A combined laboratory and microstructural investigation of the coupled hydro-mechanical failure behavior of Opalinus Clay

Winhausen, Lisa; Amann, Florian (Thesis advisor); Urai, Janos (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Shales and similar clay-rich geomaterials are considered suitable host rocks for the storage of nuclear waste in deep underground repositories. Favorable properties of these rocks are the low permeability, the high sorption capacity, and the self-sealing potential. For the assessment of the host rock integrity, a proper geomechanical characterization is required to understand the physical processes taking place during stress changes in the ground as a response to tunnel excavations. This thesis investigated the coupled hydro-mechanical behavior of Opalinus Clay, a Mesozoic clay shale formation, which is selected as the host rock for the disposal of nuclear waste in Switzerland. To gain more insights into the short-term, undrained behavior of Opalinus Clay, a combined approach of different laboratory experiments and scanning electron microscopy of deformed specimens was conducted. Therefore, a variety of different hydraulic and hydro-mechanical tests on fully saturated specimens, as well as mechanical tests on specimens at precise saturation levels and associated suctions, were conducted. Post-experimental microstructural analyses were carried out on deformed specimens by the combination of argon-beam polishing and high-resolution imaging of their microstructural fabric presented by pore space and grain structure. This way, the bulk coupled hydro-mechanical elastoplastic behavior could be linked to the micrometer-scale deformations and associated failure mechanisms.The comparison of the results from tests at different boundary conditions showed that the bulk deformation behavior is dependent on (i) the saturation and consolidation state, (ii) the effective mean stress, and (iii) the loading orientation in terms of the angle between the principal stresses and the bedding plane. In the same way, microstructural deformations showed similar dependencies by revealing deformation structures pointing to different failure modes. Under triaxial compressive load, failure in Opalinus Clay is associated with shear strain localization along elongated zones with preferred orientations. These shear zones are characterized by many different deformation markers such as shear fracturing, kink banding, grain and fabric rotation, intercrystalline sliding, and inter- and intragranular fracturing. Often, the deformed zones showed increased porosities compared to the non-sheared material. A deformation model for Opalinus Clay for undrained-unconsolidated triaxial loading was derived based on the combination of bulk-mechanical stress-strain measurements and the analyses of deformation structures. This model predicts failure by damage accumulation with ongoing shearing due to the formation of micro-fractures, their coalescence, and the final formation of dilative deformation bands, i.e., shear zones. Prominent deformation markers have been identified and incorporated into the conceptual model. By comparing results from tests at different effective consolidation stresses, the hydromechanical properties such as the elastic moduli, the magnitude of dilation, and the peak, as well as residual effective stress, exhibited a non-linear change with mean effective stress. Although all tests indicated a brittle behavior based on their stress-strain response, the dominant microscale deformation style changed from brittle to ductile mode on the grain scale, which is in agreement with changing frictional properties for different effective mean stresses at failure. The structural nature of Opalinus Clay expressed by the preferred alignment of elongated grains and sheeted clay platelets, as well as pores along the macroscopic bedding plane, creates a transverse isotropy. A systematic series of tests has shown that this anisotropy influences the total shear and tensile strength, the poromechanical response in terms of compaction or dilation, and the effective peak and residual strength. At effective consolidation stresses of 10 MPa, the effective stress paths revealed three different types of behavior classified by loading configuration in terms of the angle between maximum principal stress and the bedding orientation. Class I, defined by loading angles in the range of 90° to 60°, showed a decrease in mean effective stress due to the pronounced pore pressure development. Loading at intermediate angles between 60° and 45° (class II) showed mean effective stresses with only minor changes, and the similarity between the pre-and post-peak stress paths suggests minor dilation before and after failure. This class showed the least compressive shear strength. Specimens in class III, defined by low angles between 15° and 0°, indicated increasing mean effective stresses during shearing accompanied by dilation. Hence, more plastic strain is accommodated during the failure processes of class I and class III in comparison to class II, which is due to the formation of the shear zone, typically associated with fabric changes by the reorientation of grains and pores. In the case of class II, the preferred alignment of grains and pores is (sub)parallel to the developing shear zone, and hence, less plastic strain is required for shear straining, resulting in minor volume changes.The formation of microfractures and shear zones leads to an increase in porosity and, consequently, changes in hydraulic properties can be expected. In this thesis, a comparative study was performed using different measurement techniques to determine the hydraulic properties of Opalinus Clay. The technique proposed to capture changes in the hydraulic properties during deformation is the pore pressure oscillation method. This method yielded similar permeability values as obtained by other common transient methods and profits from continuous measurements as well as the simultaneous measurement of permeability and storativity. In view of the overall project goal to develop and calibrate a new constitutive model for Opalinus Clay, this thesis has contributed by providing an extensive data set of effective hydro-mechanical properties. The results have led to a deeper understanding of the failure processes by linking the bulk behavior to microscale deformation processes, assisting the implementation of constitutive relations in the newly developed model.

Institutions

• Division of Earth Sciences and Geography [530000]
• Chair of Engineering Geology and Hydrogeology [532110]