Insights into anisotropic compression characteristics of unsaturated compacted GMZ bentonite

Compacted bentonite has been commonly recognized as an effective buffer/backfill material in deep geological repositories for high-level nuclear waste disposal. Anisotropic microstructure can be inevitably generated during the bentonite block compaction. More importantly, this anisotropy can be intensified by the stress-induced anisotropy produced during the subsequent engineering service while the bentonite block is being submitted to external stresses. In this work, using a modified suction-controlled high-pressure consolidation setup, one-dimensional compression tests were conducted on the compacted GMZ bentonite specimens along the directions both parallel (V-type specimen) and perpendicular (H-type specimen) to the compaction surface (bedding) formed during the specimen preparation processes. Quantitative analyses on the anisotropic compression characteristic, as well as insights into the formation and evolution mechanisms of anisotropic degree, were performed. The vertical (bedding) type (V-type) specimens exhibited more significant compression deformation, lower yield stress, and higher swelling index when compared to the horizontal type (H-type) specimens. The compaction-induced anisotropy could be intensified with increasing dry density and could be either strengthened or weakened during the subsequent compression processes, in which, the anisotropy of the horizontal type specimen kept continuously increasing, while that of the vertical type specimen decreased first and then gradually turned to increase. Development and evolution of stress-induced anisotropy closely depended on the stress level applied and the stress path followed. Relationships among the pre-consolidation pressures (major/minor principal stresses) during the specimen compaction and the subsequent one-dimensional compression played a vital role in the generation and evolution of the stress-induced anisotropy of the specimen.