What is compactionin construction?

It means the field dry density of the placed soil must reach at least 95 percent of the maximum dry density established for that soil in a laboratory Proctor test. A testing technician verifies it lift by lift, usually with a nuclear density gauge. Always check which Proctor the spec references: 95 percent of Modified Proctor is a tougher target than 95 percent of Standard Proctor, because the Modified test produces a higher maximum density.

Compaction equipment only densifies a limited depth of soil. Place fill 3 ft at a time and the roller tightens the top while the bottom stays loose, which is exactly where the density probe finds it. Specs therefore limit loose lifts, commonly to 8 to 12 inches, so each layer gets full compactive effort before the next goes down. Thinner lifts mean more passes and more cost, which is why lift thickness belongs in the estimate.

The contractor reworks the lift: scarify, adjust moisture (water if dry, disc and aerate if wet), recompact, and call for a retest. No further fill goes down until the lift passes. The estimator's exposure is the rework labor and equipment time plus retest fees, which many testing agreements bill back to the contractor after a failure. Soil well wet of optimum will pump under the roller, and more passes make it worse, not better.