An Experimental Study on the Strength Behavior of Compacted Loess under the Principal Stress Axis Rotation

A series of consolidated-undrained tests was carried out on compacted loess with a hollow cylinder torsional shear apparatus to investigate the stress-strain characteristics and the strength behavior of the compacted loess under different stress paths. Two kinds of stress paths were considered in these tests, including directional shear tests with fixed principal stress axes and shear tests under pure principal stress axis rotation. Test results reveal that the stress-strain curves of the compacted loess under these two kinds of stress paths followed a similar trend, in which two shear stages were observed. In the first shear stage, with the increase of the generalized shear stress, the generalized shear strain developed relatively slowly and the specimen of the compacted loess showed a rigid response. With the further increase of the shear stress, an obvious turning point appeared and the second shear stage started. In the second stage, the generalized shear strain began to develop rapidly and the specimen showed a ductile response. The results also reveal that when the directional shear was applied to the specimens of compacted loess under different principal stress directions, there was a large difference in the generalized shear stress - strain curves and the specimens showed significant anisotropic behavior. A compared with the specimens not subjected to the principal stress axis rotation, the specimens subjected to the principal stress axes rotation had remarkably higher strength. In addition, the specimens of the compacted loess had different levels of strength when they were tested at different shear rates. The decrease of the shear rate had a slight influence on the increase of the shear strength of the compacted loess.