Time-Dependent Behavior of Opalinus Clay: Insights from six Years of CD-A Experiment at Mont Terri

This contribution addresses the understanding of the observed time-dependent behavior of Opalinus Clay (OPA) due to excavation. During six years of the CD-A experiment at the Mont Terri Rock Laboratory (Switzerland), data from two parallel, unsupported niches (approximately 11 m long and 2.3 m in diameter) was collected. The twins were excavated in 2019, with one is exposed to seasonal climate fluctuations (leading to desaturation of the clay) andthe other sealed to maintain high humidity. Long-term measurements of deformation using extensometer and laser scans show a non-elastic convergence trend, especially in the open niche. This certainly cannot be explained by elastic models, but driven by a mechanical creep mechanism or by hydraulic-mechanically (HM) coupling. Comparisons of the behavior of the two niches provide useful insights here. Further measurements of hydraulic parameters, as well as geophysical surveys and geological characterization, support the investigation. Although many long-term measurements are available today, a clear statement cannot be made yet. One reason for this is the complex behavior of the OPA, especially in the unsaturated zone affected by excavation, where shrinkage effects are pronounced and damage and inhomogeneities occur. Coupled numerical HM modeling is an important tool for gaining further insights.

In addition to the initial outcomes (Ziefle et al., 2022), this contribution provides an update. The 3D HM model has been significantly improved, and now includes a more accurate representation of the equilibrium state prior to excavation, time-dependent excavation processes, and a physics-based description of the swelling behavior. Model validation against extensometer data establishes the model as a robust basis for further investigations, including parameter studies and testing of alternative constitutive models.

 

Ziefle, G. et al (2022): Multi-disciplinary investigation of the hydraulic-mechanically driven convergence behaviour: CD-A twin niches in the Mont Terri Rock Laboratory during the first year. Geomechanics for Energy and the Environment 31.