Wiebke Baille

Effects of Temperature on Measurements of Soil Water Content with Time Domain Reflectometry

This paper analyses the effects of temperature on the quantification of soil water content by means of time domain reflectometry (TDR) method. For this purpose, the dielectric constant of soil specimens with known water content was measured at different temperatures in a range of 20–80°C. The soil types used in the present study were fine sand, sand-bentonite mixture (SBM), and sandy loam. For each soil type, the dielectric constant of at least three specimens having identical dry density but varying initial water content was measured at temperatures ranging from 20 to 80°C. The results obtained agree with previous studies showing that there are two competing phenomena during the measurement of soil water content by means of TDR: (i) The soil bulk dielectric constant increases with increasing temperature due to the release of bound water from soil solid particles and (ii) the soil bulk dielectric constant decreases with increasing temperature due to the temperature effect of free water molecules. Moreover, it has an existing equilibrium water content at which both competing phenomena compensate each other. However, for the SBM no equilibrium water content was found, but the dielectric constant increased significantly with the temperature for the whole water content range. This can be explained by the significant clay content and the high specific surface area, which leads to a dominating effect of the increase of soil bulk dielectric constant with increasing temperature.

High-density bimodal bentonite blends for hydraulic sealings at the Ibbenbüren coalmine

Abstract The two high-pressure water-retaining dams at the Ibbenbüren coalmine in Münsterland (Germany) have to perform reliably under the induced tension caused by further exploitation of the current mining area. The load-bearing and the sealing functions of the new barriers were separated and new sealing materials were developed. An innovative multilayer sealing system of bentonite and sandwiched equipotential layers (SANDWICH) supporting homogeneous swelling and sealing, independent of formation water (Nüesch et al., 2002), was applied in this project. A testing program of strain-controlled swelling pressure tests on compacted bentonite specimens and on a bentonite/sand mixture was conducted to ensure an adequate potential for swelling-pressure development. The measurements under constant volume for dry densities between 1.45 g/cm³ and 1.67 g/cm³ showed an evolving swelling pressure between 1.04 and 1.8 MPa for 100% bentonite samples. Strain-controlled oedometer tests for zero strain and step-wise applied strain up to 2% revealed that a sufficient magnitude of swelling pressure existed at maximum applied strain.

Swelling Behavior of Bentonite-Based Backfilling Materials in Nuclear Waste Repository Conditions

AbstractDuring the postconstruction phase, the stress equilibrium conditions within the proposed underground nuclear waste repositories may alter because of various factors, such as far-field mining activity, fluctuation of the groundwater table, and seepage erosion. A reduction in the transmitted stress from the host rock may destabilize the isochoric condition, leading to swelling displacement of backfilling and buffer materials. In the current study, the swelling behavior of bentonite-based backfilling materials was studied. Laboratory tests were carried out on compacted bentonite (dry density=1.26  Mg/m3) and bentonite-sand (70% bentonite, 30% sand, dry density=1.50  Mg/m3) specimens in a strain-controlled oedometer. First, compacted specimens were hydrated with distilled water under constant volume condition. Next, predetermined displacements were introduced in a step-wise manner that caused expansion of the specimens. The swelling pressures of the specimens were measured under a constant-volume cond...