Abstract:

Uniaxial consolidation of granular salt is carried out to study the mechanical behavior and fabric development in a material that deforms by microscopic brittle and intracrystalline-plastic processes. Dry granular salt is sieved to produce well-sorted size fractions. The granular salt is consolidated in a heated cell at axial stresses up to 90 MPa and temperatures of 100 - 200 °C to document stress-consolidation relationships and microstructural development. Polished and chemically-etched petrographic sections of salt samples prior to and after deformation at 150 °C are studied using transmitted- and reflected-light optical microscopy. We show that temperature has profound effect on porosity reduction during consolidation. At tested conditions, the dominant deformation mechanism is crystal plasticity; brittle deformation is largely suppressed. Samples consolidated at higher maximum axial stress develop higher overall dislocation densities. The distribution of dislocations, however, is strongly heterogeneous from grain to grain because of the complex grain-scale loading geometries and the distribution of intragranular flaws such as fluid inclusions. Static recrystallization occurs in some highly strained areas, but overall is minor at 150 °C. The experiments help to improve our understanding of consolidation, and serve to guide the fabrication of synthetic rock salt as experimental material, as well as to inform and test constitutive models of deformation of granular salt for engineering needs.

Introduction

The rheological properties of rock salt have been an important research focus because it is considered aviable geomaterial for engineered repositories of waste and energy resources, such as radioactive waste, oil, and gas (Urai et al., 1986; Carter et al., 1993). Rock salt also is studied as an analog to other geomaterials in that various deformation mechanisms, including cracking, frictional sliding, pressure solution, crystal plasticity, and dynamic recrystallization are easily activated at laboratory conditions (Zhang et al., 2007).

Extensive experimental work has been done on rock salt to determine constitutive relationships and guide numerical modeling (e.g., Watanabe and Peach, 2002; Ter Heege, et al., 2005; Zhu and Arson, 2015). Both natural and synthetic rock salt have been used for experimental investigations; artificially prepared rock salt is of higher purity and can be fabricated in a way that best serves parametric studies (e.g., Carter and Hansen, 1983; Schenk and Urai, 2004; Bourcier et al., 2013). Consolidation of granular salt is a common method used to produce synthetic rock salt samples; however, salt is highly sensitive to moisture so it is critical to have a good control of the humidity of the environment in which nominally-dry salt samples are produced, stored, and processed. Maintaining a consistently dry environment during all stages of handling salt samples can be challenging.

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