An Experimental Investigation of the Brazilian Tensile Strength and Failure Patterns of Laminated Sandstones

Excerpt

Many rocks exposed at Earth’s surface show well-defined fabric elements in the form of bedding, stratification, layering, foliation, fissuring and jointing. In general these rocks are inherently anisotropic as their physical, dynamic, thermal, mechanical and hydraulic properties vary with direction (Chen et al. 1998). Experimental results have shown that several types of rocks, such as metamorphic and sedimentary rocks, have inherent or structural anisotropy (Saeidi et al. 2013; Hoek 1964; McLamore and Gray 1967; Horino and Ellickson 1970; Kwasniewski 1993; Nasseri et al. 2003; Al-Harthi 1998). Sedimentary rocks can be isotropic or anisotropic depending on the spacing of bedding planes or lamination developed during deposition. Metamorphic rocks are mostly anisotropic because of the effects of both schistosity and cleavage (Singh et al. 1989; Ramamurthy 1993). The effect of the orientation of schistosity on the Brazilian tensile strength (BTS) of different metamorphic rocks has been studied by various authors, including Berenbaum and Brodie (1959), Hobbs (1963) and Debecker and Vervoort (2009). The effect of layer direction on the indirect tensile strength of sedimentary rocks determined using the Brazilian test was investigated by Hobbs (1963) on siltstone and McLamore and Gray (1967) on shale, Tavallali and Vervoort (2010a, b) as well as Chen et al. (1998) on sandstone. In the literature various modes of failure of rock samples have been observed and classified for anisotropic rocks. Chen et al. (1998) proposed two major modes of tensile splitting: along the loaded diameter of sandstone and by shear failure along sandstone layers. Tavallali and Vervoort (2010b) identified three types of failure of disc-shaped specimens of anisotropic rocks under Brazilian test conditions: (1) layer activation (fractures roughly parallel to the layer direction), (2) central fracture [fractures developed roughly parallel to the loading direction and in the central part of the specimen (i.e., 10 % of the diameter on both sides of the specimens central line between the two loading lines)] and (3) non-central fracture (fractures outside the central part). …