# Strata Control by Roof Blasting for Bord and Pillar Mining Method in Mechanized Depillaring Panels
## 2 Empirical Approach to the Design of Underground Structures
### Design of Rib/Snook  
Recently, empirical formulas [1; 2; 3] have been developed to estimate the strength, area, width-to-height ratio, and equivalent width of rib/snook for Continuous Miners based on case studies of 39 mechanized depillaring panels of different Indian coalfields (Equations (11)-(14)), considering safety factor of rib/snook of 0.30-0.35. \[S_{snook}=15.25\frac{w^{0.58}}{h^{1.14}H^{0.17}} \tag{11}\]  
\[A=0.16\ H^{0.89}\ R^{0.14}\ h^{0.31} \tag{12}\]  
\[\frac{w_{e}}{h}=0.18\ H^{0.15}\ R^{0.10}\ h^{-1.03}\ A^{0.60} \tag{13}\]  
\[w_{e}=0.18\ H^{0.15}\ R^{0.10}\ h^{-0.03}\ A^{0.60} \tag{14}\]  
where S\({}_{\text{snook}}\) = strength of rib/snook (MPa), w = equivalent width (m), H = depth of cover (m), and h = height of extraction (m), A = area, R = nature of roof in terms of RMR, \(w_{e}\) = equivalent width (m). An attempt is also made to estimate the strength of different sizes of rib/snook using formulations (Equations (12)-(14)). The strength of different sizes of rib/snook (Figure 10) formed during MD in the panel is also presented in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Place**} & \multicolumn{1}{c}{**Parameters**} & \multicolumn{1}{c}{**Development by Drilling**} & \multicolumn{1}{c}{**Development by**} \\  & **and blasting** & **Continuous Miner** \\ \hline \multirow{8}{*}{Gallery} & Width (m) & 3.80 & 5.50 \\  & Dry density of rock (t/m\({}^{3}\)) & 2.51 & 2.51 \\  & Rock Mass Rating & 50.40 & 50.40 \\  & Anchorage strength of resin–grouped roof bolts (t) & 10.00 & 20.00 \\  & Rock load (t/m\({}^{2}\)) & 3.27 & 4.74 \\  & Spacing between two bolts in a row (m) & 1.20 & 1.10 \\  & Spacing between two consecutive rows of bolts (m) & 0.80 & 1.10 \\  & Number of roof bolts in a row & 3 & 5 \\  & Area supported by one row of bolts (m\({}^{2}\)) & 4.56 & 6.05 \\  & Support resistance (t/m\({}^{2}\)) & 6.58 & 16.53 \\  & Support safety factor & 2.01 & 3.49 \\  & Rock load (t/m\({}^{2}\)) & 4.91 & 7.11 \\  & Area supported by all bolts (m\({}^{2}\)) & 14.44 & 30.25 \\ Junction & Number of roof bolts & 11 & 33 \\  & Support resistance (t/m\({}^{2}\)) & 7.62 & 21.82 \\  & Support safety factor & 1.55 & 3.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Support design for developed panel. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Size of Rib/Snook**} & \multicolumn{1}{c}{**Area (m\({}^{2}\))**} & \multicolumn{1}{c}{**Perimeter (m)**} & \multicolumn{1}{c}{**Equivalent**} & \multicolumn{1}{c}{**Width-to-Height**} & \multirow{2}{*}{**Strength (MPa)**} \\  & **Ratio** & & & & \\ \hline
5.00 m \(\times\) 18.50 m \(\times\) 3.00 m & 101.60 & 46.84 & 8.68 & 4.82 & 10.57 \\
4.00 m \(\times\) 18.50 m \(\times\) 3.00 m & 91.60 & 44.84 & 8.17 & 4.54 & 10.21 \\
3.00 m \(\times\) 18.50 m \(\times\) 3.00 m & 81.60 & 42.84 & 7.62 & 4.23 & 9.81 \\
2.00 m \(\times\) 18.50 m \(\times\) 3.00 m &For the panel, H = 266 m (maximum), RMR (R) = 50.40, and h = 1.80 m, the competent area of the outby rib/snook (A) = 47.83 m\({}^{2}\), having an equivalent width of 6.16 m with a width-to-height ratio of around 3.42, is required during the extraction of pillars by CM. Therefore, a rib/snook size of 2.00 m \(\times\) 18.50 m \(\times\) 3.00 m, having an area of around 71.60 m\({}^{2}\) with a width-to-height ratio of around 3.81, is found to be suitable for the considered geo-mining conditions of the panel. This size of rib/snook is the minimum area to be maintained during judicious reduction, including the effects of pillar spalling (plastic and loose zone). Therefore, a rib/snook size of 3.00 m \(\times\) 18.50 m \(\times\) 3.00 m (area of 81.60 m\({}^{2}\), equivalent width of 7.62 m, and width-to-height ratio of 4.23) is proposed to be left during MD operation, which may be further reduced up to 2.00 m \(\times\) 18.50 m \(\times\) 3.00 m (area of 71.60 m\({}^{2}\), equivalent width of 6.86 m, and width-to-height ratio of 3.81) due to induced stress-driven side spalling of pillars/fenders.