Mechanical behavior of sandy facies of Opalinus Clay under different load conditions

The mechanical behavior of sandy facies of Opalinus Clay at the Mont Terri underground rock laboratory (URL) in Switzerland was investigated with drained and undrained triaxial compression and extension, cyclic compression, and creep tests. Samples were taken from boreholes drilled parallel to bedding. Most of the samples were reconditioned to minimize sampling effects of desaturation and micro-cracking. The compression was accomplished by increasing axial stress at constant radial stress. The extension was carried out by increasing radial stress at constant axial stress. Moreover, extension was also achieved by simultaneously increasing radial stress and decreasing axial stress under constant mean stress. The test results showed elastoplastic stress-strain behavior with volumetric compaction until onset of dilatancy at high deviatoric stresses above 80%-90% of the peak failure strength. The strength is dependent upon load path and mean stress. The strength under triaxial compression is higher than that under extension. The respective strength increases with increasing mean stress. Desaturation enhances the stiffness and strength of the claystone. The deformation and strength of the claystone are time-dependent. Under constant deviatoric stress, the claystone crept continuously with time, which can be characterized by a transient phase and a following stationary phase, and even a tertiary phase at high deviatoric stresses to rupture. (C) 2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).