The effect of microstructure on m_i strength parameter variation of common rock types

A series of laboratory tests was conducted on selected intact sedimentary (sandstones and limestones) and igneous (granites and andesites-dacites) rocks obtained from several sites of Greece. Physical and mechanical properties were determined, while the investigation was focused on strength parameters determination and especially on Hoek-Brown strength parameter m_i through the triaxial compression. Mineral composition of grains, as well as microstructure (grain size, shape and interlocking) was estimated through microscopic examination using some well known petrographic indices. The alteration and weathering state for the studied igneous rocks were also microscopically quantified. Regression analysis of laboratory data have clearly shown a direct strong intrinsic influence of microstructure as quantitatively described by the petrographic indices and by textural interlocking of the grains, as well as of grain deterioration on measured m_i value. The m_i value generally increases with the increasing grain size of studied clastic and igneous rocks, but it decreases to a power expression with sparitic material of limestones. The increase of weathering and alteration state also decreases the m_i parameter of tested igneous rocks.