A series of true triaxial tests has been carried out on the quartz-rich, high-porosity Bentheim sandstone (n = 24%) using a novel loading path that maintains constant the deviatoric stress state parameter (N). We applied six different N’s, spanning from axisymmetric compression (s2 = s3) to axisymmetric extension (s2 = s1). Tests reveal the effect of mean stress (soct) and deviatoric stress state parameter (N) on failure and failure-plane angle. Regardless of N selected, the octahedral shear stress soct at failure rises with increasing soct s reaches a ‘cap’, and then drops for larger soct. A single shear band, or fault, having an angle of about 80° is characteristic at the lowest soct used (~ 20 MPa). As soct is increased, failure-plane angle decreases monotonically to about 50° when soct reaches ~ 150 MPa. Within this range of soct, rock deforms dilatantly. At higher soct compactive features start developing. In the range of soct = 150 - 200 MPa, multiple, conjugate shear-enhanced compaction bands develop at failure (angle below 45°). At soct = 200 MPa, pure compaction bands with nearly 0° angle are formed. The tests also demonstrate the effect of the deviatoric stress state parameter N on failure and failure-plane angle. As N varies from +1 (axisymmetric compression, s2 = s3) to - 1 (axisymmetric extension, s 2 = s1), soct required for failure increases monotonically, demonstrating the strengthening effect of deviatoric stress state.

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