Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

Abstract

The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates ($$\dot{\varepsilon }$$ ε ˙ ) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed pc–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and $$\dot{\varepsilon }$$ ε ˙ do not significantly affect deformation behavior of dried samples. However, σmax and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.

Discussion on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric” of Schuster, V., Rybacki, E., Bonnelye, A., Herrmann, J., Schleicher, A.M., Dresen, G.

“…A consolidation coefficient of 0.002 mm 2 /s is reported in Schuster et al (2021), which is broadly in line with measurements on Opalinus Clay obtained in oedometric or triaxial conditions in other works (Crisci et al 2019;Ferrari et al 2016;Minardi et al 2019). Computing the strain rate for drained and undrained shearing (Head and Epps 2014), values in the order of magnitude 10 -8 1/s and 10 -7 1/s would be obtained, respectively.…”

“…higher mean effective stress, with respect to S-samples. In Schuster et al (2021), while comparing the results between P-and S-samples in terms of total confining stress, the two seem to show a remarkable difference in terms of peak strength, while the discrepancy is reduced when effective stress is considered. Data from Minardi et al (2020) showed that the failure envelope for P-samples had an intercept about 70% higher than the S-samples, while the shear strength angle remains substantially unchanged.…”

“…Giger et al (2018) and Minardi et al (2019Minardi et al ( , 2020 successfully adopted this procedure for Opalinus Clay (sandy and shaly facies). The same technique was applied by Schuster et al (2021) to test wet samples of Opalinus Clay.…”

“…By applying confining stresses in the order of tens of MPa to pre-conditioned shale samples (with RH > 90%), positive pore pressure, in the order of several MPa is developed (Ewy 2018;Giger et al 2018;Minardi et al 2020). For the saturated samples in the work of Schuster et al (2021) nonnegligible pore overpressures must have been developed as isotropic confining pressure of 50 MPa was applied.…”

“…Sound testing procedures for shale have been built in recent years. In this note, we examine the recent results on wet samples by Schuster et al (2021) (Sects. 3.1.1, 3.1.5), we discuss the inhomogeneous pore pressure generation, in both drained and undrained shearing conditions, and its potential impact on the obtained mechanical properties, and the potential effect of drying-induced fissuring on the mechanical properties of samples (Sects.…”