Mechanical, hydraulic, and dielectric characterisation of fine-grained soils during densification

Abstract

Soil characterisation is of primary interest for several disciplines, including geotechnical and geo-environmental engineering, soil science, and agriculture. An upcoming and promising method for soil characterisation relies on the high-frequency electromagnetic measurement technique which provides the potential to explore and investigate soils. However, successful application of this technique requires a profound understanding of the multi-physical processes arising from interactions between soil as a porous medium and the propagation of electromagnetic waves. This thesis focuses on a coupled experimental investigation of the mechanical, hydraulic, and dielectric characterisation of fine-grained soil during densification. In the framework of this thesis, the following steps have been completed and novel findings have been made: 1. Probe designs have been developed, modified and improved to measure the dielectric soil properties in a frequency range from 1 MHz to 3 GHz during shrinkage and compaction. A low-cost in-house-manufactured probe was introduced, which was specifically designed for standardised geotechnical soil shrinkage test. During preliminary investigations, the calibration procedure and performance of these sensors were experimentally and numerically tested. The results showed agreement in the targeted frequency window. 2. Compaction and shrinkage tests on fine-grained soils were conducted in combination with dielectric and soil suction measurements. The shrinkage curve and soil water characteristic curve (SWCC) was established and successfully parameterised with physical based models. The complex permittivity was determined in a frequency range from 1 MHz to 3 GHz. The dielectric properties measured during the shrinkage test showed a sudden drop of complex permittivity which could be located in the residual shrinkage zone. 3. A theoretical electromagnetic mixture approach was used to model the measured frequency dependent dielectric properties of soils during shrinkage and compaction based on the parameterised shrinkage curve and SWCC. The theoretical results were compared to the dielectric measurements on different fine-grained soils. The comparison showed that the real part of permittivity could be well predicted whereas the imaginary part suffered inaccuracies. 4. A sensitivity analysis was performed with the theoretical electromagnetic mixture approach in consideration of the shrinkage behaviour and SWCC in order to investigate the influence of soil suction and dry density on the frequency dependent permittivity. The sensitivity analysis showed that the real part of the permittivity near 1 GHz was insensitive to changes in the soil suction and dry density. In the MHz range, the real part of the permittivity was more sensitive to changes in soil suction than it was to dry density. The imaginary part of the permittivity was generally observed to be more sensitive to soil suction over the entire frequency range. 5. The dielectric relaxation spectra obtained during compaction or shrinkage were decomposed based on a multimodal Cole-Cole model with a global optimisation algorithm by means of Markov Chain Monte Carlo algorithm. It was found that the dielectric spectrum of compacted soil could be modelled with three Cole-Cole type relaxation processes in the studied frequency range from 1 MHz to 3 GHz. The dielectric spectrum of shrinking soil was decomposed into two relaxation processes due to the reduced investigated frequency range of 50 MHz to 3 GHz.