Influence of Voids on Density Measurements of Granular Materials Using Gamma Radiation Techniques
The use of gamma radiation equipment for the nondestructive measurement of soil density originated nearly four decades ago. The typical procedure involves a careful calibration of the gamma radiation counts on specimens of known density of the same materials as those whose density are to be measured. This paper presents the results of laboratory gamma-ray density measurements on 100-mm-diameter specimens with an adapted twin-probe transmission-type density gage originally developed for field use. Primary factors affecting density calibration for soils include soil composition and soil porosity. To appreciate the soil composition factor, 100-mm-diameter specimens of solid materials similar to the soil minerals were tested, and from the data a linear ln (COUNT) versus DENSITY calibration line for solid materials was obtained. Next, aluminium cylindrical specimens with concentric holes of various diameters representing various porosities from 0 to 90% were tested. When compared to the solid material calibration line, it is clear that the bulk density calculated based on the gamma-ray path geometry gives the same calibration as the solid specimens. Subsequently, cylindrical specimens of dry sands and gravels were also made and tested. Results indicate that specimens of uniform densities and uniform void distributions must be used for calibration. Subsequently, using the calibration line obtained for solid homogeneous specimens, densities of porous soils measured by the gamma-ray device would be the bulk density of the materials confined in the path geometry of the gamma rays. Thus the gamma-ray apparatus can be a very useful tool for identifying nonuniformities due to void variation within the same specimen.