Working in Calgary, we get this question a lot on construction sites. After answering it a bunch of times, we figured we’d write up quick article so we could refer people we meet on site to it so they can take a quick read if they’d like. Known by many names on construction sites (nuke gauge, nuke, density gauge), its method of testing is fairly simple to understand.
First we need to be aware of the gauge’s three main components:
- Source Rod – the rod that is inserted into the soil being tested. The tip of this rod contains a radioactive element.
- Sensors – located at the back of the gauge, the sensors detect the amount of radiation coming from the source rod
- Display/controls – where the gauge user inputs the proctor value, sets the testing depth and time, etc
Although how each component works on its own would be very complicated, how they work together is quite simple.
How A Nuclear Density Gauge Works
We’ll cut right to the answer here, and go into more detail below if you’re interested in learning a bit more about the nuclear density gauge we use.
- Tester hammers the pin into the soil
- Tester places the gauge over the hole he’s created with the pin
- Tester pushes the source rod into the hole
- Tester begins the test and steps back
- Sensors in the rear of the gauge begin to record how much radiation they are exposed to
- The gauge calculates the material density based on how much or how little radiation the sensors detect
- Tester returns to the gauge and divides the test density by the proctor density to determine the compaction percentage
- Tester reports the compaction percent to the contractor
It really is pretty simple!
Want to know more? Keep reading below.
Nuclear Density Gauge Test Mode
There are two different modes of testing that a nuclear densometer can perform: Direct Transmission Mode and Backscatter mode. Although not apparent through regular use of the nuke, the gauge actually judges the collected radiation differently in the different modes and reports the results accordingly.
While in direct transmission mode, the sensor (located in the back of the nuclear density gauge) measures the amount of gamma radiation given off by the source rod. Simply put, the less radiation detected by the sensor the higher the material’s density. This is why it is important to select the correct depth of penetration of the source rod, as a more shallow depth will allow for more radiation to reach the sensor than if it were deeper in the same material.
In backscatter mode, the source is in the same plane as the sensor. The shielding within the gauge means that radiation given off the source must first be deflected by the material before reaching the sensor. This means that the more radiation detected by the sensor the higher the material’s density.
An important note to make is that in backscatter mode, the results are heavily weighted to the top few inches of material, where compactive efforts are strongest.
How does a Nuclear Densometer Measure Moisture Content?
The source rod uses two different types of radiation to provide complete measurements. Gamma radiation is used to measure density and neutron radiation is used to measure moisture content.
Neutrons are released from the source and emanate out, colliding with hydrogen molecules within water, or organic material within the soil. When a neutron collides with hydrogen, its speed is reduced and further collisions with hydrogen no longer slow the particle. The higher the number of slowed particles detected by the sensor, the more hydrogen is in the material.
It is important to note that water is not the only thing found in soils that can contain hydrogen. Another commonly found source of hydrogen in soil is organic material. If the material being tested feels dry but the gauge is giving a reading indicating a high moisture content and low air voids, it is very likely that the material is organic. Organic material is not suitable for construction in most cases. Another method of detecting organics in the soil being tested is to smell it and to check if the soil being tested is black. If the soil exhibits a strong odor, high gauge moisture content but feels dry, and is black, it certainly contains organic material.
Conclusion
The geotechnical professionals at Factor Geotechnical rely on many tools, one of which is the nuclear densometer. Although these tools are useful, we never rely purely on the numbers provided by our tools. We will exercise our judgement and approach issues from more than one angle so that we arrive at the best solution for you and your project.
Thankfully, the nuclear densometer gauge is a powerful tool that allows for real-time reporting of results, enabling us to provide confident recommendations about the soil being tested.
Need Calgary compaction testing? Please contact us and we’ll get back to you within 24 hours.
