Petrographic features as an effective indicator for the variation in strength of granites

• Textural characteristics of four different granites are examined
• Three of granite types have alteration Grade-II while other has Grade-III
• Strength, hardness, density, voids and ultrasonic velocity have been determined
• Statistical analyses and post-test petrography relate textures with rock strength
• Textural features have major influence on granite strength with similar alteration

The textural characteristics of four different granites from the lower Himalayan regime in north-western Pakistan have been examined in relation to their effect on the mechanical nature of rock. Detailed petrographic examination and subsequent quantitative QEMSCAN analysis provide better understanding of the difference between their textures. Three of the granite types are slightly altered (Grade-II) whereas the other has a higher degree of alteration and corresponds to alteration Grade-III. The mechanical properties determined for each granite type include: unconfined compressive and tensile strength, elastic modulus, P-wave velocity, Schmidt hardness and dry density. Statistical analyses, combined with post-test petrography, demonstrate textural control on mechanical properties. The important petrographic characteristics influencing mechanical behaviour include modal concentration and grain size of individual minerals, mean grain size of rock and distribution of grain size within a rock. Recrystallization of minerals along boundaries has a pronounced effect on increased strength of granites. Texture, however, has a significant influence on the variation of strength of granites with similar alteration grade.