THE INFLUENCE OF MEGACRYSTS WITHIN GRANITES ON ROCK STRENGTH AND PHYSICAL PROPERTIES

Granites are used within the construction and engineering industries throughout the world and it is important to understand their strength properties. This study investigates the properties of megcrystic granites, and determines how the presence of megacrysts influences rock strength. Three megacrystic granites, with differing rock properties, megacryst size and weathering conditions are evaluated by analysing Uniaxial Compressive Strength (UCS), porosity and density, p-wave velocity, core mapping (textural analysis and discontinuities), and petrographic analysis (grain size, mineralogy, alteration, micro-fracturing). The first unit is Cape Foulwind; a megacrystic granite (18-30mm alkali feldspars), in a medium grained groundmass, which has flow banding and metamorphic overprinting, and is slightly weathered. The second and third unit is Karamea; megacrystic granites, one unit is 9-13mm and the other is 14-19mm alkali feldspars. The Karamea units are in a medium to coarse sub-equiangular groundmass, and range from moderately to well weathered.

The Cape Foulwind samples have higher strength and stiffness than the Karamea samples, resulting from being less weathered and more intact. Overall the weathering extent of the Karamea granites correlates with UCS, with the more intensely weathered samples having lower strength and a higher occurrence of pre-existing micro fractures. The megacrysts in the Cape Foulwind samples are aligned along ductile and magmatic fabric, and failure during UCS testing occurred on cleavage planes of megacrysts and fabric. Correlation was also found between reduced rock strength and greater number of megacrysts.

In comparison to previous studies on UCS in megacrystic granites, pervasive weathering infiltrating through fabric and megacrysts, and associated pre-existing microfractures, has greater influence on reducing rock strength than megacryst size.