Soil compaction removes air voids from soil, increasing its density and load-bearing capacity. It's required before pouring foundations, roads, and slabs.
Testing
- Proctor test: Determines maximum dry density and optimum moisture content
- Nuclear density gauge: Field testing for compaction percentage
- Target: typically 95% Standard Proctor for fill, 98% for structures
Compaction Factor
Loose soil → compacted: Clay shrinks ~25%. Gravel shrinks ~10–12%. You need more loose fill than the final compacted volume.
Why "Optimum Moisture" Isn't Just "Wetter Is Better"
The Proctor test reveals that soil compacts to its highest density at a specific moisture content — too dry, and particles resist rearranging into a dense pack; too wet, and water fills the void spaces that compaction is trying to eliminate. Contractors add or dry out soil to hit this optimum point before compacting, rather than assuming more water always helps.
Lift Thickness
Fill is compacted in layers ("lifts"), typically 6–12 inches loose thickness per lift, rather than filling and compacting a large area all at once. Compacting too-thick a lift leaves the bottom portion under-compacted even if the surface looks and tests fine, a common cause of hidden settlement discovered only after construction is complete.