Calibration Method and Material Constants of an Anisotropic, Linearly Elastic and Perfectly Plastic Mohr-Coulomb Constitutive Model for Opalinus Clay

Nagra, the cooperative for developing and implementing a long-term radioactive waste depository in Switzerland, identified Opalinus Clay as the most suitable host rock for deep geological containment. This paper deals with those features of Opalinus Clay that are important for the design and construction of the underground structures. Consolidated drained (CD) and consolidated undrained (CU) triaxial compression tests on specimens from deep boreholes revealed that Opalinus Clay exhibits pronounced stiffness and strength anisotropy, dependency of stiffness on the initial confining pressure, slightly non-linear pre-failure stress-strain behaviour, and a drop in axial resistance after a certain amount of shearing. Within the scope of establishing a rigorous-yet practical-design approach for the repository tunnels and caverns, the simplest possible constitutive model capable of reproducing the main aspects of the Opalinus Clay behaviour is adopted. The non-associated linear elastic and perfectly plastic MC model is chosen as a starting point, on account of its wide use in tunnel engineering practice, its simplicity, and the clear physical meaning of its parameters. This paper presents a systematic and robust calibration method for an extended version of this model, which considers the pronounced strength and stiffness anisotropy of Opalinus Clay. The paper additionally provides the full suite of the equations that describe the model behaviour under triaxial CU or CD testing conditions and for any bedding orientation relative to the specimen axis. The equations are employed to determine ranges of material constants for two varieties of Opalinus Clay, based upon the results of 73 CU and CD tests. A thorough comparison between the model predictions and the experimental response is conducted, to demonstrate the versatility and limitations of the constitutive model and of the proposed calibration approach. A large number of CD and CU triaxial compression tests was evaluated.The adequacy of a simplified non-linear anisotropic material model was assessed.The prediction and calibration equations for the model were provided.The strength and stiffness constants of Opalinus Clay were determined based on the large experimental database.

Automated summary

This paper discusses the important features of Opalinus Clay for the design and construction of underground structures. The stiffness and strength anisotropy, dependence on initial confining pressure, slightly non-linear stress-strain behavior, and drop in resistance after shearing are identified through triaxial compression tests. The simplest constitutive model is adopted to reproduce the behavior, with a calibration method that considers the strength and stiffness anisotropy. The model's accuracy and limitations are demonstrated through a comparison with experimental data.