A Comparison of In Situ Unit Weight and Moisture Content Measurements Made Using a Traditional Nuclear Density Gauge and a Hybrid Nuclear-Electric Density Gauge

Soil compaction is a critical process that enhances the strength and compressibility characteristics of geomaterials, which ultimately improves the engineering performance of earthen embankments for various civil infrastructure elements including roadway and bridge foundations and earthen embankment dams. Therefore, quality-assurance and quality control (QA/QC) processes utilized during the soil compaction process are crucial to ensure that these embankments are being constructed adequately to their specified engineering design. Traditionally within the United States, after a soil has been placed and compacted, the “end-product” of the soil compaction process is assessed using a nuclear density gauge (NDG) to infer the in-place dry unit weight and moisture content. Though these types of NDG tests are relatively nondestructive in nature and can be conducted within a timely manner, NDGs utilize radioisotopes to infer dry unit weight and moisture content. The radioactivity of these isotopes presents numerous logistical and regulatory challenges when operating, transporting, and storing these devices. In response to these challenges, a new low-activity nuclear density gauge has been developed (the “EGauge”), which utilizes a smaller, more well-shielded Cesium-137 gamma radiation source to infer moist unit weight and an electrical capacitance-based moisture probe to infer moisture content. This new device emits significantly less radiation, making it safer to use for field technicians, and easier to manage as it does not have the significant requirements for transport, storage, device handling, and personnel monitoring that are typical for traditional NDGs that are governed by Nuclear Regulatory Commission (NRC) requirements. Currently, very few studies have been conducted in order to provide a direct comparison between these two devices, which are essential for assessing the reliability of this new low-activity hybrid nuclear-electric density gauge. To aid in this effort, an experimental study was conducted by testing five native soils within the state of Delaware to examine the relative differences between unit weight and moisture content measurements made utilizing these two devices. Results from this study indicated that on a point-by-point basis, both gauges are able to produce near-identical wet unit weight measurements regardless of soil type. However, a soil-specific calibration is required for the EGauge for reliable moisture content determination purposes, as results from this study showed the difference between the EGauge’s moisture content value relative to the in situ moisture can be as high as 5% when not calibrated.