Density Digs

Clayton had his basement filled in by Messier, the same company that lifted his house. The Messier crew walked a vibrating compactor over each layer of fill to make it as dense as possible. As the engineer, I had the final say as to whether the fill was sufficiently compacted. So how did I do that?

The most convenient way to test the density of a fill material in place is to take a sample. For starters, I excavated down 8 inches, because the surface doesn’t compact much at all – the vibrating device actually loosens surface fill while it compacts the fill below. Then I scooped out a sample from that depth, with a volume of about two cups. I was very careful to ensure every particle I scooped out made it into my storage container.

Compacted fill sample.

Compacted fill sample.

Density equals mass divided by volume, and I determined the mass of my sample easily on-site with a kitchen scale. To find the volume, I used the sand cone test, which looks like this:

The sand cone apparatus.

The sand cone apparatus.

The translucent white container is filled with sand. When I open a valve, the sand pours out through the metal cone and into the hole I scooped out. When the sand stops moving, I know the hole and the cone are full, so I close the valve and lift up the apparatus:

Sand cone left in the hole after the test. (No need to remove the sand - it gets buried.)

Sand cone left in the hole after the test. (No need to remove the sand – it gets buried.)

Now with just a few extra bits of information – the density of the sand (which I know ahead of time), the volume of the cone (standardized), and the mass of the apparatus before and after the test (use the scale on-site) – I can calculate the density of the fill I scooped out.

Field notes from my compaction testing.

Field notes from my compaction testing.

I’m not done yet. Proper compaction depends not on fresh-out-of-the-ground density but on DRY density. And the most convenient way to dry a sample quickly is with… wait for it… a microwave oven. I borrowed Clayton’s for the task, checking the decreasing mass of my sample after each minute until the mass stabilized. Finally, I calculated the moisture content of the original sample by comparing the fresh-out-of-the-ground density with the dry density.

As it turned out, Clayton’s compacted fill had a dry density well above the minimum density I specified. (Great work, Messier!) The moisture content however was a little low at first, meaning the fill was too dry. Fortunately, the remedy for this compaction issue is simple: just add water. Sandra spent a long morning spraying the entire excavation with a hose, and my last few samples had a moisture content exactly where I wanted.

Sandra on spray duty aims for a perfect moisture content.

Sandra on spray duty aims for a perfect moisture content.

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