Andreas Bieberstein

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.

Breaching of overtopped river embankments controlled by apparent cohesion

Laboratory experiments were conducted to obtain detailed experimental data of high temporal and spatial resolution on the breach of homogeneous non-cohesive embankments due to overtopping. These results supplement earlier data for this embankment type by providing information on the breach discharge, longitudinal and transversal breach profiles, erosion rates and pore-water pressures within the embankment. Based on detailed experimental results, two breach phases are classified. Shear and tension failure mechanisms were identified from the analysis of the obtained cross-sectional breach profiles. Furthermore, the temporal cross-sectional analysis of the used non-cohesive granular material shows a different behaviour regarding the lateral breach widening and the breach slope failure. The data analysis indicates that the apparent cohesion represented by the pore-water pressure influences the stability of the breach side slopes and hence the complete breach process. This finding is confirmed by a dimensional analysis of the experimental data and compared with information from earlier work.

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.

Preferential Water Movement in Homogeneous Soils

Preferential water movement is a frequently observed phenomenon in soil science. Especially on natural slopes hydraulic behaviour can sometimes be observed which arises from preferential water flow. If the soil consists of a more or less homogeneous material, the preferential water movement can be caused only by heterogeneously distributed water. A sprinkler irrigation test on a large scale homogeneous dyke model and corresponding measurement results using different techniques have shown on the one hand, that preferential water flow in the form of fingering possibly occurred during the experiment. On the other hand, lateral water movement due to heterogeneously distributed water content inside the dyke body could be observed. Both phenomena were also investigated in simple laboratory experiments. The contribution presents results from the above mentioned experiment which are analysed and discussed.

Long-Term Safety of Well Abandonment: First Results from Large Scale Laboratory Experiments (COBRA)

A long-term safe and reliable abandonment of wells is a crucial prerequisite for a secure abandonment of underground storage sites, while considering the long-lasting environmental impact, e.g. leakage through wells. To study the impact of the cementation on the tightness of wells, both, the cementation of a well during completion and abandonment are investigated in Full-scale laboratory experiments. Different autoclaves from small to Full-scale have been developed to test cementations under various conditions and to perform long-term tests under in situ conditions of a CO2 storage site. The experiments show that the surface-texture (e.g. roughness) of the drilled well has a significant influence on the formation of mud-channels—for rough surfaces, up to 75 % of the serrations consist of non-displaced mud and only 25 % are well hardened cement, creating possible leakage pathways. Even under idealized cementation conditions using a cement recipe characterized by a very low shrinkage, micro-annuli are formed. These micro-annuli are connected throughout the whole oil-field casing in the Full-scale experiments for which the widths of the micro-annuli in the order of 10–20 µm could be deduced. Along the micro-annuli the cement is carbonated due to the flow of CO2-bearing fluid. The fluid flow could also be verified by a Spatial Time-Domain-Reflectometry (TDR) setup embedded in the cementation, which was successfully tested as an in situ monitoring system. In the Full-scale experiments, chemical reactions in the system casing—cement—rock—fluid were examined. The geochemical analyses during and after the experiment show variations in pH, conductivity and chemical composition of the brines, which are well described by an interplay of corrosive processes and precipitations of carbonate minerals.

Optimiertes sensordesign zur bestimmung von feuchteprofilen in verlustbehafteten boden. Optimized sensor design for the determination of the spatial moisture distribution in electrical lossy soils

Spatial-TDR ermöglicht die Erfassung von Feuchteprofilen entlang von Kabelsensoren. Hierfür sind unabhängige Messungen erforderlich, wofür der Sensor beidseitig anschlossen ist. Bei ausgeprägt verlustbehafteten Böden ist der Übergang vom Sensor zur Zuleitung im TDR-Signal schwierig zu identifizieren. Diese Information ist aber für die Bestimmung von Feuchteprofilen von großer Bedeutung. Zur Verbesserung der Identifizierbarkeit des Sensorendes wurden Sensorschalter entwickelt. Spatial-TDR allows to record moisture profiles along extended flat band cables. For this purpose independent measurements are necessary where the sensor is connected from both sides. For distinctively electrical lossy materials it is difficult to identify the transition from the sensor to the input lead. But this information is absolutely necessary for the determination of moisture profiles. In order to afford a unique and reliable identification of the end of the sensor, switches are developed.

Column test apparatus for the inverse estimation of soil hydraulic parameters under defined stress condition

The experimental determination of soil hydraulic properties (soil water characteristic curve and unsaturated conductivity) over a large range of saturation is of utmost importance for any prediction of soil water dynamics and for evaluation of soil mechanical behaviour of unsaturated soils. However the hydraulic properties are expected to vary under stress, since inter alia they are mainly determined by the pore structure, which itself depends on the density of the soil and thus on the stress condition. A column test apparatus was developed to carry out multi-step outflow and inflow experiments under defined stress conditions on samples 45 cm in height and with a diameter of 19 cm. The simultaneous recording of pressure and changes in water content within the soil column at a reasonable temporal and spatial resolution is of crucial importance for the inverse parameter estimation process. A new measuring technique based on the method of Time-Domain-Reflectometry (TDR) permits the recording of continuos water profiles over the sample at a high temporal resolution. Transient in/outflow rates, water content profiles as well as tensiometer readings allow a detailed inspection of the hydraulic processes and yield a suitable data set for a more unique inverse parameter estimation. In the following paper the test apparatus will be introduced and results of an experiment will be presented.