The influence of changes in principal stress directions on the anisotropic behaviour of kaolinite powder

The reported research comprised a comprehensive laboratory investigation into the influence of changes in the principal stress directions on the behaviour of dry kaolinite powder. The experimental programme was conducted using the Directional Shear Cell, a plane-strain, biaxial, soil element testing apparatus. Initial anisotropic compaction, was followed by various loading and unloading cycles taken close to the monotonic "critical state" condition, each half cycle resulting to a 90 degree change in the principal stress directions. Cyclic loading was followed by shearing to failure along various stress paths. The deformational behaviour of the samples during cyclic loading and their post-cyclic stress-strain response and developed strength were monitored. Monotonic tests were also conducted for comparison. The data obtained showed that in the initial stages of cyclic loading significant volumetric compression and shear deformation occurred. After further cycles, the samples reached a state where small, repeatable components of resilient volumetric and shear strain occurred within each cycle. The first change in principal stress directions resulted in a significant loss of material strength, but this progressively recovered during the subsequent cycles. Samples subjected to high numbers of cycles were shown to sustain smaller strains when probed to failure. Failure took the form of either rupture planes or homogenous flow; occasionally both modes were observed. The dominant mode of failure and the failure envelope that operated were dependent on the direction of the post-cyclic stress path to failure and the number of cycles. The author explains the behaviour of kaolinite powder under cyclic loading using the progressive development and interlocking of two distinct anisotropic structures related to the two directions of the major principal stress. The particles are thought to form columns, transient in nature, that carry the loads through the sample. Deformation and subsequent failure occur through the collapse of these columns.