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A Comparison of In-Place Unit Weight and Moisture Content Measurements Made Using Nuclear Based Methods and the Drive Cylinder Method

The determination of in-place unit weight and moisture content is a critical component of earthwork construction quality assurance and quality control (QA/QC). Direct measurement methods such as the drive cylinder test, sand cone test, or rubber balloon test can be utilized to determine a soil’s in-place unit weight and moisture content, and have a long history of use in geotechnical engineering. However, these direct measurement methods are destructive in nature and the tests are time consuming to conduct in the field. As a result, nuclear density gauge testing has emerged as the dominant approach for QA/QC of soil compaction in the United States, as this device allows for relatively nondestructive testing (only a small hole in needed to run the test), and quicker testing, which allows for better coverage of a compacted soil area in the field. Conventional nuclear density gauges utilize radioactive sources to determine a soil’s inplace unit weight and moisture content, which can be hazardous to the user and others nearby if proper safety protocols are not followed. A new density gauge has recently been developed (the “EGauge”), which uses the same general principles as the nuclear density gauge to measure inplace unit weight and moisture content of soils, with relatively low emission radioactive sources being utilized in the device compared to more traditional devices (allowing for safer testing). This paper describes the results from a study that was carried out to examine the relative differences between in situ unit weight and moisture content measurements made with a drive cylinder, nuclear density gauge, and EGauge. All testing was performed on an active project site, on soil areas that had been well compacted. After calibrating the EGauge for moisture content, it was observed that all three test approaches were generally in good agreement with one another for the wet and dry soil unit weight and the soil moisture content. The nuclear density gauge consistently underestimated moisture content and consequently overestimated dry unit weight values relative to the drive cylinder measurements. Overall, the EGauge measurements were in better agreement with measurements made by the drive cylinder than they were with the nuclear density gauge test results.