ABSTRACT
ABSTRACT: The paper summarizes some of the more important rock mechanical properties that went undetected prior to the introduction of true triaxial testing. Properties such as true triaxial compressive strength and the corresponding criterion of failure, dilatancy onset, fault angle and direction, and microcrack alignment are all affected by the intermediate principal stress and this effect went unnoticed in conventional triaxial testing. True triaxial testing enabled the discovery of these and other mechanical properties. In addition, the cuboidal shape of true triaxial test specimens enables the application of three principal stresses that simulate conditions around open boreholes, where fluid pressure is applied radially directly to the exposed hole. This condition leads to a failure mode hitherto unrecognized, but similar to that leading to borehole breakouts and resembling the geological phenomenon of exfoliation.
