In stable, massive, unbroken igneous rocks, what minimum back or thickness is required when a mine is driven under a river bed?

In stable, massive, unbroken igneous rocks, what minimum back or thickness is required when a mine is driven under a river bed?

• A. 15 meters
• B. 25 meters
• C. 35 meters
• D. 45 meters

Answer: (B)

The main idea here is how much rock must be left above an underground roadway when you go under a river. That rock above, the back, acts as a cushion to keep the roof stable and to resist water pressure from the river below. In stable, massive, unbroken igneous rock, the rock mass is strong and relatively uniform, so you don’t need an extremely thick back to keep the excavation safe. But you still need a substantial amount of rock to carry loads, prevent roof fall, and allow drainage or control of any seepage pressures from beneath the river. The middle option reflects a practical balance: it provides enough cushion to maintain stability and control groundwater, while not being so large that it unnecessarily raises excavation costs in this strong rock type. Choosing a thinner back would raise the risk of roof instability and inrush, while a much thicker back would be overly conservative for this rock quality.

The main idea here is how much rock must be left above an underground roadway when you go under a river. That rock above, the back, acts as a cushion to keep the roof stable and to resist water pressure from the river below. In stable, massive, unbroken igneous rock, the rock mass is strong and relatively uniform, so you don’t need an extremely thick back to keep the excavation safe. But you still need a substantial amount of rock to carry loads, prevent roof fall, and allow drainage or control of any seepage pressures from beneath the river. The middle option reflects a practical balance: it provides enough cushion to maintain stability and control groundwater, while not being so large that it unnecessarily raises excavation costs in this strong rock type. Choosing a thinner back would raise the risk of roof instability and inrush, while a much thicker back would be overly conservative for this rock quality.