Assessment of Anisotropic Elastic Parameters Using Laboratory Triaxial and In-Situ Pressuremeter Tests in Opalinus Clay

Abstract

AbstractOpalinus Clay is a stratified shale that exhibits anisotropic deformation properties. In this work, the transversely isotropic deformation parameters of Opalinus Clay are summarized from recent undrained triaxial test campaigns. Relatively consistent values are found for Poisson’s ratios representing different orientations, regardless of the lithofacies and the effective confining stresses used in the tests. Pressuremeter tests were performed at the Mont Terri Rock Laboratory in two boreholes perpendicular and parallel to bedding, respectively. The elastic moduli normal to the borehole wall (borehole moduli) are determined using unloading data obtained at multiple diametric caliper axes and exhibit strong anisotropy for the tests in the borehole parallel to bedding. The anisotropic borehole moduli are predicted based on Amadei and Savage’s (Int J Rock Mech Min Sci 28:383–396, 1991. https://doi.org/10.1016/0148-9062(91)90077-Y) analytical solution using the laboratory-derived Poisson’s ratios and the shear modulus derived from pressuremeter tests in the borehole perpendicular to bedding. The prediction overestimates the magnitude of the borehole moduli but underestimates their anisotropic ratio compared to that from pressuremeter measurement. Drilling a borehole parallel to the bedding of Opalinus Clay is known to induce a local borehole damage zone preferentially developed normal to bedding. The results from a finite element analysis that incorporates this local damage are in better agreement with the pressuremeter measurement.