Development and Validation of a Constitutive Model for Opalinus ClayCopyright: © LIH
Opalinus clay, a Mesozoic shale formation, has been selected as the potential host rock for disposal of high- and low-level nuclear waste in Switzerland. Some of the beneficial characteristics of this formation are the low hydraulic conductivity, the high radionuclide retention potential, and the potential of self-sealing of open cracks and fissures.
The aim of this project is to develop a fully coupled hydro-mechanical (HM) constitutive model to predict the physical behaviour of Opalinus (OPA) clay under various loading and unloading conditions. The key aspect is to integrate existing HM-coupled models supported by laboratory tests, field experiments and numerical simulations, and develop, implement and validate a new constitutive model.
Phase 1.0 (2018-2022)
n the first phase of this project, we performed HM-coupled experiments using our state-of-the-art laboratory machines. Therefore, we used our in-house technology to prepare, store, monitor and perform, for instance, drained or undrained triaxial compression tests under various configurations considering the material anisotropy. The post-failure analyses were carried out by analyzing the deformation structures using electron microscopy.
A transversely isotropic elasto-plastic model coupled with damage was implemented in the finite element code “MOOSE Framework” to describe the hydro-mechanical behavior of Opalinus Clay. The model describes four de-formation regimes observed in consolidated-undrained tests, i.e., 1) transversely isotropic elastic deformation, 2) plastic deformation with cross-anisotropic peak strength, 3) post-failure regime, and 4) residual strength state.
Main results obtained from phase 1.0
An extensive testing campaign has been conducted on Opalinus Clay specimens from the shaly facies sampled from the Mont Terri URL. Hydraulic properties have been determined using different experiment methods including the pore pressure oscillation technique. Different methods have been evaluated in terms of their applicability, robustness and experimental times. Rock mechanical strength tests have been performed for different suction values and different loading orientations. Furthermore, triaxial tests under consolidated-undrained conditions have been performed under various boundary conditions to investigate the effect of effective stress and loading angles. Electron microscopy has been carried out to study the microscale deformation processes to link the bulk hydromechanical observations to the underlying mechanics at the sub-millimeter level. The high amount of experimental data and the analyses have substantially contributed to the calibration of the new constitutive model and the implementation of fundamental behavior characteristics of Opalinus Clay.
A non-associated plastic model was implemented in the “MOOSE Framework”. The implemented model was coupled with a non-local damage model to describe the failure and post-failure behavior of Opalinus Clay under undrained conditions. A series of consolidated undrained experiments were used to calibrate the model and determine the required model parameters for P-, Z- and S-specimens acquired from shaly facies of Mont Terri URL. A large-scale 3D tunnel model was also developed to evaluate the performance of the implemented constitutive model against existing field data, e.g. pore pressure evolution, deformations, and EDZ (excavation damage zone).
Phase 2.0 (2022-2025)
In the second phase of this project, we perform long-term THM-coupled triaxial experiments for observations of the time-dependent deformation behavior of fully saturated samples under drained conditions. Equivalent to the first project phase, the deformed specimen will be subjected to microstructural analysis (BIB-SEM) for a better understanding of creep mechanisms on the microscale in clay rocks. Multistage creep tests will be conducted with consideration of the structural anisotropy and the effect of temperature on the creep behavior. Therefore, different specimen configurations with the bedding oriented either parallel or perpendicular to the loading axis will be tested under elevated temperature conditions. Additionally, sample material from the EDZ will be the subject of creep investigations to analyze the influence of damage on long-term creep behavior.
The existing constitutive model will be calibrated along experiments performed on sample material from greater depth, i.e. from higher effective consolidation stresses. Additionally, a creep model will be implemented in MOOSE to simulate the time-dependent deformation observed within the creep tests. The focus will be mainly on the steady-state and tertiary creep deformation under the influence of stress and temperature.
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- Khaledi, K., Hamdi, P., Winhausen, L., Jalali, M., Jaeggi, D. and Amann, F., 2021. Unloading induced absolute negative pore pressures in a low permeable clay shale. Engineering Geology , 295 , p.106451.
- Winhausen, L., Klaver, J., Schmatz, J., Desbois, G., Urai, J.L., Amann, F. and Nussbaum, C., 2021. Micromechanisms leading to shear failure of Opalinus Clay in a triaxial test: a high-resolution BIB–SEM study. Solid Earth , 12 (9), pp.2109-2126.
- Winhausen, L., Khaledi, K., Jalali, M., Urai, J.L. and Amann, F., 2022. Failure mode transition in Opalinus Clay: a hydro-mechanical and microstructural perspective. Solid Earth , 13 (5), pp.901-915.