mesh hole is selected. The drill bit specification used for excavation in the Makeng Iron Mine is \(\varphi\)38 mm, with the actual construction hole diameter ranging between 38 mm and 40 mm. For analysis, the 115 m level, section 28# through the vein, is chosen as the application site. Based on the loosening circle calculation, the loosening circle radius at the 115 m level for section 28# is 1.4 m. The roadway specifications are 3.9 m in width and 3.6 m in height. According to the suspension theory, after the excavation of the roadway, the original in situ stress in the surrounding rock redistributes, changing from the original three-dimensional stress to two-dimensional corresponding stress and creating a stress concentration area. In this area, the original joints, fissures, bedding, faults, and other discontinuities in the rock mass may further develop. Affected by this development, some rock loosening, sliding and fracture phenomena will occur. Further, new discontinuities may be generated, and caving may occur under the action of self-weight. The suspension function of the anchor bolt is to give sufficient restraint to the subregion at this time and anchor these loose and broken rocks in the hard and stable rock stratum inside the surrounding rock (Figure 4). Using the anchor suspension theory and inputting these parameters into the calculation formula (Equation (1)), it is determined that the distance should not be less than 1.02 m. Based on the mesh specifications, the distance between anchor rods is set to 1 m \(\times\) 1 m.  
\[\text{a}<\sqrt{\frac{\text{G}}{\text{kL}_{\text{b}}\gamma}} \tag{1}\]  
where: a--anchor bar spacing, row spacing, m;  
G is the anchor design anchoring force, taken as 70 KN/root;  
k is the safety factor, take 2;\(\rm L_{b}\) is the thickness of the unstable rock layer to be suspended, determined by the radius of the circle of loose surrounding rock, here taken as 1.4 m;  
\(\rm\gamma\) is the bulk weight of the rock, determined by the nature of the surrounding rock, here take 24 \(\rm KN/m^{3}\). The total length L of the anchor consists of three parts: the anchoring length \(\rm L_{a}\) of the anchor deeper into the more stable rock layer, the thickness \(\rm L_{b}\) of the unstable rock layer to be suspended and the thickness \(\rm L_{c}\) of the upper pallet and anchorage, where the thickness \(\rm L_{c}\) of the upper pallet and anchorage is taken as 0.15 m.  
\[\rm L=L_{a}+L_{b}+L_{c} \tag{2}\]  
The anchorage length \(\rm L_{a}\) for deeper penetration into the more stable rock layer is determined by Equation (3). The anchorage length is calculated to be 0.45 m, so the total length of the anchor L is 2 m.  
\[\rm L_{a}\geq K\frac{df_{s}}{4f_{c}} \tag{3}\]  
where: K is a safety factor, generally taken as 2;  
\(\rm d\) is the diameter of the anchor, 0.02 m;  
\(\rm f_{s}\) is the tensile strength of the anchor, taken as 315 MPa;  
\(\rm f_{c}\) is the bonding strength of anchor and cement paste, taken as 7 MPa. Spray layer damage is mainly dominated by cut layer damage and bonding damage.