Strength Testing of Sandstone Under Multi-Axial Stress States

A true triaxial apparatus was designed and fabricated to achieve multi-axial stress states with prismatic specimens. The device, which is a modification of the University of Minnesota plane-strain apparatus, is equipped with piston assemblies that generate the intermediate principal stress. A closed-loop, servo-hydraulic load frame is used to control the major principal stress, while the minor principal stress is developed by fluid pressure. A stress path was developed to perform experiments under various states of stress at constant mean stress. Results from strength testing on Dunnville sandstone under conventional compression and extension are compared with the results performed under multi-axial stress states. Failure data were plotted in principal stress space to investigate the effect of intermediate stress on strength. A plane fitting method was used to obtain the material parameters of two, six-sided Paul–Mohr–Coulomb failure surfaces, which capture the strength characteristics of the sandstone over a range of mean stress.