Index Properties MCQ with answer and solution

Concept

Rock porosity depends on grain shape, sorting, packing and on mechanical processes like compaction, cementation and fracturing.

Formula

n = V_v / V × 100; σ_v = γ × z, where γ = G × γ_w × (1 - n)

Step-by-step solution

1. Check A: porosity changes with grain size distribution (sorting) and grain shape. Correct. 2. Check B: specific gravity gives unit weight, and unit weight × depth gives overburden stress. Correct. 3. Check C: compaction, crushing and fracturing (mechanical processes) change porosity. So C is NOT correct. 4. Check D: cubic packing of equal spheres gives about 47.6% porosity, the loosest packing. Correct. 5. Answer is C.

Explanation

Mechanical processes like compaction under load and fracturing change the pore space in rock. So the claim that porosity does not depend on them is wrong. All other statements are true.

Why the other options are wrong

A: True, size distribution and shape affect porosity. B: True, unit weight from specific gravity is used to find overburden stress. D: True, cubic packing gives the highest porosity (about 47.6%).

Shortcut

Cubic packing gives about 47.6% porosity (loosest). Rhombohedral packing gives about 26% (densest).

Common mistake

Thinking D is wrong. Cubic packing is the loosest packing, so its porosity is high.