Numerical Study of Neutron Probe Measurement of Water Content in Expansive Soils

Measurement of water content in expansive soil using a neutron moisture meter is strongly influenced by the moisture reactive property of this type of soil. It is particularly difficult to calibrate a neutron moisture meter when the dry density changes and the soil cracks during drying. This numerical study models the neutron probe measurement of soil moisture, based on a 7-group diffusion theory and it uses finite element method to solve boundary-value problem of the diffusion equations. The results demonstrate that reliable theoretical calibration curves can be generated for a variety of complicated conditions that are likely to be encountered under field conditions, such as changes in soil dry density and the development of shrinkage cracks. It is apparent that the calibration relationship is significantly affected by such conditions, and when the effects of density change and crack development are both taken into account in the numerical modelling, that nonlinear calibration relationships (in terms of volumetric water content) are obtained. The results demonstrate that theoretical calibrations can easily be obtained for situations that would be difficult to create and control under field or laboratory conditions.