Use of Low-Activity Nuclear Density Gauge in Iron Oxide-Rich Soils

Nuclear density gauges (NDG) have been widely used for field compaction control. However, their shortcomings include the burden of licensing with nuclear regulatory agencies, gauge maintenance, storage and transportation restrictions, requirement to wear dosimeters, and radiation safety training. A recently developed low-activity NDG (LANDG) alternative utilizes a low-strength gamma ray source to measure soil density that is Nuclear Regulatory Commission exempt in the U.S.A. and its territories. An accompanying non-nuclear moisture probe yields the moisture content by measuring the dielectric constant of the soil. This study aims to compare the LANDG results with those of the NDG in two common soil types on the island of Oahu, Hawaii: volcanic soils and coralline soils. Coralline soils are partially broken down remains of coral comprised primarily of calcium carbonate. Volcanic soils are basalt derivatives rich in iron oxide and the question arises as to whether the iron oxide will affect the dielectric value resulting in erroneous moisture content measurements. This research shows that the LANDG and NDG measurements of total unit weight are in good agreement for both soils. The moisture content measurements from the LANDG’s accompanying moisture probe are in good agreement with those of the NDG in coralline soils but are less satisfactory in volcanic soils. A plausible explanation is offered. Moisture offsets may be applied to the moisture probe’s measurements to reduce scatter and increase accuracy. Instances of when this is required are suggested.