The detection of porosity changes within a soil matrix caused by internal erosion is beneficial for a better understanding of the mechanisms that induce and maintain the erosion process. In this paper, an electromagnetic approach using Spatial Time Domain Reflectometry (STDR) and a transmission line model is proposed for this purpose. An original experimental setup consisting of a coaxial cell which acts as an electromagnetic waveguide was developed. It is connected to a transmitter/receiver device both measuring the transmitted and corresponding reflected electromagnetic pulses at the cell entrance. A gradient optimization method based on a computational model for simulating the wave propagation in a transmission line is applied in order to reconstruct the spatial distribution of the soil dielectric permittivity along the cell based on the measured signals and an inversion algorithm. The spatial distribution of the soil porosity is deduced from the dielectric permittivity profile by physically based mixing rules. Experiments were carried out with glass bead mixtures of known dielectric permittivity profiles and subsequently known spatial porosity distributions to validate and to optimize both, the proposed computational model and the inversion algorithm. Erosion experiments were carried out and porosity profiles determined with satisfying spatial resolution were obtained. The RMSE between measured and physically determined porosities varied among less than 3% to 6%. The measurement rate is sufficient to be able to capture the transient process of erosion in the experiments presented here.

Knowledge of the frequency-dependent electromagnetic properties of coarse-grained materials is imperative for the successful application of high frequency electromagnetic measurement techniques for near and subsurface monitoring. This paper reports the design, calibration and application of a novel one-port large coaxial cell for broadband complex permittivity measurements of civil engineering materials. It was designed to allow the characterization of heterogeneous material with large aggregate dimensions (up to 28 mm) over a frequency range from 1 MHz–860 MHz. In the first step, the system parameters were calibrated using the measured scattering function in a perfectly known dielectric material in an optimization scheme. In the second step, the method was validated with measurements made on standard liquids. Then the performance of the cell was evaluated on a compacted coarse-grained soil. The dielectric spectra were obtained by means of fitting the measured scattering function using a transverse electromagnetic mode propagation model considering the frequency-dependent complex permittivity. Two scenarios were systematically analyzed and compared. The first scenario consisted of a broadband generalized dielectric relaxation model with two Cole–Cole type relaxation processes related to the interaction of the aqueous phase and the solid phase, a constant high frequency contribution as well as an apparent direct current conductivity term. The second scenario relied on a three-phase theoretical mixture equation which was used in a forward approach in order to calibrate the model. Both scenarios provide almost identical results for the broadband effective complex relative permittivity. The combination of both scenarios suggests the simultaneous estimation of water content, density, bulk and pore water conductivity for road base materials for in situ applications.

A purpose-built permeameter was used to explore the transient evolution of porosity during the mixing process in filtration experiments. The experiments considered upward seepage flow and explored the influence of base and filter particle sizes, along with different hydraulic conditions. The permeameter acted as a coaxial transmission line enabling electromagnetic measurements based on spatial time domain reflectometry, from which the porosity profile was obtained using an inversion technique. Quantitative characteristics of the onset and progression of the mixing process were extracted from a porosity field map. The limiting onset condition was influenced by geometric and hydraulic factors, with the critical flow rate exhibiting a strong dependence on the base particle size, while the critical hydraulic gradient exhibited a stronger dependence on filter particle size. The progression of the mixing process was characterised by both the transport of base particles into the filter layer, as well as the settlement of the filter particles into the base layer due to the reduction of the effective stress at the base-filter interface leading to partial bearing failure. The rate of development of the mixture zone was strongly dependent on the hydraulic loading condition and the base particle size, but the final height of the sample after complete mixing was independent of the hydraulic loading path.

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.

 Design of prototype to measure online dynamic water content profile in porous media.  An existing reactor is transformed into a large coaxial cell.  First time TDR measurements are performed at high temperatures.

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Internal erosion is one of the major threats for water retaining structures like embankment dams or levees. Numerous design criteria were developed in the last decades for this reason. As the main concern of the geotechnical engineer is to prevent erosion, it is not astonishing that a vast majority of the criteria are focussed on the onset or initiation of erosion, or more precise, to avoid it. However, many existing structures have already experienced the one or other event of internal erosion. Thus, it is beneficial to understand not only the initiation, but also the progression of the erosion process. This process is though still not completely understood. Among the different types of internal erosion, contact erosion is characterised by two soils (base and filter) of different grain size distributions (PSD) forming an interface to each other. The erosion process is triggered by a water flow, which can be either parallel or perpendicular to the interface. The latter configuration is sometimes referred to as filtration and is in the focus of this research. If erosion occurs, base particles are transported into the pores of the filter by a water flow, forming a mixture zone with a lower porosity and permeability. For a better understanding of the contact erosion process, the formation of this zone must be understood. My special thanks are dedicated to my main supervisor Assoc. Prof Alexander Scheuermann. Without his believe in my capabilities as a researcher I would never have come to Australia for a PhD. Giving me the freedom to develop my own ideas, he was always present if advice was needed. Without Dr Thierry Bore, I would still try to disclose some of the secrets of high frequency electromagnetic measurement techniques. It was a great luck that he joined the team of the Geotechnical Engineering Centre at UQ during my candidature resulting in a fruitful collaborative work on the Coaxial Erosion Cell. Prof. Dr.-Ing. Karl Josef Witt made the collaboration with the Bauhaus University in Weimar, Germany, possible and enabled generously the use of the laboratory equipment and erosion cell located there. This was another great luck for me and my research progress. Not to forget his counsel based on his great expertise. My third supervisor, Dr.-Ing. Andreas Bieberstein always had an advice when necessary. The realisation of the experimental setup wouldn't have been possible without the work of the faculty workshop and the performed experiments without the support of the laboratory staff of both universities. It was a pleasure to work together with these great persons. The PhD-experience would be lacking an important part without my wonderful fellow students both in Brisbane and Weimar. I will always remember the time we spent together, not to forget the coffee connection and our lunch break filled with discussions of every aspect of a PhD but much more often philosophising about all and nothing. It was great to share the nice moments and it was always a help knowing ones problems during the candidature are not uniq...

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