# Strata Control by Roof Blasting for Bord and Pillar Mining Method in Mechanized Depillaring Panels
## 5 Strata Control by Roof Blasting  
Design of Roof Blasting for Inducing Caving  
Borehole CMPS 22 (Figure 2) is considered for the design of roof blasting to induce caving inside the goat. Considering 10% of the bulking factor, the height of the caving (18 m) will be 10 times the height of the extraction (1.8 m). The overlying strata mostly contain 12 m of fine-grained sandstone, 1-2 m medium-grained sandstone, and 2-3 m of shale/carbonaceous shale. The average core recovery is more than 92%, and RQD is also well above 90. The Cavability Index of overlying strata of around 10 times the working height, is found to be between 7000 and 8000, which is categorized as roof cavable with difficulty in nature, and requires induced caving by the blasting of the roof in case of overhang. The fine-grained sandstone of 12 m in the roof will delay the caving of the roof in the goat due to its massiveness of 12 m and a uniaxial compressive strength (40 MPa). It will also cause the development of higher induced stress along the line of extraction at around 8700 m\({}^{2}\) cumulative area of goat. It is required to blast this roof horizon to induce caving for regular roof fall. Furthermore, it is reported [38] that the nature of the immediate roof strata up to 20 m plays a significant role till the caving starts. It explained the process of  
Figure 21: RLH at goat edge during mechanized depillaring of panel.  
cantilevering after the first roof fall (break of the beam) and further collapse of the hanging cantilever of the roof strata with the progress of the extraction.  
Considering the geo-mining conditions of the panel, roof blasting of up to 9 m thickness includes 2 m shale, 1 m medium-grained sandstone, 3 m carbonaceous shale, and 3 m of fine-grained sandstone roof horizon, which is required to induce fracturing in the rock mass to facilitate regular caving. Roof blasting may be conducted around the junction toward the barrier pillar right from the start of pillar extraction, as shown in Figure 22. In order to induce fractures in the roof horizon, a vertical blast hole of up to 9 m of 42 mm diameter is to be drilled toward the inby of the junction in the goat to avoid any sort of damage to RBBLS and instruments installed at junctions, as shown in Figure 23. Details of the drilling pattern and explosive charging are shown in Figure 24 and Table 8. Six blast holes should be drilled in a row at a distance of 0.25 m from the adjacent edge of the pillar and at a distance of 1.0 m from each other. Additionally, they should be directed toward the inby of the junction in the goat to ensure the efficacy of RBBLS and remote observations of instrument readings. Drilling and charging at the junctions should be completed before they shift into goat. The blast hole is to be charged with a 32 mm diameter of explosive cartridge and a 6 mm detonating cord together, using plastic spacer pipes of 40 mm outer diameter, 38 mm inner diameter, and 6 m in length. The proposed method of charging blast holes is given in Figure 25.  
Figure 22: Schematic diagram of induced caving by roof blasting during extraction of second row of pillar after completion of extraction of Pillar No. 5.  
Figure 23: Sectional view of the drilled hole with explosive charging pattern.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Blast Design Parameters** & **Value** \\ \hline Diameter of blast hole & 42 mm \\ Number of holes per ring in a single blast & 6 \\ Depth of hole & 9.0 m \\ Spacing between holes in a row & 1.0 m \\ Distance from the side of the pillar & 0.25 m \\ Explosive charge length & 6 m \\ The diameter of each emulsion permitted cartridge & 32 mm \\ Length of each permitted emulsion cartridge & 200 mm \\ Number of cartridges in one hole of 6 m length & 30 \\ Explosive charge per hole & 6 kg \\ Total explosive charge per ring & 36 kg \\ \hline \hline \end{tabular}
\end{table}
Table 8: Proposed blast design parameters for induced caving.  
Figure 24: Plan view of drilling pattern for induced caving by blasting along the drilling plane.