Particle breakage, deformation and shear strength of conglomerate rockfill material: a case study of Masjed Soleyman Dam cracking and settlement

Masjed Soleyman rockfill dam, with a height of 177 m, has experienced significant settlement during its construction, impoundment and operation stages. A possible contributing factor in the excessive deformation is the weak conglomerate rockfill material used in building the dam shells. This research work focuses on the experimental study of the mechanical behavior of the conglomerate rockfill material of the dam shells, including the point load, Brazilian, oedometer, and direct shear tests. The results of the point load and Brazilian tensile tests showed moisture and size effects, and suggested that the strength of the rockfill made of this rock falls within the medium to weak range. The oedometer test results demonstrated that the specimen moisture, density, gradation and applied stress impact the particle breakage, strain, and saturation collapse of the rockfill material. Our findings suggest that the excessive dam settlement and saturation collapse (wetting deformation) could have been substantially decreased and controlled by wet compacting the rockfill layers during the dam construction. It was shown that particle breakage in the direct shear tests is greater than that in the oedometer tests, suggesting that for dams with excessive shear deformation (like Masjed Soleyman Dam), a more realistic estimate of particle breakage can be obtained using either direct shear test or triaxial tests. The ratio of strain to particle breakage index was revealed to be independent of the applied stress and moisture content of the specimen, but it was affected by the material gradation and density.