Effects of Weathering on Mechanical Properties of Granite Gneiss and Dolerite: A Case Study of Oban and Obudu Basement Rocks South-Eastern Nigeria

Abstract

This paper describes the geotechnical characteristics of granite gneiss and dolerite from Oban and Obuduregions in South-eastern Nigeria. The suitability of these rocks for civil construction purposes and the effects of weathering on their mechanical properties are examined. The results indicate that compressive strength ranges from 19.31 to 77.37 MN/m² and 40.5 to 140.0MN/m² for the granite gneiss and dolerite respectively. Their respective Young Modulus ranged 13.60 to 84.20GPa and 24.45 to 102.50 GPa.The low values obtained for compressive strength and Young Modulus in this work are the direct effects of the humid tropical conditions that cause weathering of the rocks at the base of the overburden aided by water. In addition to weathering, structural defects such as fractures, joints, faults and the mechanical drilling process tend to introduce micro-pores/fractures in the samplesthereby leading to a decrease in the strength and modulus of these rocks.Furthermore, the moisture content (W%) and the degree of weathering decrease with increasing hardness (H), specific gravity (SG),compressive strength (α) and Young Modulus(E). The relationship between moisture content, the degree of weathering and the mechanical properties – particularly compressive strength (α) and Young modulus (E)also showsthat the fresh (FR) to slightly weathered(SW) rocks are generally suitable for most civil construction works than the extremely weathered(EW) to highly weathered (HW) rocks.

KEYWORDS: BasementRocks,Mechanical Properties,Weathering Grades(FR-EW),Oban/Obudu.