Abstract:

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.

This content is only available via PDF.
You can access this article if you purchase or spend a download.