Experimental Investigation on Principal Stress Rotation in Kaolin Clay

A combined axial–torsional testing system was developed to investigate the effect of rotation of principal stresses on the three-dimensional mechanical behavior of Kaolin clay. Uniform and reproducible cohesive specimens having a specimen shape of a hollow cylinder were obtained using a two-stage slurry consolidation technique. Precise stress paths (triaxial compression to pure torsional shear to triaxial extension), corresponding to a fixed rotation of the major principal stress axis, were achieved by using the proportional-integral-derivative (PID) feedback control technique. Kaolin clay specimens were tested under a variety of stress paths associated with a constant principal stress rotation angle (β) under undrained conditions. Typical test results, such as effective friction angle, undrained shear strength, stress–strain relationship, pore pressure evolution, and stress paths are presented as a function of β . During shearing, the procedure to use advanced servo-hydraulic control (using PID algorithm...