Compaction behavior — key statements: Which of the following statements about dry density versus water content behavior is/are correct? Select the best combined answer.

Introduction / Context:
Laboratory compaction tests (Standard/Modified Proctor) produce a characteristic dry density–water content curve. Recognizing its features is crucial for field compaction control and specification compliance.

Given Data / Assumptions:

• Soil is compacted at different water contents with fixed compactive effort.
• Dry density γd is measured after compaction.
• Zero air-voids (ZAV) line represents the theoretical upper limit of γd at a given water content.

Concept / Approach:
As water content increases from very dry conditions, a thin water film lubricates particles, aiding rearrangement and increasing γd, reaching a peak at OMC. Further addition of water fills voids without improving particle interlock, reducing γd. The ZAV line (na = 0) is given by γd(ZAV) = γw / (w + 1/G) and is an asymptotic upper bound rarely achieved in practice.

Step-by-Step Solution:

Verification / Alternative check:
Compare Standard vs Modified Proctor: higher effort shifts the curve up and left (higher γd, lower OMC) but preserves these qualitative statements.

Why Other Options Are Wrong:
Each individual statement A–D is correct; therefore the combined correct choice is “All the above”.

Common Pitfalls:
Confusing ZAV line with air-void lines of constant nonzero air content; assuming γd keeps increasing with water content indefinitely.

Final Answer:
All the above