Compression, swelling and rebound behavior of GMZ bentonite/additive mixture under coupled hydro-mechanical condition

The GMZ bentonite/additive mixtures have been proposed as the buffer/backfill media for the high-level radioactive waste geological disposal in China. In this study, two types of mixtures were prepared by mixing variable amounts of crushed granite powder or quartz sand (a range of 10% to 50% by dry mass of aggregate addition) with pure GMZ bentonite. To investigate the volumetric deformation characteristics of the bentonite/additive mixtures under coupled hydro-mechanical conditions, sequential oedometer tests comprising four phases were carried out, including preliminary compression, swelling under constant vertical stress, re-compression, and unloading-rebounding. Experimental results indicate that the volumetric deformation characteristics of the mixture largely depend on the initial condition, additive content and type. The evolutions of compression index and swelling index are governed by the degree of saturation and the state of soil matrix during different loading phases. During the compression phase, the soil compressibility relates to the formation of additive skeletal structure, while the rebound behavior during the unloading phase is determined by the competing mechanical and physico-chemical effects. The correlation between swelling index and bentonite dry density is not unique and dependent on the nature of the additive. The impact of additive properties on the volumetric behavior is further confirmed through tracking swelling indicators including final swelling strain and swelling pressure. This study provides an improved fundamental understanding of the mechanical behavior of GMZ bentonite/additive mixture under coupled conditions and optimizes their engineering applications for nuclear waste disposal.