Study the following statements with respect to compaction of soils and identify the correct option. Statement A: Fine-grained soils attain a much higher density and lower optimum water content when compared to well graded coarse-grained soils. Statement B: Compacted density of soil increases as water content increased, till a maximum dry density is achieved.

Asked in SSC JE 06/11/2024 (Morning)

Detailed Explanation

Soil type strongly affects the compaction curve. Well-graded coarse-grained soils (gravels and sands with a good range of sizes) let smaller particles fill the gaps between larger ones, so they pack very densely and need little water to do it. Fine-grained soils (silts and clays) have a huge surface area that holds water, so they need more water and still reach a lower dry density.

Why “Statement B is correct and A is incorrect” Is Correct

Statement A is incorrect because it states the opposite: it is the well-graded coarse soil that attains the higher maximum dry density and the lower OMC. Statement B is correct: on the dry side, increasing water content increases the compacted dry density until the maximum dry density is reached at the OMC. Hence ‘B correct, A incorrect’.

Why the Other Options Are Wrong

• B. Both statements are correct: Both cannot be correct, because statement A swaps the behaviour of fine and coarse soils.
• C. Both statements are incorrect: Statement B is a correct description of the rising (dry-side) part of the compaction curve.
• D. Statement A is correct and B is incorrect: This option keeps the wrong statement and rejects the right one.

Key Concepts for Students

• Typical MDD: well-graded gravelly sand about 2.0–2.2 g/cm³; clay about 1.5–1.7 g/cm³.
• Uniformly graded (poorly graded) sands give flatter, lower curves than well-graded ones.
• Plasticity and clay mineral type (e.g. montmorillonite) raise the OMC further.

Practice more: this is question 9 of Compaction of Soil MCQs – Part 1. Try the whole set and check every answer instantly.