Alexander Scheuermann

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full-scale levee model.

Spatial time domain reflectometry (spatial TDR) is a new measurement method for determining water content profiles along elongated probes (transmission lines). The method is based on the inverse modeling of TDR reflectograms using an optimization algorithm. By means of using flat ribbon cables it is possible to take two independent TDR measurements from both ends of the probe, which are used to improve the spatial information content of the optimization results and to consider effects caused by electrical conductivity. The method has been used for monitoring water content distributions on a full-scale levee model made of well-graded clean sand. Flood simulation tests, irrigation tests, and long-term observations were carried out on the model. The results show that spatial TDR is able to determine water content distributions with an accuracy of the spatial resolution of about ±3 cm compared to pore pressure measurements and an average deviation of ±2 vol % compared to measurements made using another independent TDR measurement system.

Numerical 3-D FEM and Experimental Analysis of the Open-Ended Coaxial Line Technique for Microwave Dielectric Spectroscopy on Soil

Open-ended coaxial line probes (OCs) are systematically analyzed by means of numerical 3-D finite element calculations in combination with experimental investigations for microwave dielectric spectroscopy on fine grained soils. The probes, based on conventional coaxial lines and connectors (N, SMA), are broadband characterized in the frequency range from 1 MHz to 10 GHz. The sensitive region for dielectric measurements is ±7-mm lateral and 7-mm perpendicular to the midpoint of the sensor aperture. The spatial spreading of the sensitive zone is stable for the investigated low-loss and high-loss strongly dispersive standard liquids, as well as the saturated and unsaturated soils. Dielectric spectra are determined based on a bilinear relationship between effective permittivity and complex reflection coefficient of the probe after probe-calibration with known standards. The mean relative error of the real part of the complex permittivity from 100 MHz to 10 GHz is smaller than 3.5% and is less than 10% for the imaginary part. A lower limit of the measurement range of 50 MHz with the used procedure and materials is suggested. Complex effective permittivity of saturated fine-grained soils is determined with the developed probes and procedure. The soil dielectric spectra were analyzed with a broadband relaxation model, as well as a novel, coupled hydraulic-dielectric mixture approach. The results demonstrate the suitability of the investigated OCs for the determination of high resolution soil dielectric spectra.

Soil water content monitoring on a dike model using electrical resistivity tomography

We explore the viability of electrical resistivity tomography (ERT) to quantify soil water content in a full-scale dike model. While ERT is an established method to detect changes in soil water content, the retrieval of two-dimensional water content distributions from surface-based surveys is a much more challenging application. We conducted a series of experiments on a full-scale dike model comprising rainfall and flooding events. The dike was surveyed with an ERT line down the land-side slope and monitored with permanently installed spatial time-domain reflectometry (TDR) cables for comparison. Water content changes were calculated by applying Archie’s equation and agreed well with comparison measurements from spatial TDR measurements. The saturation exponent in Archie’s equation was estimated by combining spatial TDR and ERT measurements and agreed well with literature data of comparable sands. The choice of the inversion type, e.g., time-lapse inversion or least-squares versus robust constraint, as well as the influence of inversion parameters, such as regularization and the choice of convergence criterion, is discussed in the context of an apparent inversion artefact for the rainfall experiment, using forward modelling of synthetic data sets. The results indicate that ERT may be a feasible method to quantify water content changes under certain conditions, provided an initial water content estimate is available.

On the Feasibility of Pressure Profile Measurements With Time-Domain Reflectometry

Many applications in geotechnical engineering require knowledge of total pressure distributions. So far, only measurements at single points at individual locations of interest can be carried out with conventional geotechnical measurement devices. Time-domain reflectometry (TDR) has already been used to assess the amount of shearing at multiple distinct locations along coaxial cables grouted in a rock or soil mass. However, pressure profile determination has not been investigated up to now. A novel approach has been investigated to determine pressure profiles from the time-domain reflection data of transmission lines. Mechanical forces change the distance between rubber-insulated conductors of transmission lines and, therefore, affect the spatial distribution of capacitance and inductance. These variations lead to partial reflections of an incident step pulse from which physical parameter distributions along the inhomogeneous transmission line are reconstructed. A reconstruction algorithm that was originally developed for soil moisture profile determination (Spatial-TDR), was adopted and applied in laboratory experiments. A further experiment illustrates TDR signals that can be expected in actual applications under different load conditions.

A Lattice Boltzmann model for studying transient effects during imbibition–drainage cycles in unsaturated soils

This paper presents a numerical model based on the Lattice Boltzmann Method (LBM), developed for studying dynamic responses of an unsaturated porous medium to periodic imbibition and drainage induced by a cyclic water table movement. The model includes gravity which helps defining an hydraulic head. The model predicted an incremental increase of the overall water content in the medium over each cycle prior to a quasi-steady oscillatory state, a hydraulic ratcheting effect that has been previously observed in laboratory experiments. An empirical model was proposed to combine the transient and harmonic variations of the volumetric water content. The parameters of this empirical model were examined against physical quantities including the frequency of the driving water table oscillations and the porosity of the porous medium. The findings presented here may help to improve the formulation of constitutive models that are able to describe hydraulic processes of unsaturated soils.

