Micro-textural effects on crack initiation and crack propagation of 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 considerable variations in unconfined uniaxial compressive strength and the crack initiation (σ_CI) threshold. The UCS ranges between 108-289MPa, and σ_CI ranges between 46-130MPa. Detailed quantitative petrographic and mineralogical analyses were carried out on each tested specimen to understand these strength variations. Additionally, 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 initiation and propagation, and (2) the orientation and distribution of micro- and macro-cracks formed during laterally 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 200μm) to amphibole and plagioclase phenocrysts. In addition σ_CI and UCS decrease with an increasing normalized area of biotite phenocryst.