Nuclear Gauges for Soil Density (Compaction) Testing By Deborah D. Parkins, PE, Western Technologies Inc.
Soil is a building material that supports everything built on the earth and soil density testing using a nuclear density gauge or “nuke gauge” is a simple and accurate way to obtain these measurements. Historically, gauging soil compaction was strictly empirical. In days of old, soil compaction was considered adequate if the wall or road did not fail due to the ground moving. Foot and animal traffic were used to compact soil and in 3500 BC, the innovative Incas rolled a 5 ton stone wheel to achieve compaction. These early non-standardized methods of compacting soil were often successful but left to chance relying on trial and error as. It was not until the 18 th century, during a time of increased trade, that the need for stable roads and infrastructure forced standardization of building practices birthing the first “civil engineers” in France. Since that time, advancements have been made in measuring soil compaction yielding real time results during the earthwork and foundation preparation process. A nuke gauge for soil density testing is one of the widely utilized standardized methods for measuring compaction and is performed per American Society for Testing and Materials D6938 (ASTM D6938).
Before getting into the details of soil density testing with nuke gauges, it is important to understand soil properties and what happens during the compaction process. Soil consists of solids, air known as “voids” and water. Compaction introduces additional stress to the soil redistributing the soil by reducing voids resulting in densified soil.
The degree of compaction is dependent on the soil properties, type and amount of energy administered during the compaction process and the soil’s water content. For every soil, there is an optimum moisture content for which the soil reaches its maximum compression. In other words, there is a sweet spot between achieving the highest level of compaction and the moisture content of the soil. Standardized laboratory testing (e.g. Proctor test ASTM D698) is used to determine the soil’s moisture-density relationship (Proctor curves along with rock correction factors) and these results are used along with field testing to gauge compaction levels.
The nuclear gauge’s source rod uses two types of radiation, gamma and neutron, to measure density and moisture content respectively. The nuclear gauge is used in two modes: direct transmission for soils and “backscatter” mode for asphalt compaction testing.
The direct transmission method is typically performed during soil density testing. A retractable rod is lowered in to a pre-drilled hole (about 2 inches below the soil profile that you are measuring) through which gamma radiation is emitted. The radiation travels through the soil and the gauge detector reads the radiation that remains or the radiation that was not absorbed by the soil. Therefore, the detector count is inversely proportional to the material density or, the lower the detector count, the higher the density.
The opposite is true during the backscatter method which is often used during asphalt density testing. During the backscatter method, the gauge remains at ground level without any penetration into the test material and as the source emits radiation, the detector counts the radiation that scatters or bounces back. Therefore, with the backscatter method, the detector count is directly proportional to the density. With both methods, calibration factors are used to correlate the count to the actual density. During moisture content testing, the source emitted neutrons collide with hydrogen molecules in the water which slows the neutrons. In this case, “slowed” neutrons are counted by the detector and the slowed neutrons are in direct proportion to the amount of water in the soil.
Nuke gauges provide convenience, speed and accuracy due to ease of operation, quick response times and reliable and repeatable results. Alternatively, nuke gauges are expensive and regulatory requirements govern storage, usage and transportation and handling. The initial cost for a nuke gauge is approximately $7000 and with annual expenses of $700 for calibration, shipping and badge testing for exposure monitoring. Regulations include operator certification and regular operator exposure monitoring and gauges have to be closely tracked and securely stored. Due to the safety concerns surrounding nuclear devices, non-nuclear alternatives to nuclear gauge density testing are becoming popular with the Federal Aviation Association (FAA), Federal Highway Association (FHWA) and state highway departments.
In the meantime, as non-nuclear alternatives to nuke gauge density testing evolve, nuke gauge density testing is an industry-standard for soil compaction testing being very popular and preferred due to its convenience and accuracy.