Density And Moisture Content Measurements By Nuclear Methods

Sources of error that have been identified on the gamma-ray soil density gauge are: (1) sensitivity to local density variations, especially at the soil surface, (2) sensitivity to soil composition, and (3) inaccurate calibration techniques. An attempt was made to solve these problems by developing for each gauge calibration models that enable stable and homogenous non-soil standards to be used for calibration. Calibration standards and calibration models for each gauge were found and tested. In the process of developing and testing the new nuclear gauge calibration model, the dual-gauge principal of compensating these gauges for variations in soil composition was discovered. The air-gap method of using this principal is explained and was optimized by using the calibration model developed in the current study.

Data is provided from an evaluation of the accuracy of the nuclear gages in comparison with the currently used non- nuclear methods of determining soil density and moisture content. The research study was principally theoretical in nature and was conducted in two parts with each part respectively considering density and moisture content. Two existing commercial gages were evaluated, and a nominal experimental program employing non-soil standards of known density and composition was conducted to verify the density results. The results of the moisture content study were checked against data in the literature.

The objective was to minimize the errors identified with measurement of density and moisture content of soils using nuclear gauges. To provide a reasonable basis for optimization, all identified errors were combined to yield a single criterion, the quality factor, which can be used to evaluate the over-all performance of a nuclear density gauge. Research aimed at minimizing moisture content measurement errors involved using the Monte Carlo or random walk method to simulate gauge response, checking the results of the simulation against experimental studies, and attempting to generalize the Monte Carlo results.

Recent advances in the design of nuclear equipment and a better understanding of the nuclear principles involved have led to increasingly widespread use of nuclear gages in earth construction control work. This report describes surface-type nuclear equipment, procedures, and various test methods used for making shallow-depth moisture and density determinations in place on soil and soil-aggregate mixtures. In general, a 6-in. direct transmission density test using a properly operating nuclear gage and an up-to-date factory calibration curve will yield test results slightly better than those of conventional density tests. The nuclear test is simpler to perform than conventional tests and requires only about 15 min to obtain both a density and moisture test result. (Modified author abstract).