Effect of granular structure and initial suction on shear strength of GMZ bentonite for deep geological disposal

Abstract

Granular bentonites have recently attracted increasing attention to serve as buffering/backfilling material alternatives in deep geological repository for disposing high-level radioactive waste (HLRW). In this paper, GMZ bentonite was prepared in the forms of compacted powder (CP), powder/pellet mixture (PPM) and pellet mixture (PM), and a series of direct shear tests, compression tests, soil-water retention tests and microstructural observation were conducted. Results show that the water retention capacity is independent of granular structure and dry density at suction s > 10 MPa. Below this suction, variation in water content should be related to the difference in microstructure. As s decreased, the specimen became more compressible and its shear resistance decreased while showing unimportant changes at s > 100 MPa. Besides, differences in compression and shear behavior between three types of specimens became gradually less obvious with decreasing suction. This indicates that wetting-induced comparable fabric could gradually cause similar compression and shearing response. Both high-density pellets and gradation are considered to affect the internal friction mode and mechanical interlocking phenomenon, so that the peak shear strength and relevant parameters varied. The unified effective stress equation considering both capillary and adsorptive mechanisms was used to predict the peak shear strength, and the predicted values agreed well with test data mostly for the high suction range. The results and ideas obtained in this study are conceived to provide helpful reference for material selection and assessment of bentonite barriers for deep geological disposal.