Evolutions of Dynamic Elastic Properties of Opalinus Clay Under Varying Stress Paths

Abstract

ABSTRACT: Understanding the elastic stiffness evolution of the host rock under stress is important in the prediction of ground deformation during the construction and operation of a radioactive waste repository. The relationship established between dynamic elastic properties and stress also allows for an accurate interpretation of seismic survey and borehole sonic logging data in the field. In this work, the dynamic elastic properties of Opalinus Clay are determined using axial ultrasonic surveys at varying stress steps in triaxial tests. The evolutions of the P- and S-wave velocities are investigated along different stress paths, respectively with constant mean stress (CMS) and constant radial stress (CRS). For CMS testing, a cycle of triaxial compression and extension is performed. It is shown that P-wave velocity gradually reduced at the stage of triaxial compression and reversing the stress path in triaxial extension does not fully recover the velocity changes. The onset of specimen damage is identified at a deviator stress threshold where the velocity change diverts from its current trend in either triaxial compression or extension. The stress threshold differs between triaxial compression and extension, and it is inversely related to the water saturation of the Opalinus Clay. Before damage initiation, dynamic Young’s modulus shows a minor degradation under the CMS path, whereas it increases under the CRS path in triaxial compression. Both trends agree with the measured evolutions of static Young’s modulus of argillaceous claystones at similar stress levels. This study provides additional experimental guidance for constitutive modeling of stress-dependent elastic stiffness for this rock type. 1. INTRODUCTION Opalinus Clay is a shale with low permeability and has self-sealing potential upon the intake of moisture. It is considered as a possible host rock for future radioactive waste disposal in Switzerland (Bossart et al., 2017). Understanding the geomechanical response of the Opalinus Clay allows for the safe construction and operation of a disposal repository (Amann et al., 2017). One of the key geomechanical properties that have been under investigation is elastic stiffness.