Abstract: The crushed Callovo-Oxfordian (COx) argillite powder is proposed as an alternative backfilling material in France, which will be used in the engineering barrier of high-level radioactive waste (HLW) repository. In this investigation, one-dimensional oedometer tests are carried out on highly compacted (dry density of 2.0 g/cm3) crushed COx argillite powder. The evolution of axial stress and the volumetric deformation characteristics of the samples are studied during saturation process. The effects of the initial axial stress level and the grain size distribution (GSD) on the hydro-mechanical behaviors are discussed. The results show that the volumetric deformation characteristics of samples greatly depend on the applied initial axial stress during saturation. Under the initial condition of higher axial stress, the monotone collapse behaviors are observed for the coarser powder samples. Whereas the finer powder samples present collapse-swelling-collapse behaviors, and the axial stress decreases firstly, then increases and finally decreases again. The initial lower axial stress causes obvious swelling behaviors for both the coarser and finer powder samples and the corresponding axial stress increases gradually. After saturation, the equilibrium axial stresses of finer powder samples are higher than those of coarser powder samples. The hydraulic conductivity is less than 10-11 m/s for all the studied samples after the dry density of 2.0 g/cm3 is reached.