Abstract:

The mafic rocks of Tarbela Alkaline Complex are excavated in the intake area of the Tarbela Dam for raising of Tunnel 3 and 4. These rocks are studied in terms of their petrographic characteristics and geotechnical properties. The bulk samples were collected at different elevations both from open and underground excavations. The petrographic study reveals that rocks are medium to coarse grained with euhedral to subhedral grains texture. These rocks essentially contain pyroxene, amphibole, plagioclase and accessory amount of chlorite, olivine, biotite and opaque phases. Modal mineralogical analysis categorized them as plagioclase bearing hornblende pyroxenite. The parameters investigated during current research for geotechnical assessment include unconfined compressive strength, Los Angeles abrasion, specific gravity, water absorption, porosity, soundness using sodium sulfate, flakiness/ elongation index and alkali-silica reactivity. Comparison with standard results revealed that the studied mafic rocks are suitable to use both as aggregate source or foundation material for construction of small- and large-scale civil structures. The regression analysis shows strong negative linear relationship of strength with weight loss during Los Angeles Abrasion, Porosity and water absorption. Due to the positive geotechnical results, the studied rocks are recommended to use as primary raw material for Tarbela 5th Extension Hydropower project.

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