Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

Abstract

The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servocontrolled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress-strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.

Influence of Water Saturation on the Mechanical Behaviour of Low-Permeability Reservoir Rocks

“…The crack closure threshold appears to be around 5.675% of the peak strength, and the ratios for the crack initiation thresholds derived from stress-strain curves and AE responses are 32.135% and 30.660%, respectively, while they are 83.299% and 82.600% for the crack damage thresholds determined from the stress-strain curve and AE performance, respectively. The normalized percentages for crack initiation and crack damage are consistent with other researchers' findings [15,17]. There are no large differences between the thresholds obtained from the stress-strain curves and AE responses, except that the values obtained from the AE activities are always slightly lower than those from the stress-strain curves.…”

“…In general, an arch shape for axial stiffness is shown for all water saturation conditions. However, the average axial stiffness between 20% and 40% of the stress range is always higher than that between 40% and 60% of the stress range, which is reasonable considering that crack initiation starts at around 30-50% of the UCS according to Amann et al [17]. Therefore, theoretically the stress The development of the axial stress-strain slopes discussed above is certified by the variation of the axial stiffness, which increases significantly at first, then remains stable, followed by a decreasing trend.…”

“…Eberhardt et al [15] suggested that the elastic modulus should be calculated in the phase between crack closure and crack initiation thresholds, as the lateral stress-strain and volumetric stiffness curves are linear in this stage, and the value calculated in this stage is higher than those from the ASTM recommended approaches. Amann et al [17] showed that the crack initiation threshold ranges between 30% and 50% of the rupture stress, and the crack damage thresholds occur at around 70% of the rupture stress. Therefore there are basically five stages in the compression: (i) the crack closure stage; (ii) crack closure to crack initiation threshold (30%-50% of the peak stress) without any cracking; (iii) the middle range of the stress at half of peak stress, where the normal elastic moduli are obtained; (iv) from the middle range to that before the crack damage threshold; and (v) period after the crack damage threshold until failure.…”

“…Eberhardt et al [15] proposed that the stress-strain curves are characterized by the crack closure, crack initiation and damage stress thresholds and demonstrated methods to obtain these thresholds. Amann et al [17] showed that the crack initiation threshold was about 0.3 of the UCS and the crack initiation process was accompanied by the onset of inelastic strain and acoustic emission, while the crack damage threshold characterized the volume deformation transition from the compaction to dilation [18]. Studies show that water diffusing to the tips of the fractures can lower the thresholds for crack propagation for rocks, ceramics and single crystals of quartz [18].…”

“…The deformations or failure patterns also vary with water saturation [6]. The weakening effects of water on rocks are generally attributed to the chemical reactions between water and minerals, especially the clay-water reactions [7], pore pressure [17], reduction of friction between grains [23], the surface energy [12] and the test conditions. However, most of the experiments have been conducted on high-permeability rocks like sandstones, or only consider the totally dry and full saturation levels.…”