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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.
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.
Evaluates methods for measuring soil moisture and describes the equipment used.
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.