Rock Mechanics and Rock Engineering | 2019
Influence of Pore Water (Ice) Content on the Strength and Deformability of Frozen Argillaceous Siltstone
Abstract
Bedrock freezes deeply in high-latitude or high-elevation regions; for instance, permafrost thickness has been estimated to exceed 900 m in Arctic Canada and can even reach 1400 m in Siberian Russia (Sammis and Biegel 2004). Excavation of frozen bedrock therefore presents one of the most difficult challenges faced by engineering constructions in such areas. Understanding the mechanical properties of frozen rocks is the key for safe and efficient excavation. Due to unsettled weather and complex topography, moisture states in natural rocks may vary greatly; this variation can be more severe and complex at subzero temperatures in cold regions (Sass 2005). It is well known that the water content can influence the mechanical responses of rock (Roy et al. 2017). Therefore, investigating the mechanical properties of frozen rocks of various water contents is essential. Rocks of various water contents are basically strengthened after freezing. Static strength of rocks, including the uniaxial compressive strength, tensile strength and pointload strength, are reported to increase at subzero temperatures compared to at room temperature. Several rock types have been tested, including limestone, basalt, granite, sandstone, andesite, marble and welded tuff (Heins and Friz 1967; Mellor 1970; Inada and Yokota 1984; Dwivedi et al. 1998; Kodama et al. 2013). Fracture toughness of rocks (limestone, basalt, granite (Heins and Friz 1967), dolerite, dolomite, quartz–mica schist and agglomerate (Dwivedi et al. 2000) also increases with the decrease of temperature. Deformability of frozen rocks, usually measured by elastic modulus (e.g., Young’s modulus, initial tangent modulus, etc.), is a function of temperature as well. Young’s modulus of limestone, granite (Heins and Friz 1967), and initial tangent moduli of Berea sandstone, Indiana limestone and Barre granite show a significant increase with decreasing temperature (Mellor 1970). Previous research on the mechanical properties of frozen rocks, in most cases tested samples are pre-saturated, has focused on the influence of temperature, while the influence of the initial water content has drawn limited attention, despite the fact that unsaturation is normality. Thus, several fundamental issues remain unfathomed: (1) to what degree is initial water content altering the mechanical behaviors of frozen rocks, (2) what are the microscopic mechanisms of this influence, and (3) what is the essential difference that initial water content introduces to frozen rocks? Trying to answer the above questions, we investigated the influence of the initial water content on the mechanical properties of frozen argillaceous siltstone (at − 20 °C). Both strength (uniaxial compressive strength, tensile strength and point-load strength) and deformability of frozen argillaceous siltstone with six saturation degrees were tested. Moreover, the postfreezing phase composition of pore water (i.e., the relative amount of water, ice and gas) in rocks with nine saturation degrees was measured using the nuclear magnetic resonance (NMR) method. The potential roles of unfrozen * Fan Zi [email protected]