Abstract: Artificial specimens of cemented rockfill materials with high rock block proportions (70%， 80%， 90%) were fabricated for uniaxial compression to understand the effects of rock block proportion and size effect on the failure modes and mechanical characteristics. The observed experimental phenomena were explained by numerical simulation. Results show that with the increase of rock block proportion， zones of both tensile stress and compressive stress develop as staggered distribution so that failure mode is dependent on the splitting failure and the shear failure. Increasing rock block proportion changes a tortuous failure surface contributed by splitting failure to multiple failure surfaces and detachment of rock particles along the periphery of specimens to form the compound failure mode combining splitting failure and shear failure. The size effect does not play any role in failure modes， but do play prominent role in mechanical characteristics. Mechanical properties of bigger samples are worse than those of smaller samples.

Key words: cemented rockfill materials, high rock block proportion, uniaxial compression, size effect