Nuclear Density Testing

Keil Krieg (CMET Department Manager at geotechnical consultancy SAECO) shows you how to test soil density by nuclear method.

Why run nuclear density tests?

Geotechnical technicians perform this test to determine soil’s compaction.

“When we're building a structure or we're building anything on earth for that matter, we want to make sure a contractor has compacted it and pre-smashed it down so that in the future it's able to hold the weight of a structure,” Keil explains.

The test also reveals how the soil’s current density compares to its maximum density—in other words, how much more contractors can squish it down to make it sturdy enough to build on.

How it works

Engineers perform nuclear density tests with a gauge that’s specialized for this test. The test involves driving a pin into the ground, then inserting a rod that has a radioactive element at the bottom that connects to the receiver on the gauge. Then the machine automatically calculates the density in pounds per cubic foot in this location.

Step 1: Standardize the gauge

Every day engineers begin with their gauges. Keil sets his gauge on a block that knows this machine. The machine is calibrated with this block, and he standardizes it every day to make sure the gauge has determined what “mood” it's in and what external variables exist in the field. Standardizing the gauge takes four minutes for Keil’s machine.

Step 2: Prepare the location

Engineers want to make sure they have maximum surface area contact between the nuclear gauge and the ground, so Keil preps an area and makes sure that it's as flat as possible using a rod guide. Then, he drives a pin into the ground to create a hole for the radioactive rod.

Step 3: Perform the test

Now, Keil inserts the radioactive rod into the hole he made in the ground. With the push of a button, his gauge automatically performs the test in one minute. This soil’s dry density is 87.6 pounds per cubic foot.

Step 4: Analyze the results

For Arizona, 87.6 pounds per cubic foot is very low. In fact, this soil probably failed its density test—meaning if a contractor wanted to place material here, they probably couldn’t do it. The soil wouldn’t support the structure.

Since the soil’s current density isn’t good enough, Keil needs to find out if the contractor can compact the soil enough to make it work for their project. To do that, he will have to determine the soil’s maximum pounds per cubic foot.

“Typically, you're shooting for about a 95% to 100% maximum dry density. At 87.6 pounds per cubic foot, you’re typically going to be running about 80% right here,” Keil says.

Fortunately, most nuclear density gauges also calculate the soil’s percent moisture content, which is also a factor in compaction. Moisture helps the soil compact better. The closer you are to optimum moisture content, the higher percent compaction you can achieve with relatively less effort.