EVALUATION OF GEOMECHANICAL PARAMETERS OF MIGMATITES FROM DIFFERENT GEOLOGICAL ENVIRONMENTS IN PARTS OF SOUTHWESTERN NIGERIA

Abstract

The evaluation of physical and mechanical parameters of migmatitic rocks from two different geological environments were investigated. Migmatitic rock samples were collected at random from Ayere (Igara schist belt) and from two different locations at Akure. Rebound hardness test was carried out on the field with the use of Schmidt hammer in line with International Society for Rock Mechanics procedure. Thin sections of the samples were prepared and studied under the petrographic microscope. Photomicrographs of the different views of the sections were captured and studied with Image J programme. Physical and mechanical tests were carried out on the rocks in line with American Standard for Transport Material Standard Test Method C97-83, D2216-80. The results showed that mineralogically, Ayere samples consisted of quartz, plagioclase, hornblende and biotite with a modal composition of 40%, 27%, 24% and 9% respectively; Igbatoro samples consisted of plagioclase, microcline, quartz, biotite, hornblende and opaque mineral with a modal compostion of 32%, 31%, 28%, 4%, 4% and 1% respectively while Aponmu samples consisted of plagioclase, quartz, biotite, hornblende, muscovite and opaque mineral with a modal composition of 48%, 35%, 9%, 6%, 1% and 1% respectively. The results of the physicomechanical parameters of the migmatite gneiss from Ayere are as follow: average moisture content 0.051%, average porosity 0.70%, average specific gravity 2.78, average rebound hardness 52.6, average slake durability 97.46%, average point load 8.38MPa, average AIV 22.3%, average ACV 23.8%, average UCS direct 189.2MPa while average value from Schmidt rebound hardness and point load are 183MPa and 201.1MPa respectively. The results of the physico-mechanical parameters of the migmatite gneiss from Igbatoro junction are as follow: average moisture content 0.076%, average porosity 0.74%, average specific gravity 2.58, average rebound hardness 55.2, average slake durability 98.28%, average point load 7.64MPa, average AIV 23.0%, average ACV 24.0%, average UCS direct 172.2MPa while those derived from Schmidt rebound hardness and point load are 166MPa and 183.4MPa respectively. The results of the physico-mechanical parameters of the migmatite gneiss from Aponmu are as follow: average moisture content 0.102%, average porosity 0.78%, average specific gravity 2.65, average rebound hardness 48.4, average slake durability 97.63%, average point load 7.48MPa, average AIV 24.0%, average ACV 26.9%, average UCS direct 148.5MPa while those derived from Schmidt rebound hardness and point load are 140MPa and 179.5MPa respectively. The petrographic study shows that migmatite gneiss from Ayere has the highest Equivalent Quartz Content (EQC) and there are inclusions of finer grains of zircon and quartz in the larger grains of plagioclase and quartz. The results of the physicomechanical tests indicate that the migmatite gneisses from all the locations are of high strength with Ayere having the highest. The inclusions are indications that migmatite gneiss from Ayere might have undergone more phases of orogenesis than those from Akure, which further strengthened the rock. Conclusively, migmatite gneisses from the three locations have been proven to be of high strength but that of Ayere are more reliable for any civil engineering works and construction purposes. .