ASTM D6938 Standard Test Methods for In-Place Density and Water Content of Soil and Soil Aggregate by Nuclear Methods (Shallow Depth)

Get Certified ASTM D6938 Testing for Reliable Compaction Quality Verification 

Certification confirms that the in-place density and water content of compacted soil and soil-aggregate meet the requirements of the applicable ASTM D6938. ASTM D6938 certified testing determines the density and moisture content of compacted fill or subgrade before construction crews accept it for further paving or construction operations. Testing and certification help ensure ASTM D6938 compliance and reduce the risk of structural failure from settlement. 

The main application of ASTM D6938 is to measure the in-place density and water content of soil and soil-aggregate for construction work quality assurance. This test is used in industries for compaction acceptance testing, construction quality assurance, and geotechnical research and development. 

• Material type: Soil and soil-aggregate, at shallow depths up to 300 mm (12 in.). 
• Purpose: Measure in-place density and water content for evaluation. 
• Properties measured: Wet density, water content, and dry density. 
• Method: Technicians deploy a nuclear gauge on or into the soil and measure gamma-ray attenuation by direct transmission, backscatter, or the backscatter/air-gap ratio. 
• Result: Mass per unit volume measures density, and water content is measured as a percentage. 
• Test Limitations: The test method applies only to shallow depths of up to 300 mm (12 in), and results depend on correct gauge calibration using known reference standards. 
• Environmental conditions: Soil texture, surface roughness, and calibration of the gauge may influence the accuracy of the measurement. 
• Applications: Acceptance of compacted soil in construction, compaction verification during roadway and embankment construction, geotechnical research and development.  

What are the Uses of ASTM D6938 Testing?

ASTM D6938 testing quickly verifies compaction quality by applying a nondestructive nuclear gauge technique to benefit the construction team. Engineers then use the results to ensure compaction is at the correct level for the project and to aid in statistical analysis of repeated measurements. This standard-

• Evaluates wet density of compacted soil and soil-aggregate as it is being placed.
• Uses controlled gamma radiation attenuation to artificially measure density.
• Estimates surface water content based on backscatter mode measurement.
• Helps to calculate dry density using the wet density and water content.
• Aids in quality control and acceptance testing of compacted fill and subgrade.
• Helps carry out research and development activities that involve multiple field measurements.
• Helps to select appropriate direct transmission or backscatter mode for the site. 

What Materials Can Be Tested Under ASTM D6938? 

ASTM D6938 is used for soil and soil-aggregate at shallow depths; direct transmission measurements are made to a depth of 300 mm (12 in) below the surface. Common uses range from compacted fill, subgrade, and roadway base materials tested in construction through to laboratory testing of subsidence and separation in asphalt. The gauge is calibrated with reference standards, and the sand cone test is often used to standardize the gauge for a particular soil type before making repeated measurements in the same soil. 

Why is ASTM D6938 Important? 

ASTM D6938 provides a fast, nondestructive method for quality control of compaction that would not require the time and disturbance of the conventional excavation-based method. Construction teams use this test method for quality control and acceptance testing because its nondestructive nature allows repeated measurements at the same test location and statistical analysis of the results.

ASTM D6938 Testing Process and Data Collection

The test starts after the technician prepares a flat, level test surface on the compacted soil, removing any loose material that could affect the reading. Next, the technician adjusts the nuclear gauge to the desired mode, either inserting the source probe into the depth or using backscatter mode with the source and detector on the surface, and establishes the specified depth for the source probe. The transmitter sends out gamma radiation, and the detector measures the radiation intensity after it is absorbed or scattered by the soil. When the technician sets the mode for density, the gauge also measures a separate backscatter reading at the surface to provide an estimate of water content. Finally, the gauge compares the detected radiation rates against previously established calibration data to calculate wet density, water content, and dry density. 

Common Challenges and Troubleshooting

Readings are often incorrect because the surface is not adequately prepared. The gaps between the gauge and the irregular surface of the soil may distort the path of gamma radiation transmission. Similarly, if technicians do not regularly check the gauge against reference standards, improper or outdated gauge calibration can distort density and moisture results. Variation in soil composition, including unusual minerals or chemical constituents, can influence gamma radiation attenuation in ways not fully accounted for in standard calibration. In the laboratory, these problems are overcome by preparing a smooth, level test site, checking the gage calibration against reference standards before using it in the field, and cross-checking the reading against the sand cone test if the soil composition is questionable. Regular calibration of equipment and trained technicians following the qualification criteria of Practice D3740 ensures MaTestLab’s consistent and repeatable ASTM D6938 results. 

• Laboratories report water content, dry density, and wet density from gauge readings obtained with gamma radiation.
• The technician states the material type being tested (soil, soil-aggregate, or fill) and the depth or measurement mode.
• Analysts record the readings taken during the indicated direct transmission or backscatter and compare them with the compaction lift being tested.
• Engineers consider surface preparation, gauge calibration, and soil composition when interpreting density and moisture results.
• Engineers use the measured density and water content values to support compaction quality control and construction acceptance decisions. 

Link to ASTM D6938