PETROLOGICAL AND MECHANICAL PROPERTIES OF THE DAHOMEYAN GRANITIC ROCKS – A CASE STUDY AT GREEN VALLEY AND AKROFU

Abstract

The main aim of this project is to determine the structures, petrography, strength and competence of Dahomeyan granitic rocks at Green Valley and Akrofu in Ghana and compare them from both localities in terms of their mineralogical composition and strength properties. Thin sections were prepared with rock samples from the field at the KNUST Geological Engineering Laboratory and petrographic microscope used to determine the different types of minerals in the samples and also the rock type. The mechanical strengths of the rocks were determined at the KNUST Civil Engineering Laboratory using Schmidt Hammer device. The studied Dahomeyan rocks at both Akrofu and Green Valley are respectively classified as granodiorite and biotite granitic gneiss. The mineralogical composition of different modal percentages includes quartz, feldspars, biotite, muscovite and hornblende. The presence of fractures and foliations due to a mafic biotite mineral indicate deformations rocks. The granodiorite is classified as weak rock with compressive strength of 23.50 MPa. This is due to higher fractures in the rock. In conclusion, the higher the presence of high grade of fractures and foliations due to biotite mineral, the weaker the strength of the rock. The biotite granite gneiss at Green Valley can thus be useful as crushed stone in road construction as well as for building material. For the rock texture, coarser mineral fabric corresponds to higher uniaxial compressive strength. The strength of these granitoids were influenced by the climatological changes as well as the intensity of weathering they had undergone within the area.