The given question is about identifying the main load-bearing elements in bord and pillar mining. Let's break it down to understand the concept thoroughly:
Bord and Pillar Mining: This is a mining method used particularly in coal mines. It involves creating a grid of rooms (bords) separated by coal pillars. These pillars are crucial in maintaining the structural integrity of the mine. Here's a detailed step on why pillars are the main load-bearing elements:
- Definition: Bord and pillar mining involves cutting into the coal seam in a grid pattern and leaving behind pillars of coal that support the roof.
- Function of Pillars:
- Pillars are left between mined-out areas to support the overhead rock strata and prevent collapses. This makes them fundamental components in bearing the load.
- They act as primary supports to the roof, maintaining stability throughout the mining operation.
- Comparison with Other Options:
- Roof bolts: These are supplementary supports used to stabilize the roof but are not the main load-bearing elements.
- Timber supports: Like roof bolts, they provide additional support but do not bear the main load.
- Side walls: They are generally isolated during the mining process and do not serve as primary load-bearing structures.
- Conclusion: Based on the above explanation, it is clear that pillars are the main load-bearing elements in bord and pillar mining.
Thus, the correct answer is Pillars.