# Strata Control by Roof Blasting for Bord and Pillar Mining Method in Mechanized Depillaring Panels
## 7 Conclusions and Recommendations  
Mechanized depillaring with continuous miner technology has proven its effectiveness in Indian coalfields by improving strata control management, primarily due to the faster extraction rate. An attempt has been made in this paper to predict the safe design of underground structures involved for better strata control while working with Continuous Miner Technology based on available empirical and numerical simulation studies. A numerical modeling study helped in fixing the gallery width to 5.5. m and COD of 11 m, considering 5 mm as the permissible limiting value of roof sagging to induce permanent movement (plastic/loose in nature) in the split/original galleries to be driven. A stress-strain curve was observed on the numerical models to estimate an optimum size of rib/snook such that it fails in a controlled manner without inhibiting roof falls after the completion of the slicing operation. Here, 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) was proposed to be left during MD operation, which was 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 side spalling of pillars based on permissible limiting FOS of 0.30-0.35. It was found to be matching with the values estimated using empirical formulations. RLH was measured at different locations for the design of breaker-line support in the numerical model at a distance of 1.0 m outby from the goat edge without using applied supports in the model, which was found to be 1.5 m (3.75 t/m\({}^{2}\)). The SSF of the proposed support plan at the goat edge was found to be 2.50 for an applied support resistance of 67.87 t/m\({}^{2}\), and considering the dynamic effects at the goat edge, it was found to be matching with the values estimated using empirical formulations. An empirical study estimated an average hanging roof span of 32 m (around 8700 m\({}^{2}\) for a panel width of 271 m) was found to be inducing the first major fall of the beam roof to form a cantilever. Furthermore, a roof span of around 21 m (around 5687 m\({}^{2}\) for a panel width of 271 m) was found to be causing the subsequent goafing and caring of the cantilever. However, roof falls will not take place during the initial five rows of pillar extraction, even with a roof span of 8700 m\({}^{2}\) or more, due to insufficient panel width of one/two pillars. The first main roof fall was expected to occur during or before the extraction of the pillar in the eighth row. The Cavability Index of overlying strata of around 10 times the working height was found to be between 7000 and 8000. This categorized the roof as cavable with difficulty in nature and required induced caving by the blasting of the roof to avoid overhang. Fine-grained sandstone, measuring 12 m in the roof, was expected to delay the caving of the roof in the goat due to its massiveness of 12 m and uniaxial compressive strength (40 MPa). Roof blasting of up to 9 m in the roof horizon, which includes 2 m of shale, 1 m of medium-grained sandstone, 3 m of carbonaceous shale, and 3 m of fine-grained sandstone roof horizon, was required to be conducted to induce fracturing in the rock mass to facilitate regular caving. In order to induce fractures in the roof horizon, a vertical blast hole up to 9 m of 42 mm diameter was required to be drilled toward the inby of the junction in the goaf to avoid any sort of damage to RBBLS and instruments installed at junctions. Six blast holes were required to 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 toward the inby of the junction in the goaf to ensure the efficacy of RBBLS and remote observations of reading of instruments. Drilling and charging at the junctions were required to be completed before their shifting into goaf with the progress of pillar extraction.