Constitutive models are used to describe the mechanical behavior of materials (Karev et al., 2020;Nadai, 1963;Puzrin, 2012). These consist of mathematical equations that can be used to relate physical quantities and are defined by material-specific constants (Davis & Selvadurai, 2005). The design of civil engineering structures relies on the precise calibration of these constants (Alonso et al., 2010;. Triaxial tests are commonly carried out to obtain the constitutive parameters of geomaterials (Jaeger et al., 2007). It is generally assumed that the triaxial test is a representative volume test, meaning that the developed stresses and strains are uniform in the sample. Accurate detection of a sample's strain is important for the determination of the mechanical properties. Linear variable differential transformers (LVDT) and strain gauges are common technologies to measure the strain response. The drawback is that the measurement is limited to the point or small zone where they are installed, forcing the assumption of a homogeneous sample response in accordance with the representative volume assumption. However, recent numerical and experimental studies have shown strain localization in the samples, even at early test stages (McBeck et al., 2019;Van der Baan & Chorney, 2019). This implies that when point measurement methods are used, the mechanical parameters obtained are dependent on the sensor location.
<p>Clay-rich rocks occur in a wide range of tectonic settings. They are of great interest, for example, for the mechanical properties of shallow subduction zone interfaces, but also for natural barriers in nuclear waste deposits or as subsurface caprocks for CO<sub>2</sub> storage. In contact with a polar fluid (e.g., water), the interaction between clay minerals and fluid can lead to swelling or, under confined conditions, build-up of swelling stress. Many studies have focused on the closure of cracks in clay-rich sedimentary rocks by swelling (also referred to as &#8217;self-sealing&#8217;). However, less is known about how water-clay interactions affect the stress state of clay-rich rocks and whether they may induce slip along pre-existing faults. We try to address this knowledge gap in the present study by conducting triaxial shear experiments.</p> <p>The experiments are performed using oblique saw-cut cylindrical samples, where the top half consists of a clay-rich rock (Opalinus claystone) and the bottom half of a permeable sandstone (Berea sandstone). To estimate the frictional properties of the sandstone-claystone interface, dry experiments are performed at 4 to 25 MPa confining pressure and constant axial displacement of 0.1 mm/min. Fluid injection experiments, where fluids are injected through the permeable footwall sandstone, are performed at 10 and 25 MPa confining pressure, constant piston position (no axial displacement), and an initial differential stress of about 70 % of the expected yield stress. The effect of water-clay interactions on the stress state is estimated by comparing the fluid pressures required to initiate slip when a non-polar fluid is injected (no water-clay interactions are expected) and when a polar fluid is injected (water-clay interactions will occur). In some experiments, the sample assemblage is equipped with fiber optics strain sensors glued to the surface of the sample to distinguish between (poro)elastic deformation of the matrix, deformation due to water-clay interaction, and elastic relaxation due to slip along the saw-cut.</p> <p>For fluid injection experiments with a non-polar fluid (decane), the mechanical data indicate that slip along the saw-cut occurs at fluid pressures close to what is expected based on the friction slip envelope determined for the dry state. For fluid injection experiments with a polar fluid (deionized water), a differential stress drop already occurs when the water initially reaches the sandstone-claystone interface at ambient fluid pressure (0.1 MPa), which is not expected based on the dry friction slip envelope. The fiber optics strain sensor data indicate that swelling of the claystone is followed by a microstructural collapse before slip along the saw-cut likely occurs. In summary, our data suggest that water-clay interactions may initiate slip due to (1) the alteration of the friction slip envelope, (2) build-up of swelling stress, and (3) collapse of the claystone microstructure. However, to what extent these three mechanisms contribute to the according differential stress drop requires further research.</p>
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