THE EFFECT OF WEATHERING ON THE ENGINEERING PROPERTIES OF IJARE AND EWU CHARNOCKITE, SOUTH WESTERN, NIGERIA

Abstract

This research examines the effect of weathering on the engineering properties of Ijare and Ewu charnockite, Southwestern, Nigeria. To achieve the objectives, un-weathered (fresh) and weathered samples were collected from the study areas for the determination of physical and mechanical properties of the rocks. The physical properties determined are specific gravity, density, porosity, water content and water absorption. The mechanical properties investigated included point load strength index (Ijare having the point load index of 5.74 MPa and 3.72 MPa for un-weathered and weathered respectively while Ewu having the point load index of 7.33 MPa and 4.74 MPa un-weathered and weathered respectively), unconfined compression test (Ijare having the average uniaxial compressive strength of 136.90 MPa and 65.50 MPa for un-weathered and weathered respectively while Ewu having the average uniaxial compressive strength of 219.85 MPa and 99.10 MPa unweathered and weathered respectively), slake durability (Ijare having the slake durability of 98.80% and 97.80% for un-weathered and weathered respectively while Ewu having the slake durability value of 98.80% and 98.1% for un-weathered and weathered respectively), Schmidt hardness test (Ijare having the Schmidt rebound value of 49 MPa and 45 MPa for un-weathered and weathered respectively while Ewu having the Schimdt rebound value of 52 MPa and 47 MPa un-weathered and weathered respectively). The rock samples were also subjected to petrographical analysis to see the effects of mineralogical composition within the rocks as a result of weathering. Laboratory test and results obtained from the un-weathered and weathered samples were varied to know the changes in the properties of the rock as a result of the effect of weathering using Excel and the results were subsequently correlated. The results show that weathering increases some physical properties investigated; porosity, water content and water absorption of the rock while it decreases others, specific gravity and density. The result of the tests on Ijare and Ewu samples respectively showed the bulk density and dry density increases for both the weathered and un-weathered samples. Increasing degree of weathering result in a major reduction in the strength of the charnockite rock which is directly associated with changes in mineralogy and associated porosity. The point load strength, uniaxial compressive strength test and rebound hardness result values decrease as a result of weathering. A multilinear regression model was developed using OriginPro software for the prediction of UCS and analysis of variance (ANOVA) was also carried out. The multilinear regression model serves as means of estimating UCS from physical properties and ANOVA shows that the independent parameters have strong influence on the UCS as the calculated F value is greater than the F value set at 0.05 confidence interval. The laboratory tests conducted indicate that weathering reduces strength of the rocks which could affect slope stability and performance of these rocks in engineering applications.