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