Compaction
What does 95 percent compaction actually mean?
"95 percent compaction" means the fill has been compacted to 95 percent of the soil's maximum dry density as determined by a Proctor test in a lab. An inspector verifies it in the field, usually with a nuclear density gauge.
Loose fill settles. Whatever gets built on top of it, a slab, a trench patch, a foundation, settles with it, which is why specs require compaction and why inspectors test for it.
The vocabulary
- Proctor test: a lab test that finds a soil's maximum dry density and the moisture content where it compacts best. The field target ("95 percent of standard Proctor") is a percentage of that number.
- Lift: one layer of fill placed and compacted before the next goes on. Compactors only densify so deep, so specs require thin lifts, commonly 6 to 12 inches loose, depending on the machine and soil.
- Moisture: soil compacts best near its optimum moisture. Too dry it won't tighten; too wet it pumps. Water truck or wait, but don't test-roll soup.
Passing the first time
Thin lifts, the right machine for the soil (vibratory for granular, padfoot for clay), moisture close to optimum, and full coverage in passes. Guessing with thick lifts saves an hour and buys a failed test, a dig-out, and a re-do.