Insight into the brittle failure behavior of clay shales in unconfined and confined compression

摘要

The brittle failure behavior of clay shale from the Mont Terri Underground Rock Laboratory in Switzerland was investigated in confined and unconfined compression. The unconfined compression strength of the tested samples averaged 6.60 MPa. The crack initiation occurred on average at approximately 2.0 MPa based on analyzing both acoustic emission measurements and stress-strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress when the specimens were unconfined. In confined compression the crack initiation threshold was only slightly dependent on confining stress. The deviatoric stress at crack initiation was on average 2.2 MPa. The failure behavior is observed to be dependent on the confining stress. At zero confinement axial splitting was observed. With increasing confinement the angle of the macroscopic failure surface measured from horizontal flattens. At a confining stress >2 MPa the overall inclination of the stepped failure surface was 40-45°. Axial meso-fractures disappeared entirely when confinement was larger than 2 MPa. Instead of axial fractures, 50-55° inclined fractures appeared. The transition from axial splitting to macroscopic shear failure was associated with a continuous increase in rupture stress.