Simulation of homogenization behavior of compacted bentonite containing technological voids using modified penalty-based contact model

Abstract

Low-density zones generated during the bentonite blocks/voids homogenization process in the repository may serve as potentially preferential paths for radionuclide leakage. More importantly, void closure during homogenization process involves complex contact problems, where the stiffness at the contact interface typically undergoes significant fluctuations. In this work, with contact interface stiffness addressed through a step function approach, a modified penal.

ty-based contact model was proposed to simulate the contact behavior involved at the gap closure stage of the bentonite/gap assemblage homogenization process. Then, unsaturated infiltration swelling tests on bentonite block (1.7 Mg/m³)/gap (width: 2 mm) assemblages were performed, and the variation of dry density at different hydrated times (0, 24, 72, 168, and 720 h) in specific areas were measured. Based on the results, time-dependent swelling pressure profiles of the assemblage were acquired, while the homogenization process was evaluated. Results reveal that after approximately 40 h of hydration, the gap is completely closed, and the radial stress condition of the compacted bentonite transits progressively from the initial free swelling into a constant volume expansion state. The swelling pressure correspondingly develops quickly to a peak value at 1.8 MPa once the hydration starts, then decreases to a valley value of 1.4 MPa at the complete gap closure, and subsequently begins to increase to the final stable value of 1.8 MPa. Further examination reveals that as hydration advances, dry density of the assemblage converges to the expected final dry density with a maximum residual inhomogeneity of about 2 %. Finally, validations demonstrate that the proposed model can accurately reproduce deformations of the assemblage during the free swelling stage, and the swelling pressure profiles. A comparative analysis was made with the previous approach of identifying gaps as highly deformable materials, revealing that the proposed model overcomes the traditional limitations associated with the separation or penetration behavior occurring between the compacted bentonite and contact boundaries during the gap closure.