Abstract:
The time-dependent deformation of shale is a phenomenon that impacts several industries but remains rather poorly understood. The Pierre Shale, a poorly indurated geological unit (unconfined compressive strength of less than 5 MPa), is an important formation that can be used to better understand the behavior of intermediate geomaterials and, specifically, time-dependent deformation, or creep. A series of long-duration (weeks to months) triaxial constant-stress-difference tests were performed on recently obtained core samples from the Pierre Shale. Samples were hydrostatically consolidated and then subjected to variable constant-stress differences. The stress differences in the samples were increased in stages until sample rupture occurred. The apparent steady-state creep rate was observed to generally increase with increasing stress differences for each test and spanned a range of approximately two orders of magnitude. There appears to be no creep threshold in the Pierre Shale, as measureable steady-state creep was recorded at stress differences as low as 1 MPa. Additionally, time-dependent deformation appears to include a component of damage accumulation. Although creep rupture was observed to occur at stress differences similar to the peak strength of the shale as determined by constant strain rate triaxial tests in some samples, other samples ruptured at stress differences of only 25 to 50 percent of the expected peak strength.
Introduction
Fine-grained geological units are abundant and occupy over 50 percent of all sedimentary rocks in the preserved geological record (Boggs, 1995). These widespread geological units present unique engineering problems that include the consideration of time-dependent deformation and strength loss. Nuclear repository science is often the state of the art when describing phenomena and processes, and the current understanding of the behavior of fine-grained geological units, such as shale, has advanced over the years through laboratory testing, numerical modeling, and experiments in underground research laboratories. However, several recent authors, both domestic and international (Argonne National Laboratory, 2011; Nuclear Energy Agency, 2010; Hansen et al., 2010; Mazurek et al., 2008), have indicated topics where important knowledge gaps still exist. For example, understanding the time-dependent behavior of shale and the sealing of fractures that develop within the Excavation Damage Zone of the formation are current knowledge gaps that are often cited as having high importance.
The time-dependent deformation of shale has applications in several other industries outside of nuclear repository science. Underground mining of sedimentary units, such as coal, aggregates, and evaporites, often encounters shale near the mined seam, and a better understanding of time-dependent behavior can aid in hazard recognition and control. The oil and gas industry also encounters challenges dealing with time-dependent deformation of shale, both in terms of wellbore stability and hydraulic fracture propagation. Civil engineering projects encounter the impact of shale deformation in tunneling, foundation design, and slope-stability projects.
