What is the purpose of a field compaction test on subgrade or subbase?

• A. To verify color of soil
• B. To measure shear strength
• C. To confirm the compacted lift meets the specified density requirements
• D. To check for moisture content only

Answer: (C)

The key idea is to verify that each compacted lift on the subgrade or subbase has been compacted to the required density. Density is what determines how firm the soil is and its ability to support loads, resist settlement, and perform well under traffic and weather. A field density test measures the in-place density after compaction and compares it to the project’s specified target (often a percentage of the maximum dry density from a Proctor test). If the measured density falls short, adjustments—such as adding moisture, increasing the number of passes, or reworking the area—are made to meet the specification. This test isn’t about color or simply measuring moisture content by itself, and while moisture content affects compaction, the objective is to confirm the lift meets the required density.