Micro-textural effects on crack initiation and crack propagation of andesitic rocks Ömer Ündül a,b, ⁎, Florian Amann a , Namık Aysal b , Michael L. Plötze c a Swiss Federal Institute of Technology, Zurich, Institute of Geology, Engineering Geology, Sonneggstrasse 5, 8092 Zurich, Switzerland b Istanbul University, Engineering Faculty, Geological Engineering Department, Avcilar, 34320 Istanbul, Turkey c Swiss Federal Institute of Technology, Zurich, Institute for Geotechnical Engineering, Environmental Engineering and Clay Mineralogy, HIF D20, 8093 Zurich, Switzerland abstract article info Article history: Received 27 August 2014 Received in revised form 18 March 2015 Accepted 26 April 2015 Available online xxxx Keywords: Crack initiation Crack propagation Uniaxial compressive strength Quantitative petrography Andesitic rocks The onset of dilation and uniaxial compressive strength (UCS) of rocks are known to be affected by mineralogical, petrographic and microstructural features. Therefore micro-textural variations are essential for understanding variation in strength and failure behaviour of rocks. In this study volcanic rock samples with different micro- textural characteristics from different locations in Turkey were evaluated. The mechanical tests revealed consid- erable variations in unconfined uniaxial compressive strength and the crack initiation (σ CI ) threshold. The UCS ranges between 108–289 MPa, and σ CI ranges between 46–130 MPa. Detailed quantitative petrographic and min- eralogical analyses were carried out on each tested specimen to understand these strength variations. Addition- ally, microstructural studies were carried out on thin sections before and after mechanical perturbation to analyze (1) the effect of the main constituents of the rock specimens and mineral heterogeneity on crack initia- tion and propagation, and (2) the orientation and distribution of micro- and macro-cracks formed during later- ally unconfined compression. The synthesis of mechanical, mineralogical, petrographic and microstructural data suggests that both the crack initiation threshold and peak strength are influenced by the ratio of the area of fine grain groundmass (grains smaller than 200 μm) to amphibole and plagioclase phenocrysts. In addition σ CI and UCS decrease with an in- creasing normalized area of biotite phenocryst. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Variations in strength and failure of rocks depend on many factors including micro- and macro-scale features such as pre-existing microcracks and heterogeneities. Such variations in strength are well known for heterogeneous rocks (e.g., veined rocks; Amann et al., 2014). On the other hand, such variations in strength have also been ob- served in rock types where macroscopic heterogeneities could not be identified and the mineralogical constituents were homogeneously dis- tributed (Hatzor and Palchik, 1997; Eberhardt et al., 1999; Prikryl, 2006, etc.). Since Brace (1964) and Bieniawski (1967) the failure of rocks under uniaxial compressive tests is known to occur in a sequence of (1) crack closure (CC; i.e., in case of pre-existing damage), (2) linear elastic deformation, (3) crack initiation (σ CI ) and stable crack propaga- tion, (4) unstable crack propagation when a critical crack density is exceeded, and (5) eventually coalescence of cracks and rupture (Fig. 1). The crack initiation threshold which has been shown to be an estimate of the lower limit of the long term strength at low confinement is of particular interest in this study. Martin (1997), Diederichs et al. (2004), Damjanac and Fairhurst (2010), Diederichs (2007), Andersson et al. (2009), Rojat et al. (2009), Hidalgo and Nordlund (2012), Nicksiar and Martin (2013a) and many others showed for tunnels in hard brittle rock that for tangential stresses in excess of σ CI spalling is initiated in the low confinement zone around the excavation. They also showed that stresses beyond σ CI often lead to rapidly developing instabilities. The stress magnitudes for spalling initiation at low confine- ment are typically in a range of 0.3–0.6 of the unconfined compressive strength (UCS). In addition to the macroscopic failure behaviour, micro-cracks that form around tunnels can substantially enhance the permeability in the tunnel near-field causing potential fluid or gas mi- gration pathways as shown for host rocks for nuclear waste repositories (Bossart et al., 2002), or enhance the swelling potential in swell prone rock types such as anhydrite bearing clay rocks (Amann et al., 2014). Thus, the crack initiation threshold and factors influencing the initiation, propagation and uniaxial compressive strength of rock types are of great interest for understanding a large range of engineering application and research. For the crystalline rocks, Eberhardt et al. (1998) showed that the crack initiation stress is independent of loading rates. Thus microtextural properties (including grain sizes and distribution of minerals) are con- sidered to be important factors for controlling the crack initiation and crack damage thresholds. This corroborates with the early work of Brace (1961, 1964) and Olsson (1974) who found a decrease of peak strength with increasing grain sizes. Similar results were found by Engineering Geology xxx (2015) xxx–xxx ⁎ Corresponding author at: Istanbul University, Engineering Faculty, Geological Engineering Department, Avcilar, 34320 Istanbul, Turkey. E-mail address: oundul@istanbul.edu.tr (Ö. Ündül). ENGEO-04032; No of Pages 9 http://dx.doi.org/10.1016/j.enggeo.2015.04.024 0013-7952/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo Please cite this article as: Ündül, Ö., et al., Micro-textural effects on crack initiation and crack propagation of andesitic rocks, Eng. Geol. (2015), http://dx.doi.org/10.1016/j.enggeo.2015.04.024