WP 1: Pathways through swelling and shrinkage processes
A second borehole BAD-2 with a depth of 29.2 m was drilled in May 2019 at Mont Terri. Sampling was performed by scientists from BGR and CAU Kiel. While the first borehole BAD-1 was drilled parallel to the geological layers to obtain samples from the sandy facies only, the BAD-2 was drilled perpendicular to the layering to investigate the spatial extent of the different facies. At CAU Kiel, shrinking and swelling experiments were run on claystone under applied free boundary suction and temperature conditions. When the water content of clay changes, the clay often reacts with shrinking or swelling. This can be both a problem for the sealing of underground repositories as well as a tool. The IfG conducted experiments and simulations to analyze the process and the related changes of the stress state.
In geotechnical/geomechanical laboratory of CAU Kiel, the prepared two cylindrical thin sections of sandy Opalinus claystone are used to determine the drying and wetting paths. The saturated salt solutions are used to apply different osmotic suctions, such as Magnesium Chloride (MgCl2), Magnesium Nitrate (MgN2O6), Sodium Chloride (NaCl), Potassium Chloride (KCl) and Potassium Nitrate (KNO3). The suction values range from 3.2 up to 367 MPa, which insures both drying and wetting paths. The room temperature is around 20 °C and the fluctuation of the temperature is negligible. The first sample is used to determine the linear axial strains along the embedded layers, which are arranged in a parallel and perpendicular orientations. The second sample is used to determine the water content change during the wetting and drying paths. The samples dimension is 100x10 mm (DxH). The equilibrium inside the desiccator is reached when the change of the samples water content is zero. Fig. 1a. depicts the change of suction and axial linear strains in parallel and perpendicular directions. Similarly, Fig. 1b. shows the change of water content with applied suction using salt solutions. In drying path, the results indicate a higher strain for a strain perpendicular to the embedded layers. When the suction is higher than 150 MPa, the strains in perpendicular direction are almost 4.5 larger than parallel ones. Interestingly, in the wetting path, the differences between the strain gauges are much less. According to the water content data, the air-entry pressure for a sandy Opalinus claystone is found to be around 25 MPa.
Fig. 1. The change of (a) suction and axial linear strains in parallel and perpendicular directions, and (b) water content with applied suction using salt solutions
The experimental campaign “Experimental-Numerical Analysis of Discontinuities (AD)” in the Mont Terri Rock Laboratory was continued by the BGR in collaboration with the CAU Kiel. In particular, a second borehole BAD-2 with a depth of 29.2 m was drilled in May 2019. Sampling was performed by scientists from BGR and CAU Kiel. While the first borehole BAD-1 was drilled parallel to the geological layers to obtain samples from the sandy facies only, the BAD-2 was drilled perpendicular to layering to investigate the spatial extent of the different facies. The boreholes have been analyzed by means of Electrical Resistivity Tomography (ERT) and miniseismic techniques (Kneuker et al., 2017). The BAD-2 borehole yielded an additional 13 m of samples from the sandy facies, which will be further analyzed in the laboratory of CAU Kiel to investigate discontinuities such as the propagation of desiccation cracks.
Furthermore, the documentation of numerical simulation campaign to investigate swelling and shrinkage in an idealized niche of the Mont Terri URL was finished. This work will become a chapter in the upcoming GeomInt Book (WP4).
Kneuker, T., Hammer, J., Shao, H., Schuster, K., Furche, M., & Zulauf, G. (2017). Microstructure and composition of brittle faults in claystones of the Mont Terri rock laboratory (Switzerland): New data from petrographic studies, geophysical borehole logging and permeability tests. Engineering geology, 231, 139-156.
At the geomechanical laboratory at CAU Kiel, shrinkage and swelling behavior of claystone under applied free boundary suction and temperature conditions are investigated. Shrinkage and swelling are induced by preparing two thin section samples and storing them in a desiccator, in which a salt solution with a specified osmotic suction value is placed. The salt solutions under consideration include the following: magnesium chloride (MgCl2), magnesium nitrate (MgN2O6), sodium chloride (NaCl), potassium chloride (KCl) and potassium nitrate (KNO3).
The change in water content in the second sample during a two-week period for each salt solution (long enough to reach equilibrium) is noted and a suction-water content behavior is obtained. At the same time, two linear strain gauges with a dimension of 10 mm are glued onto the first sample such that the axial strains parallel and perpendicular to layering alignment can be measured. Finally, a water content-axial strain as well as suction-axial strain behavior of claystone during the shrinkage and swelling process are captured and are implemented in the developed THM lattice model.
