jsa

Okay, let's break down this problem and why "overriding of pillars" is the correct answer. Here's the reasoning: * **The Scenario:** A massive sandstone roof *delays* the immediate caving of the goaf (the area from which coal has been extracted). This means the roof is essentially bridging across a larger unsupported span than normal. This, in turn, builds up a larger stress concentration on the pillars. * **Why other options are incorrect:** * **Coal Bump:** A coal bump is a sudden, violent failure of coal, often involving a release of energy. While increased stress on pillars *can* contribute to bump potential, the *delay* in roof fall is a key factor leaning toward overriding in this scenario. Bump is a sudden occurrence, whereas overriding is a more gradual process under this specific circumstance. * **Stiffening of Pillars:** Stiffening means that the pillars are gaining strength or resisting deformation, but it would mean the opposite of what happens in the question. * **Spalling of Pillars:** Spalling is the breaking off of small pieces or flakes from the surface of the pillar. While spalling *can* occur under high stress, it's not the primary mechanism described in the scenario. * **Why Overriding of Pillars is Correct:** * **Delayed Caving & Stress:** Because the sandstone roof is delaying the goaf fall

The correct answer is **overriding of pillars**. Here's why: * **Overriding of Pillars:** This occurs when the roof strata, particularly a massive and strong layer like sandstone, doesn't immediately cave in the goaf (the mined-out area). This results in a large overhang or cantilever. The weight and pressure from this massive overhang concentrate on the pillars closest to the goaf edge. This concentrated stress can cause the pillars to fail in compression, and the roof literally "overrides" them, pushing down and potentially causing the pillars to buckle and collapse. Here's why the other options are less likely: * **Coal Bump:** Coal bumps are sudden, violent failures of coal that can occur due to high stress concentrations. While a massive roof can contribute to high stresses, bumps are usually localized and not directly tied to crushing of pillars at the *outbye* (away from the working face) side due to a large goaf overhang. * **Stiffening of Pillars:** Stiffening implies an increase in resistance to deformation. Pillars don't "stiffen" under excessive load; they yield or fail. * **Spalling of Pillars:** Spalling is the breaking off of thin layers or fragments from the surface of the coal pillar. While spalling can occur under high stress, it's not the primary mechanism of failure when a massive roof causes pillar crushing due to a large goaf overhang. It'