Soils & Compaction
Compaction is the heart of earthwork: densifying soil so it carries load, resists settlement, and sheds water.
The Proctor test
A Proctor (moisture-density) test determines a soil’s maximum dry density and the optimum moisture content at which that density is reached. The modified Proctor uses a higher compaction energy than the standard Proctor, giving a higher maximum density and a lower optimum moisture.
Key ideas: relative compaction = field density as a % of the lab maximum dry density (e.g., a 95% spec) · density peaks at the optimum moisture content · modified Proctor = higher energy than standard · compact in thin lifts so energy reaches full depth.
Getting compaction in the field
In the field, density is reported as relative compaction — a percentage of the lab maximum. To hit it, the soil’s moisture is kept near optimum, and fill is placed in thin lifts so the roller’s energy reaches the full depth of each layer. The strength of the compacted subgrade largely sets how thick the pavement above it has to be.
Practice: Soils & Compaction
Frequently asked
What does 95% relative compaction mean?
What is the difference between the standard and modified Proctor tests?
Why does moisture matter so much in compaction?
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