Identify the behaviour study of soils in which the zero air void unit weight is found.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 04/12/2023 (Morning)
Detailed Explanation
The zero air voids unit weight is the dry unit weight a soil would have if all its air were expelled and its voids were completely full of water (degree of saturation S = 100%). It is calculated for each water content and drawn as the zero-air-voids line on the same graph as the compaction curve.
\[ \gamma_{d,\text{zav}} = \frac{G\,\gamma_w}{1 + wG} \]
Why “Compaction test” Is Correct
Compaction works by expelling air from the soil. The zero-air-voids line shows the theoretical limit of that process: no amount of compaction at a given water content can push the dry unit weight above it. That is why it belongs to the compaction test, where it is plotted alongside the measured compaction curve to judge how close the soil got to full saturation.
Why the Other Options Are Wrong
- B. Shear strength test: Shear strength tests (direct shear, triaxial) measure strength parameters c and φ; they do not produce a zero-air-voids line.
- C. Permeability test: Permeability tests measure how fast water flows through a saturated sample (coefficient k), not dry unit weight.
- D. Consolidation test: Consolidation tests study volume change of saturated clay under load over time (e–log p curve, cv, Cc).
Key Point
Every point of the compaction curve lies below the zero-air-voids line; at the OMC the soil typically still has about 2–5% air voids.
Key Concepts for Students
- The ZAV line uses the 100% saturation condition; lines for S = 80% or 90% can be drawn the same way using wG = Se.
- The wet side of the compaction curve runs roughly parallel to the ZAV line.
- A compaction curve that crosses the ZAV line indicates a mistake in G or in the test data.
Practice more: this is question 2 of Compaction of Soil MCQs – Part 1. Try the whole set and check every answer instantly.