Characterizing and Modeling Elastic Stiffness Variation of Opalinus Clay  Based on Laboratory and In-situ Measurements
Creators: Liu, Lang
Publisher: name: University of Alberta Library
Year: 2022

<p>Opalinus Clay is the designated host rock for future radioactive waste disposal in Switzerland. Characterizing the elastic stiffness of Opalinus Clay and its variation is essential in the ground deformation prediction and geophysical survey during the construction and operation of disposal repositories. In this thesis, an in-situ method, pressuremeter testing (PMT), is employed in three boreholes at the Mont Terri Rock Laboratory to determine the in-situ elastic stiffness of the Opalinus Clay. The shear modulus is measured by PMT in Oplainus Clay using the unloading steps at different expansion pressures, and its dependence on pressure is shown to be significant when the expansion pressure is less than 5 MPa. The interpreted shear modulus values agree with those measured from triaxial tests on core specimens when small-strain nonlinearity, elastic anisotropy, shear mode, and loading path are considered. A representative modulus value of approximately 3 GPa is established for intact Opalinus Clay at the Mont Terri site for the shear strain magnitude at 0.1%. The prediction using the anisotropic elasticity parameters obtained from laboratory tests reasonably agrees with the anisotropic borehole modulus measured in a borehole drilled parallel to bedding planes.<br>
The evolution of elastic properties is measured using an ultrasonic survey during triaxial tests on laboratory specimens. A cycle of triaxial compression and extension under constant mean stress is applied to the specimens to approximate the stress condition during borehole unloading and reloading. The measured evolution of dynamic elastic modulus differs from that obtained from the triaxial tests under constant lateral stress. Compared to the static modulus, the sensitivity of the dynamic modulus to stress is not significant for the applied stress range. The damage initiation threshold is identified from the measured evolution of the elastic properties and can be distinguished for different triaxial shear modes and specimen saturations.<br>
The variation of static elastic modulus during triaxial loading was modeled using three elastic stiffness functions. Isotropic damage and plastic yielding are incorporated in the constitutive formulation. The model reproduces the nonlinear stress-strain relationship observed in multi-staged triaxial tests. The axisymmetric borehole response in a cycle of drilling unloading and pressuremeter loading is also predicted by implementing the proposed model into a finite element (FE) code. The modeling study demonstrates that the modulus values obtained from pressuremeter tests can be underestimated due to drilling-induced borehole damage even when the stress at the borehole wall is recovered to the initial state.<br>
The borehole damage can be localized by (1) overstressing and/or (2) the instability of geological structures, e.g., bedding planes, after drilling unloading. Identifying the localized BDZ by overstressing may enable the determination of the far-field stress orientation at the borehole cross-section plane. The in-situ methods, including rotational interval velocity measurement (ROT-IVM) and PMT, both show the azimuthal variation of elastic properties in the borehole nearfield. The ROT-IVM data can be further analyzed using tomography inversion, which allows for an estimation of the extent of BDZ. The variations of P-wave velocity and its anisotropy, however, are not likely linked to the estimated variation of the stress field but to the varying saturation of Opalinus Clay at different measurement locations. The anisotropic borehole modulus measured by PMT at a lower pressure range may indicate the orientation of BDZ, but the indication can be complicated by the local pore fluid dissipation in the near-borehole field. Alternatively, using the PMT data after borehole yielding under high expansion pressure is shown to be promising in capturing the far-field stress orientations in the Opalinus Clay, regardless of the initial borehole disturbance.</p>