When the roof of the mine working is broken or fractured, what minimum back or thickness is required?

• A. 25 meters
• B. 50 meters
• C. 75 meters
• D. 100 meters

Answer: (B)

When the roof is broken or fractured, the rock above the working becomes unstable and can fail further into the opening. A protective back—a resisting thickness of rock behind or above the working—is needed to carry the load, bridge over fractures, and stop additional rock from dropping into the space. The standard minimum back required in this situation is fifty meters. This thickness provides enough material to maintain stability and reduce the risk of a sudden collapse while work continues or temporary supports are in place. Smaller back would not reliably resist the loads, while larger back is more than needed for the minimum requirement.