How to Compact Fill Dirt for Building

The process to appropriately compact fill dirt for building includes a record-keeping step to protect you in the event of building issues in the future.

1. Dampen the surface of the fill dirt you wish to compact. Don't over saturate it to the point where pools of water form or it becomes muddy. The surface should have a uniform dark color, and wind should not be able to raise a cloud of dust from the area.

2. Set up string line 8 inches above the grade you want your dirt to be at. Make sure the string is level. Place it every 8 feet so you can see how uniformly level the surface of your fill dirt is. Fill in any obvious low points with a few shovels of extra fill. When satisfied, take the string line down.

3. Tamp the soil, using a hand tamp or machine tamp, working back and forth in a grid pattern. Tamp the fill dirt for at least 20 minutes. Then put the string line back up and eyeball the fill dirt to make sure it is still level. Add more dirt if needed and tamp again.

4. Test the soil with the compaction-testing machine. Contact your local zoning board to learn what range of results is acceptable for fill dirt on your site. If you fail the compaction test, dampen the earth again, re-tamp and re-test.

Tip

Give yourself enough time to tamp adequately to compact the fill dirt; don't leave it to the last minute with concrete on the way. Once a soil has passed compaction, it is considered good for weeks, so don't worry about time passing—worry about not leaving yourself enough time to get the job done.

Warning

Hire a compaction-testing company to perform your compaction tests. The testing machines use a radioactive material that can be dangerous if mishandled.