How to Determine Moisture Content in Soil

Keil Krieg, CMET Department Manager at SAECO, teaches us how to determine soil’s moisture content following a sand cone test.

Why test the soil’s moisture content?

Geotech engineers need to know the soil’s moisture content to determine its dry density and calculate its percent compaction out in the field. They usually have to determine moisture content after a sand cone test.

Like nuclear density tests, sand cone tests help determine the soil’s total density. But unlike nuclear density tests, the sand cone method doesn’t account for the moisture content. So that’s what Keil is figuring out in this video.

How it works

Keil will run a moisture content test using a calcium carbide pressure method. He’ll take calcium carbide—which reacts with moisture—and put it in a chamber with the soil. It will give him a reading of the percent moisture content on its dial.

Step 1: Prepare the testing chamber

Keil makes sure his testing chamber is clean and in good working condition.

Since his soil sample is granular, Keil puts two scoops of calcium carbide into the chamber. He’s also added two spherical pulverizers in the chamber (these will come in handy later).

Step 2: Prepare and insert the soil

Keil takes a representative sample of material that came from the sand cone testing hole and weighs it. This particular chamber requires 20 grams of soil.

However, Keil isn’t ready to introduce it directly into the chamber because he doesn’t want the chemical reaction to occur without a pressure tight seal on the chamber.

So he turns it horizontally. The sample is in the cap he'll place over the top of the chamber. Since it’s horizontal, it has not fallen into the chamber and hasn't touched any calcium carbide yet. Once the pressure tight seal is in place, Keil can introduce it from the cap into the chamber.

Step 3: Pulverize and mix the soil

Once the soil is in the chamber, Keil has to pulverize it and mix it with that calcium carbide to get a pressure reaction. That’ll drive the needle on the side of the chamber and tell him the soil’s percent moisture.

Pulverizing means that Keil shakes the chamber for approximately one minute so that the two spheres roll around with the soil, mixing it with the calcium carbide thoroughly.

“It’s kind of like a hula hoop—you gotta get in the rhythm of it,” Keil jokes.

Step 4: Read the results

After the reaction occurs, Keil takes a reading and records it in his notes. This soil’s moisture content is at 7.4.

Step 5: Make any corrections

Finally, Keil has to correct for the rock that was in the sample. Remember, he removed the rock during the sand cone test—so he’ll use the percent rock from that test to mathematically change this number to get a more accurate moisture content.

Correcting for rock content helps him get a test results that's more comparable to an oven-dry moisture test. Oven-dry tests contain all of the aggregates and all of the fine particles in the soil, but unlike this field test, they’re performed in a lab. (More on lab testing later in the series.)

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