Experimental investigation of the dielectric properties of soil under hydraulic loading

Abstract

An experimental set-up was developed in order to determine the coupled hydraulic, dielectric and mechanical properties of granular media under hydraulic loading. The set-up consisted of a modified column for permeability tests involving a flow meter and pressure transducers along the sample to quantify the hydraulic gradient. A newly developed open-ended coaxial probe allowed the measurement of the frequency dependent dielectric permittivity of the material under test. The shear strength of the sample within the column was measured using a conventional vane shear device. In this paper, the overall set-up is introduced with focus on the open-ended coaxial probe. The design and calibration of the probe are introduced in detail. A numerical study showed that the sensitive cylindrical volume of the probe was approximately 150 mm in diameter with a depth of 65 mm. An investigation with glass beads showed that the set-up allowed the parameterization of the hydraulic, mechanic and dielectric parameters of granular materials under the influence of vertical flow. A satisfactorily good correlation between porosity and the real part of the dielectric permittivity was detected. The critical hydraulic gradient defining the transition of a fixed bed of particles to fluidization was characterized by a sharp peak in the evolution of the hydraulic conductivity and could easily be determined from the measurements. The shear strength of the material under test reduces linearly with increasing hydraulic gradient. Future investigations will be carried out to provide the required parameterizations for experimental and numerical investigations of the internal erosion of granular media.

The Technical Challenges for Applying Unsaturated Soil Sensors to Conduct Laboratory-Scale Seepage Experiments

“…The in/outflow of a one-dimensional soil column could be measured using versatile methods: (1) the flow rate logging using flowmeters; (2) the accumulative soil moisture discharge logging by applying conventional TDR analysis for an FRC sensor; (3) the weight logging for the entire soil column or outflow tanks using digital bench scales. The flowmeters have successfully been applied to measure in/outflow rates by many single-phase seepage experiments integrated with the TDR sensor technique in previous studies [ 19 , 20 , 21 , 28 , 83 , 84 ]. All those studies applied a high-precision electromagnetic flowmeter for flow rates logging at higher flow rates, which were generated for a single-phase flow seeping through perfectly spherical glass beads in various sizes under unconsolidated conditions.…”

Physical observations of the transient evolution of the porosity distribution during internal erosion using spatial time domain reflectometry

“…The third step is the conversion of the dielectric permittivity profile to a porosity profile, n(x), using theoretical mixing equations. Prior studies indicated the applicability of the Lichtenecker-Rother model (Bittner et al, 2017(Bittner et al, , 2019 for saturated glass beads, where the dielectric permittivity of the mixture is computed as the weighted sum of the dielectric permittivity of the water and glass beads by their respective volume fraction:…”

“…Test samples were prepared with soda-lime glass beads as an idealised granular soil. The suitability and advantages of using glass beads as a replacement material for soil has been highlighted in several prior studies (Tomlinson and Vaid, 2000;Scheuermann, 2012;Zakaraya Alhasan et al, 2015;Bittner et al, 2017). From the perspective of spatial TDR, an additional benefit of considering glass beads is the constant dielectric permittivity of the glass beads, as well as the fact that mineralogy of real soil particles do not have to be taken into account.…”