Investigation on healing behavior of unsaturated GMZ bentonite pellet mixture based on compressibility: Fid-Bau Portal

Investigation on healing behavior of unsaturated GMZ bentonite pellet mixture based on compressibility

Zhang, Zhao / Ye, Weimin / Wang, Qiong / Chen, Yonggui

Bentonite pellet mixture has been considered as an important candidate buffer/backfill material in deep geological repository for disposal of high-level radioactive waste. During the long-term operation of a repository, large inter-pellet pores inside the pellet mixture will dominate the formation of engineering barrier and even work as preferential migration pathways of water, gas and radionuclides. In this study, GMZ bentonite pellet mixtures were first wetted to various suctions under a constant-volume (CV) or constant-stress (CS) condition, and then submitted to suction-controlled compression tests. Results reveal that all compression curves of the pellet mixtures are nearly bilinear, while the unloading curves of the pellet mixtures appear to be linear in high-suction stage (s > 4.2 MPa) and bilinear in low-suction stage (s < 4.2 MPa). Normal consolidation line of compacted bentonite block could be proposed as healed state line (HSL) of pellet mixtures. As the vertical stress increases, the compression curves of pellet mixtures gradually approach and finally coincide to the HSL, indicating that the healed state can be reached by mechanical loading. The healed stresses for saturated pellet mixtures are closely equal to their pre-consolidation pressures, while for unsaturated pellet mixtures, the healed stresses are higher than the pre-consolidation pressures. Simultaneously, the healed stress of pellet mixtures reduces with the decrease of suction. Furthermore, the healed stress is independent of wetting boundary conditions (CV and CS) in high-suction stage (s > 4.2 MPa), while the obtained healed stresses under the CS condition are lower than those obtained under the CV condition in the low-suction stage (s < 4.2 MPa).