## 3 Quality Grade Survey of Surrounding Rock Mass
### Design of Roadway Support Parameters in Broken Rock Mass Areas  
#### 3.4.1 Shotcrete Support Design  
The critical aspect of the concrete design is to determine the thickness of the spray layer based on the survival conditions of the deep roadway in Shilu Iron Deposit. Shotcrete supports prevent dangerous rocks from falling, which maintains roadway stability. Therefore, the thickness of shotcrete is determined based on the stability of these dangerous rocks. In other words, the bond strength of the concrete should surpass shear strength. The thickness can be calculated using Equation (5):  
\[T=\frac{GK}{0.75R_{t}S}, \tag{5}\]  
where \(T\) is the thickness of the spray layer, m; \(G\) is the gravity of the dangerous rock, \(N\); \(K\) is the full coefficient (generally, \(K=3\)); \(S\) is the circumference of the contact surface between the dangerous rock and the spray layer, m; and \(R_{t}\) is the tensile strength of the spray layer, Pa.  
Radius Rr of the broken ring is calculated according to the block caving theory (Equation (6)) [17].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Rock Mass Quality Grading** & **II** & **III** & **IV** & **V** \\ \hline \multirow{3}{*}{Support forms} & Plain spray or single anchor & Bolt-shotcrete or bolt-mesh & Anchor mesh spray & Combined support \\  & Unstable roadway engineering or roadways with a short service life & Locally fractured one body or rock mass & Partially broken roadways and long service life and poor stability & Severely broken roadways and unstable roadways with a long service life and poor stability & Severely broken roadways and unstable roadways \\ \hline \hline \end{tabular}
\end{table}
Table 5: Rock mass quality grading and support forms.  
\[R_{\rm r}=R_{0}\bigg{[}(1-\sin\varphi_{0})\frac{\gamma_{1}Z+c_{0}.ckg\varphi_{0}}{c _{0}.ckg\varphi_{0}}\bigg{]}^{\frac{1-\sin\varphi_{0}}{2\sin\varphi_{0}}}, \tag{6}\]  
where \(\gamma_{1}\) is the bulk weight of the overlying rock formation (weighted average); \(Z\) is the distance from the roadway center to the surface, m; \(c_{0}\) is the adhesion of the rock, N; \(\varphi_{0}\) is the internal friction angle of the rock, \({}^{\circ}\); and \(R_{0}\) is the radius of the outer circle of the roadway, m.  
The top pressure per meter of roadway is calculated (Equation (7)) [17]:  
\[Q_{d}\approx 2a\cdot\eta(R_{w}-h)\gamma, \tag{7}\]  
where \(a\) is half of the upper width of the roadway excavation section, m; \(\eta\) is the correction coefficient of roadway width; \(R_{w}\) is the radius of the surrounding rock crushing ring, m; \(h\) is the height from the circle center to the roof, m; and \(\gamma\) is the bulk weight of the roof rock, kg/m\({}^{3}\).  
The roof pressure per unit area is calculated (Equation (8)) [17]:  
\[q_{d}=\frac{Q_{d}}{2a}=\gamma\cdot\eta\big{(}R_{p}-h\big{)}. \tag{8}\]  
The internal friction angle of the two overburden rock masses is 24.1\({}^{\circ}\) at the top and bottom of the roadway; the average bulk weight of the overburden rock formation \(\gamma=2.70\) t/m\({}^{3}\); the radius of the three-centered arch of the roadway \(R_{0}=2.47\) m; and \(h=2.13\) m. According to Equation (6), crushing ring's radius \(R_{p}=4.16\) m; roadway's roof pressure \(q_{d}=7.18\) t/m\({}^{2}\); and \(Q_{d}=0.18\) MPa.  
The thickness of the sprayed shotcrete layer in Shilu Iron Deposit is calculated to be 53.02 mm and determined as 55 mm using Equation (8) when \(G=15\) kN and the roadway roof is almost load-free.  
The shotcrete parameters of Shilu Iron Deposit align with standard support specifications. Wet concrete injection machinery is used for shotcrete application. The raw materials consist of Portland cement, medium-fine sand with particles smaller than 2.5 mm, and stones no larger than 15 mm. These components are mixed in a weight ratio of 1:2:2. The water-cement ratio is between 0.3 and 0.4. A quick-setting agent is added at 3-5% of concentration to optimize coagulation.