STATE-OF-THE-ART: EXPERIMENTAL STUDY ON MECHANICAL PROPERTIES OF ROCKFILL MATERIALS UNDER TRUE TRIAXIAL CONDITIONS

This paper reviews the development of geotechnical true triaxial apparatus and the research status of the mechanical properties and mechanism of rockfill materials. Then, a large-scale geotechnical static-dynamic true triaxial apparatus developed by this research team and the preliminary experimental results on the mechanical properties of rockfill materials are introduced. According to loading modes, the true triaxial apparatus for soil testing can be divided into three categories, i.e., the rigid loading type, the flexible loading type, and the hybrid loading type. True triaxial testing of rockfill will be more challenging than that of sand and of clay due to the large specimen size, high pressure, strong mutual interference in different directions, difficulty in load alignment after large deformation, strong friction between the load plate and the specimen, and the complexity of specimen installation and measurement. Thus, the true triaxial apparatus applicable to rockfill materials, as well as the related results, are still relatively rare currently. To study the mechanical properties and to develop corresponding constitutive models of rockfill materials by considering the particle breakage and anisotropy under true triaxial conditions and complex stress paths have become a trend of this research topic. The large-scale static-dynamic true triaxial apparatus at Tsinghua University adopts the specimen sealing device equipped with oval caps and a specially designed latex membrane, which effectively solves the problems relevant to the sealing under high pressure, assembly, disassembly, and the measurement of rockfill specimen and the durability of latex membrane. A series of true triaxial tests with complex stress paths were carried out on rockfill materials. Studying results show that:the rockfill in a true triaxial test presents significant stress-induced anisotropy and stress-state dependence, especially for the minor principal stress-strain relation, the mean stress-volumetric strain relation, and the generalized shear stress-shear strain relation. Also, the mean stress and generalized shear stress have a significant cross-effect on the volumetric strain and generalized shear strain.