Use of Radioactive Material to Measure Soil Moisture and Density

Abstract

The need for convenient instruments to measure soil moisture and density in place in a quick, reliable, and possibly continuous manner has long been felt by soils engineers. This paper summarizes the work done up to this time by the Civil Aeronautics Administration and Cornell University on the nuclear meters that are designed to solve the problem. The method presented here makes use of the physical principles of the scattering of neutrons and gamma rays. A source of proper radiation, radium D-beryllium or cobalt 60, is contained in cylindrical probes along with a detector of slow neutrons or of gamma rays, respectively. The probe is inserted into the soil mass through a 1-in. diameter pipe, and the radiation penetrates the nearby soil and is scattered back toward the detector, the amount and character of the scattered radiation giving a measure of the characteristics. The design and testing of such probe meters that measure soils at depth and of the related surface meters that measure thin-layered soils are described in detail. With suitable care, the probe meters measure moisture to ±1 lb of water per cu ft, and measure density to ±5 lb of soil per cu ft. The surface meters are in an earlier experimental stage and are not so accurate. The time required to take a complete set of readings is about 15 min. Further laboratory and field testing is being conducted to perfect these meters to fulfill specific requirements of different engineering projects.

New Method for Quality Assurance of Asphalt Pavements Using a Low-Activity Nuclear Density Gauge

“…Nuclear density gauges (NDGs) use a gamma-ray method for measuring density (7)(8)(9)(10). The gamma-rays traverse through a large volume, and the scanning volume can be controlled.…”