Fast time domain reflectometry (TDR) measurement approach for investigating the liquefaction of soils

Time domain reflectometry (TDR) was used for the spatial observation of transient density changes in a laboratory shake-box apparatus. Shake-box experiments are conducted to investigate the liquefaction of soils due to seismic excitation. As a sensor, a flat ribbon cable was placed within a water saturated sand column two metres in height. In the experiment, the soil column was excited at its base with a sinusoidal signal with low frequencies and moderate amplitudes. During the tests, TDR measurements were conducted with a repetition rate of 30 s−1. Temporal density changes during the liquefaction and reconsolidation process of the quartz sand along the sensor could be shown through signal analysis. The mean porosities calculated from the relative dielectric permittivity using a soil-specific calibration function could be determined with an RMSD-value of 0.02 compared to the porosities determined from the height of the sample. With the metrological set-up for the TDR measurements being used, fast temporal changes in the density profile could be qualitatively visualized indicating the movement of the reconsolidation front during the liquefaction.

Determination of the Soil Water Retention Curve and the Unsaturated Hydraulic Conductivity from the Particle Size Distribution

Because of the complexity of the metrological determination of the soil water retention curve (SWRC), so-called pedotransfer functions (PTF) have been developed for several years. Mostly these PTF are based on a more or less simple regression analysis using a limited set of data. In such methods the SWRC is predicted with data on the amount of soil components sometimes supplemented by values regarding the density or the amount of organic materials. Only few PTF deal directly with the particle size distribution. In many cases empirical factors are necessary to obtain a prediction for the water retention curve. A new method for determining the soil-hydraulic properties using the pore constriction distribution of a soil has been developed, whereby the pore constriction distribution is derived from the particle size distribution depending on the density of the soil. The contribution will present the new pedotransfer method and shows results in comparison to experimental investigations.

Broadband electromagnetic analysis of compacted kaolin

The mechanical compaction of soil influences not only the mechanical strength and compressibility but also the hydraulic behavior in terms of hydraulic conductivity and soil suction. At the same time, electric and dielectric parameters are increasingly used to characterize soil and to relate them with mechanic and hydraulic parameters. In the presented study electromagnetic soil properties and suction were measured under defined conditions of standardized compaction tests. The impact of external mechanical stress conditions of nearly pure kaolinite was analyzed on soil suction and broadband electromagnetic soil properties. An experimental procedure was developed and validated to simultaneously determine mechanical, hydraulic and broadband (1 MHz-3 GHz) electromagnetic properties of the porous material. The frequency dependent electromagnetic properties were modeled with a classical mixture equation (advanced Lichtenecker and Rother model, ALRM) and a hydraulic-mechanical-electromagnetic coupling approach was introduced considering water saturation, soil structure (bulk density, porosity), soil suction (pore size distribution, water sorption) as well as electrical conductivity of the aqueous pore solution. Moreover, the relaxation behavior was analyzed with a generalized fractional relaxation model concerning a high-frequency water process and two interface processes extended with an apparent direct current conductivity contribution. The different modeling approaches provide a satisfactory agreement with experimental data for the real part. These results show the potential of broadband electromagnetic approaches for quantitative estimation of the hydraulic state of the soil during densification.

Probability of transportation of loose particles in suffusion assessment by self-filtration criteria

Suffusion occurs when soil particles are loosened, detached, and transported away by seepage flow through a series of pores and pore constrictions. While traditional suffusion assessment methods are often based on particle size distribution analysis only, modern assessment methods are focused on constriction size distributions, which are derived from particle size distributions based on certain assumptions. This paper provides a new assessment method, which employs the probability of loose particles being transported to the next pore. The new approach can introduce the influence of an overlapping zone between the two fractions of loose particles and soil primary fabric. This overlapping zone was often overlooked in prior studies. The constriction size distribution of the primary fabric can be calculated approximately by two-dimensional or three-dimensional methods. The three-dimensional method used in the presented study even offers the opportunity to study the influence of particle arrangements on suffusion. The results of the new assessment methods show satisfactory agreement with experimental results.

Broadband dielectric measurement methods for soft geomaterials: coaxial transmission line cell and open-ended coaxial probe

Broadband dielectric measurement methods based on vector network analyzer coupled with coaxial transmission line cell (CC) and open-ended coaxial probe (OC) are simply reviewed, by which the dielectric behaviors in the frequency range of 1 MHz to 3 GHz of two practical geomaterials are investigated. Kaolin after modified compaction with different water contents is measured by using CC. The results are consistent with previous study on standardized compacted kaolin and suggest that the dielectric properties at frequencies below 100 MHz are not only a function of water content but also functions of other soil state parameters including dry density. The hydration process of a commercial grout is monitored in real time by using OC. It is found that the time dependent dielectric properties can accurately reveal the different stages of the hydration process. These measurement results demonstrate the practicability of the introduced methods in determining dielectric properties of soft geomaterials.