# Strata Control by Roof Blasting for Bord and Pillar Mining Method in Mechanized Depillaring Panels
## 2 Empirical Approach to the Design of Underground Structures
### Width of Gallery
#### 2.2.2 Safe Cut-Out Distance  
Safe Cut-Out Distance (COD) is defined as the maximum length of the unsupported span between the advancing face and the nearest supported area, or the width between the side pillars, i.e., the width of excavation. As per the available literature review, [PERSON] [18] system (Figure 6) is found to be a suitable approach for estimating the safe COD (unsupported maximum span) during MD in the panel. The observed RMR value of the immediate roof strata of BCS is 50.40, with adjustments, as the seam was developed through solid blasting. The roof of the BCS is sandy shale, with an average value of elastic modulus of the immediate roof (E) as 2.0 GPa, and the width of the gallery has been fixed at 5.5 m.  
[PERSON] [18] used the Rock Mass Rating (RMR) developed by [PERSON] [19; 20; 21] for stand-up time and unsupported span of roof (Figure 6). The Coal Mine Roof Rating (CMRR) developed by [PERSON] and [PERSON] [22] was used to design cut-out distances in US underground mines. [PERSON] [23] suggested an empirical formulation (Equation (4)) to design cut-out distance based on the Coal Mine Roof Rating (CMRR), the width of the gallery, and depth. \[D=2.47+0.172R-0.046W-0.0008H \tag{4}\]  
where D is the cut-out distance (m), R is the Coal Mine Roof Rating, W is the width of the gallery (m), and H is the depth of cover (m). Using this relationship (Equation (4)), the cut-out distance is found to be 10.67 m for an RMR of 50.40, gallery width of 5.5 m, and maximum depth of cover of 266 m.  
[PERSON] et al. [24] proposed a relationship (Equation (5)) to estimate the safe convergence value based on COD, RMR, and width of the gallery (m). \[C=2.09\ W^{1.23}\ R^{-0.39}\ D^{0.11} \tag{5}\] \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multirow{2}{*}{**Structure**} & **H** & **W\({}_{1}\)** & **W\({}_{2}\)** & **W\({}_{e}\)** & **B** & **h** & **\(\sigma_{e}\)** & **Strength** & **Load** & **Safety** \\  & & **(m)** & **(m)** & **(m)** & **(m)** & **(m)** & **(m)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **Factor** \\ \hline Drilling and & \multirow{2}{*}{Pillar} & \multirow{2}{*}{266} & 35.7 & 31.2 & 33.30 & 3.8 & 1.8 & 23 & 41.14 & 8.25 & 4.98 \\ Blasting & & & 34 & 29.5 & 31.60 & 5.5 & & & 39.19 & 9.17 & 4.28 \\ \cline{2-11} CM & \multirow{2}{*}{Fender-I} & \multirow{2}{*}{266} & 10 & 29.5 & 14.9 & 5.5 & 1.8 & 23 & 20.09 & 12.23 & 1.64 \\ \cline{2-11}  & & & 18.5 & 29.5 & 22.7 & 5.5 & & & 29.04 & 10.24 & 2.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Safety factors of the developed pillars by drilling and blasting and reduced pillars by widening of galleries by Continuous Miner and fenders after their splitting. Figure 6: Stand-up time and unsupported span based on Rock Mass Rating (RMR) and Joint factor (Jr) [18]. where C is the convergence of the immediate roof (mm), W is the width of the gallery (m), R is the Rock Mass Rating, and D is the cut-out distance (m). For a safe convergence of 5 mm, this relationship estimates a cut-out distance of 16 m for a gallery width of 5.5 m and an RMR of 50.40. [PERSON] et al. [25] used the concept of [PERSON] [18] and [PERSON] [26] to design COD for three Indian coal mines. [PERSON] et al. [27] attempted to define the safe and limiting value of roof sagging of 5 mm for a safe design of COD. [PERSON] et al. [27] proposed a relationship (Equation (6)) to estimate the COD based on elastic modulus and width of the gallery, considering a few case studies of MD in Indian coalfields. \[D=14.61+1.98\mathrm{E}-2.12\mathrm{W} \tag{6}\]  
where D is the cut-out distance (m), E is the elastic modulus (GPa), and W is the width of the gallery (m). Using this relationship, the cut-out distance is found to be 9 m for a 5.5 m gallery width and 2.0 GPa elastic modulus of the immediate shale roof. Based on the (Figure 6) by [PERSON] [14], the estimated maximum unsupported span, i.e., COD for the development of galleries comes to be around 12.0 m for the existing conditions of roof rock mass above BCS.