Mechanical behaviors of a sandstone and mudstone under loading and unloading conditions

This study focuses on the effects of loading and unloading conditions on mechanical behaviors of a sandstone and mudstone. Two-type triaxial tests were carried out. One is the conventional triaxial test, in which the condition is an axial pressure controlled loading. The other is the unloading triaxial test, in which the condition is an axial pressure controlled preloading phase until reaching some 85% of estimated failure strength followed by a confining pressure controlled second loading phase until reaching failure strength. The experimental data indicate that the mechanical behaviors obtained from the unloading triaxial tests, such as the specimens’ failure surface, strain–stress curve, triaxial compressive strength, and triaxial shear strength, are different from ones obtained from the conventional triaxial tests. The failure surface observed in the conventional triaxial tests is a simple inclined slope, but, in the unloading triaxial tests, it is complex including two or more intersecting faces. The crest of deviatoric stress and corresponding axial strain obtained from the unloading triaxial tests are smaller. Some indexes to denote mechanical behaviors of rocks, such as triaxial compressive strength, angle of internal friction, and angle of shearing resistance, obtained from the unloading triaxial tests, are greater.