Coupled hydraulic, mechanical and dielectric investigations on kaolin

A new integrated experimental and theoretical framework to simultaneously measure mechanical, hydraulic and high frequency dielectric properties of soil during shrinkage was designed aiming at the development, verification and validation of constitutive electromagnetic material relationships. The electromagnetic response of a soil to a high frequency (radio to microwave) signal contains information that can be harvested to characterise soil including its state variables such as water content, density and soil suction. The developed framework allows decoding of this response. In the presented study on kaolin (i) shrinkage tests ii) shrinkage tests with non-destructive determination of dielectric spectra from 50 MHz to 3 GHz, and (iii) determination of the soil suction relationship over a broad range of water content, have been performed. The results of this investigation show a nonlinear evolution of the permittivity at 1 GHz as a function of water content with a sharp transition. None of the advanced mixture approaches that were applied in a forward approach could quantitatively characterize this relationship between the permittivity and the volumetric water content. With a recently developed advanced mixture equation (Augmented Broadband Complex dielectric Mixture model - ABCM) applied in an inverse scheme, the saturated part of the shrinkage curve was able to be estimated from the permittivity measurements. A quantitative relationship between the cementation exponent within this mixture equation and the soil suction have also been established. The conditions and reasons for the drop in permittivity remains subject of future research.