Instationärer Wassertransport in quellfähigen Böden unter gekoppelten hydraulischen, mechanischen und thermischen Randbedingungen: experimentelle und numerische Untersuchung
Zusammenfassung der Projektergebnisse
In Germany, a bentonite-sand mixture (50:50) compacted to bricks is proposed to be used as a backfill material in nuclear waste repositories. As an expansive clay, bentonite swells on exposure to water and fills the available constructional gaps and technical voids. For the mechanical stability of the entire disposal system, the rate of saturation and the resulting swelling pressure are crucial from the design point of view. Hence, an understanding of the coupled hydro-mechanical behaviour of backfill materials and the ability to model their behaviour is paramount for ensuring long-term safety. The present research work funded by DFG aims at improving the understanding of the coupled hydromechanical behaviour of compacted bentonite-sand mixtures at conditions relevant for repositories using a holistic approach combining experimental, theoretical and numerical investigations. A column-type experimental device was designed to investigate the behaviour of soil subjected to a repository-relevant hydraulic gradient. The experimental device facilitates the transient measurements of swelling pressure in both axial and lateral directions, and the simultaneous measurements of temperature, water content and relative humidity at various pre-selected locations. Validation tests were performed to ensure the reliability of transient measurements. A water infiltration test was performed with the pre-compacted sample of a Calcigel bentonite-sand mixture (50:50) using the newly designed device for a period of 349 days. The experimental set-up mimics the transient hydration of backfill material in a nuclear waste repository. The test results highlighted the hydration-induced heterogeneity in the material and its effect on the lateral swelling pressure development along the height of soil sample. The axial swelling pressure measurements revealed the factors which affect the stress-transfer mechanism between both the ends. The simultaneous measurements of relative humidity and water content indicated the porosity redistribution close to the hydration-end during the test. A fully coupled hydro-mechanical analysis of the water infiltration test was conducted using the finite element method. The modified Barcelona Basic Model (BBM) was used to describe the mechanical behaviour of soil along with the double structure water retention model for an indirect coupling. For identifying the model parameters, elementary tests were conducted with the Calcigel bentonite-sand mixture (50:50), which include water retention tests under constant volume condition and the suction-controlled oedometer tests on small-scale samples. The simulation results showed good agreement with the experimental measurements pertaining to the material hydraulic behaviour and captured the water transport mechanism in the material during the hydration process. Considering the mechanical behaviour, the modified Barcelona Basic Model successfully predicted the axial swelling pressure evolution at the bottom-end, while for the inner sections, the simulation results were not able to predict the measured lateral total stresses.
