Joint project GeomInt 2: Geomechanical integrity of host and barrier rocks - Experiments, modelling and analysis of discontinuities

Country / Region: Germany

Begin of project: October 1, 2020

End of project: September 30, 2022

Status of project: March 10, 2022

(Follow-up on GeomInt 01.07.2017 – 30.09.2020)

German Version

The safe storage of toxic and radioactive waste requires an interdisciplinary and careful geoscientific research and prognosis studies to avoid harmful effects on the environment. Interventions in the geological subsurface lead to deformation and physico-chemical alterations of the rocks. Thus, the integrity of geologic reservoirs and barriers can be unfavorably affected: fissures and cracks can develop or be activated. We summarize these diverse micro- and macromechanical structures under the term “discontinuities” whose origin is mostly insufficiently understood and cannot be adequately modeled with the commercial simulation systems currently available.

Processes such as swelling/shrinking, percolation/healing, and stress redistributions can influence or lead to discontinuities. BGR's work in the project GeomInt 2 will focus on investigating and modeling the swelling and shrinking processes observed in the Mont Terri CD-A experiment. Here, two 11-m-long niches were excavated and extensively instrumented during the fall in 2019. While one niche is ventilated and thus exposed to seasonal variations in humidity, the second niche is kept closed by a bulkhead. First geophysical measurements as well as observations of desiccation cracks on the walls already indicate the influence of desaturation in the ventilated niche, while the sealed niche, where a constant high humidity has been established, does not show such cracks. BGR will perform in-situ sampling, geophysical measurements in laboratory and in-situ, and numerical analyses to investigate the hydraulic-mechanical processes in the niches (Fig. 1). Here, the numerical methods developed in GeomInt will be used and, if necessary, further refined to model and analyze the observed differences between the two niches. In a first step, the various influences on the hydraulic-mechanical behavior will be numerically investigated using a two-dimensional model. Furthermore, the possibility of modeling shrinkage-induced cracking in hydro-mechanical context is investigated on the mechanical side using the phase field method and plasticity models and on the hydraulic side using multicontinua models. A quantitative evaluation will be performed by comparison with measured data to enable a basic validation of the modeling approach at field scale.

Fig. 1: BGR activities in the joint project GeomInt2 Source: BGR

The research project is part of the geoscientific research funding programme "GEO:N – Geosciences for Sustainability" of the Federal Ministry of Education and Research (BMBF). It aims at an intensive international collaboration with the Mont Terri Rock Laboratory as well as the DECOVALEX 2023 and STIMTEC2 projects. Discontinuity leading processes were investigated in the first GeomInt project phase (Link) using both numerical and laboratory methods. the main results of the first project phase are reported in the open access book Kolditz et al. (2021, https://doi.org/10.1007/978-3-030-61909-1). Based on the results of this project, the GeomInt 2 project was launched in October 2020.

The focus of the GeomInt2 research project is on the evaluation of recent in-situ experiments in the Mont Terri (Opalinus clay), Springen (salt) and Reiche Zeche (crystalline) rock laboratories. Selected laboratory experiments are planned to further address targeted remaining knowledge gaps such as mechanical anisotropy of claystones. The research project is divided into three work packages dealing with pathways due to (1) swelling and shrinking processes, (2) pressure-driven percolation, and (3) stress redistribution. In-depth laboratory tests for detailed process understanding and numerical process modeling are planned in all packages. The BGR is mainly involved in work package 1 and contributes its experience at the interface between geological and numerical issues.

In connection with the Cyclic Deformation (CD-A) experiment at Mont Terri, several numerical models were created and computed using the open-source finite element software OpenGeoSys (OGS 6, https://www.opengeosys.org/). The focus of the computational models is on understanding coupled hydromechanical processes and shrinkage-induced cracking. Currently, the applicability of the developed approach is being tested in both laboratory and in situ scales (Fig. 2).

Fig. 2: Numerical models related to the CD-A Experiment Source: BGR

Funded by the
Federal Ministry of Education and Research
Program: Geo Research for Sustainability (GEO:N)
Funding number: 03G0899F
Project website: https://www.ufz.de/geomint

The results of the project GeomInt2 are published as Open Access-Book.

Literature:

• Kolditz, O.; Görke, U.-J.; Konietzky, H.; Maßmann, J.; Nest, M.; Steeb, H.; Wuttke, F.; Nagel, Th. (Eds.): GeomInt–Mechanical Integrity of Host Rocks. Series: Terrestrial Environmental Sciences. Springer, 2021. ISBN 978-3-030-61908-4, DOI: 10.1007/978-3-030-61909-1
  
  
• Olaf Kolditz, O.; Yoshioka, K.; Cajuhi, T.; Günther, R.-M.; Steeb, H.; Wuttke, F.; Nagel, T. (Eds.): GeomInt-Discontinuities in Geosystems From Lab to Field Scale. Springer, 2023. ISBN 978-3-031-26492-4, DOI: 10.1007/978-3-031-26493-1

Partner:

• Helmholtz-Zentrum für Umweltforschung (UFZ) Leipzig, Department Umweltinformatik (Verbundprojektleitung)
• Christian-Albrechts-Universität zu Kiel, Department of Geosciences, Geomechanics and Geotechnics
• Technische Universität Bergakademie Freiberg, Institut für Geotechnik
• University Stuttgart, Institue of Applied Mechanics (Civil Engineering) (en)
• Institut für Gebirgsmechanik GmbH (IfG), Leipzig (en)