Soil compaction behavior: In a standard or modified Proctor compaction test, how does the optimum moisture content (OMC) change when the compaction effort is increased?

Introduction / Context:
Compaction improves soil density and strength by expelling air and rearranging particles. The relationship between water content and dry density produces a characteristic curve with a maximum dry density (MDD) at the optimum moisture content (OMC). Understanding how OMC shifts with effort is essential for selecting field compaction specifications.

Given Data / Assumptions:

• Same soil tested with different compactive efforts (e.g., Standard vs. Modified Proctor).
• Compaction energy is increased while test method stays consistent.
• Soil responds typically (no unusual cementation).

Concept / Approach:

Higher compactive effort drives particles closer with less need for water as a lubricant/dispersant, increasing MDD and shifting the peak to a lower moisture content. Thus, as effort increases, OMC decreases and MDD increases. This trend holds for most fine-grained and granular soils used in earthworks.

Step-by-Step Solution:

Verification / Alternative check:

Comparative lab data between Standard (600 kN-m/m^3) and Modified (2700 kN-m/m^3) consistently show lower OMC for the Modified test.

Why Other Options Are Wrong:

• Remains same/increases: Contradict common compaction curves.
• Increases then decreases: Not typical for the same soil with monotonic energy change.

Common Pitfalls:

• Assuming field moisture targets from Standard Proctor when specifications require Modified Proctor densities.

Final Answer:

decreases