Laboratory and Field Calibration of a Neutron Depth Moisture Gauge for Use in High Water Content Soils

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Courtesy of ASTM International

In recent years radioisotopes have been increasingly used for determining site values of the density and the water content of soils. Since field measurements obtained by neutron moisture gauges are relative, these instruments must be calibrated. Calibration is usually carried out in the laboratory by plotting neutron counting rates versus known water content values of several soil media, homogeneous as well as heterogeneous.

The accuracy of the factory calibration curve of a widely used, commercial neutron depth moisture gauge was evaluated both in the field and in the laboratory by comparing probe readings to measured volumetric water contents of several soil mixtures. The analyses of the laboratory results indicate that the factory linear calibration curve is satisfactory for volumetric water contents not exceeding 40%. However, for volumetric water contents in the range from 40 to 100%, the relationship between probe readings and volumetric water contents is nonlinear. This nonlinear curve was found to be quite different from that obtained in the field calibration campaign carried out in a deposit of sensitive Champlain clay having volumetric water contents which compare well with those of the laboratory-prepared clay mixtures.

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