Influences of real-time temperature and stress cycle on gas permeability and deformation characteristics of unsaturated compacted bentonite

The multi-field coupling effect of underground high-level waste repository is complicated. As the first choice of buffer materials, unsaturated compacted bentonite needs to be able to effectively discharge the gas generated on the surface of the metal tank to the outside to avoid pressure accumulation. The quasi-stationary flow method and strain gauges are employed in this paper to measure the volume deformation and the effective gas permeability of two sets of unsaturated compacted bentonite samples under real-time temperature and confining pressure cycle. The influences of real-time temperature and stress cycle on the gas permeability and deformation characteristics of the samples are analyzed. Results suggest that the temperature increment will increase the compression deformation of the samples during loading, but has little effect on the deformation recovery during unloading. And there is a good exponential relationship between the effective gas permeability and the connected porosity of the samples during entire stress cycle under the same temperature condition. Combined with the NMR scan results, the mechanism of decline in gas permeability caused by temperature increasing is explained as: (1) the temperature rise increases the compression deformation of the samples and reduces the connected porosity, (2) the increase in temperature enhances the slippage effect of gas molecules and increases the resistance of air flow through pore channels, and (3) thermal expansion of pore water changes its distribution in the samples and further compresses the effective throat diameter of the connected pores. The present work deepens the understanding of the multi-physics coupling mechanism of buffer materials and provides data support for engineering design and numerical simulation. © 2022, Science Press. All right reserved.