Experiment: Closure of pathways in clay through swelling
A dry sample of red salt clay from the Zechstein T4 series was exposed to water (2 bar) at its top and bottom surfaces. A mantle pressure of 4 bar was applied, and the permeability and volume change of the sample were monitored. Photo of setup:
A swelling of the sample was observed through the amount of liquid inside the apparatus, which decreased over the course of weeks:
During the first week of the experiment, the permeability of the sample decreased because the swelling closed or narrowed pathways in the sample. After that, an increase in the permeability is observed as the water dissolves part of the salt in the clay, opening up new pathways.
A second experiment using brine instead of water is currently running.
BGR / UFZ
Numerical investigations at BGR and UFZ have been focused on the implementation of algorithms that account for discontinuities in the Hydraulic-Mechanical (HM) modeling framework of OpenGeoSys (OGS6). To this end, extensive tests have been carried out with OGS6 at BGR to compare the simulation results generated by its predecessor OGS5 that were meant to reproduce in-situ experiments of seasonally induced cyclic deformation of a gallery in the Mont Terri Rock Laboratory (Fig. 1).
In the meantime, the UFZ has implemented the Phasefield Method in OGS6 to account for discontinuities as a damage variable, which will enhance the mechanical coupling of the HM modeling by accounting for crack propagation in the rock. As a next step, these efforts will be combined to model the cyclic desiccation and saturation of clay rocks to understand the effects of discontinuities on the mechanical properties of a gallery in the Mont Terri Rock Laboratory.
In CAU Kiel and with the application of experimental and numerical approaches, the material behavior during the shrinkage and swelling phases will be investigated. Within a free shrinkage and swelling boundaries and taking the microscopic images from the surface of the claystone, the movement of predefined mesh points on the sample surface with time will be observed. The micro-fracking and opening of the fracks will be interpreted and a shrinkage coefficient will be then deployed in the shrinkage lattice model. The 2D and 3D CAU Lattice code are able to predict the stochastic frack initiation and propagation under THM processes. The shrinkage coefficient will be then implemented in LEM to simulate the aggregate-shrinkage model.
According to aggregate-shrinkage model , the new length of an element (L_(t+1)) in each time step is:
Where α_s is a shrinkage or swelling coefficient and T is the total time where the evaporation rate is equal to zero. The shrinkage rate depends on the amount of saturation (S) and the water pressure (p) in the interparticle porous medium. The dual Lattice model  is utilized to determine the water flow and developed water pressures in a porous medium .
 J. Sima, M. Jiang, and C.Zhou (2014). Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling. Computers and Geotechnics 56, 168–180.
 P. Grassl (2009). A lattice approach to model flow in cracked concrete.Cement & Concrete Composites 31 (2009) 454–460
 T. D. Vo, A. Pouya, S. Hemmati, A. M. Tang (2017). Numerical modelling of desiccation cracking of clayey soil using a cohesive fracture method. Computers and Geotechnics 85, 15–27
After detailed planning from BGR, IfG and CAU, a BGR team conducted the first drilling campaign in the underground research laboratory Mont Terri, Switzerland. 15 m cores of Opalinus Clay, sandy facies, have been gained by double core barrel. The cores have carefully been separated and put into liners, flooded with nitrogen, in order to prevent drying of the clay. They are now in the laboratories of IfG and CAU for laboratory testing. The drilling has been done in the framework of the Mont Terri Experiment “Experimental-Numerical Analysis of Discontinuities (AD)”.
Concerning numerical investigations, the BGR and UFZ are working jointly on the development of methods for shrinkage and swelling of clayey materials in the framework of hydraulic-mechanically coupled processes. With a swelling stress, depending on water saturation, in-situ observed phenomena can be explained. In order to get a better fit to measurements, incorporation of non-linear mechanics, as Non-Local Damage (NLD) and Phase-Field (PF), will be tested.
Within WP1, the UFZ team improved existing elasto-plastic constitutive formulations for fault evolution in single-phase hydromechanical settings, and implemented cohesive zone models for fault propagation as well as specific hydraulic fracture flow laws into numerical methods in OpenGeoSys using enriched finite element spaces. These models are currently tested in preliminary simulations in the context of fault slip experiments in Opalinus clay in Mt. Terri. Preparations for extensions to multiphase settings are ongoing.
Regarding the laboratory investigations at the IfG and CAU, one main topic during past months was how to get claystone samples from Mt. Terri underground laboratory. At an WP1 meeting, which was held at the BGR in Hannover in December 2017, the framework of experimental an theoretical topics was discussed. This includes the number of samples, the type of claystone and the time schedule for drilling and transporting the core samples. BGR will propose an experiment at the Mt. Terri Technical Meeting (February 2018, Porrentruy, Switzerland), to build a bridge between the Mont Terri and the GeomInt project.