The 25-meter back thickness rule applies under which type of rock?

The 25-meter back thickness rule applies under which type of rock?

• A. Massive, unbroken igneous rocks
• B. Sedimentary rocks
• C. Metamorphic rocks
• D. Fractured rocks

Answer: (A)

The 25-meter back thickness rule is about the stability of rock directly behind the working in a solid, continuous rock mass. It applies most reliably when the rock is massive and unbroken, such that it behaves like one solid block. In this situation, a 25-meter thickness of back can be assumed to carry loads without premature failure, given there are no significant fractures or weaknesses. When rock is fractured or has planes of weakness—such as many sedimentary or metamorphic settings—the rock mass can fail along joints or bedding planes well before reaching that thickness, so relying on a single 25-meter limit would be unsafe. Therefore, this rule is best applied to massive, unbroken igneous rocks.

The 25-meter back thickness rule is about the stability of rock directly behind the working in a solid, continuous rock mass. It applies most reliably when the rock is massive and unbroken, such that it behaves like one solid block. In this situation, a 25-meter thickness of back can be assumed to carry loads without premature failure, given there are no significant fractures or weaknesses. When rock is fractured or has planes of weakness—such as many sedimentary or metamorphic settings—the rock mass can fail along joints or bedding planes well before reaching that thickness, so relying on a single 25-meter limit would be unsafe. Therefore, this rule is best applied to massive, unbroken igneous rocks.