THE EFFECTS OF WEATHERING ON SOME PHYSICAL AND MECHANICAL PROPERTIES OF SELECTED CARBONATE ROCKS IN SOUTH-WESTERN NIGERIA

Abstract

This study is on the investigation of the effects of weathering on physical and mechanical properties of Ewekoro limestone and Igbeti marble rocks with the aim to establish the strength of the rock samples for design and engineering purposes. In order to achieve the set objectives, twenty representative samples were collected from each of the study areas representing particular group of unweathered, slightly weathered, moderately weathered and highly weathered rocks using the field study indicators of colour, texture and friability, and International Society for Rock Mechanics (ISRM) classifications for the laboratory determination of physical and mechanical properties of the selected rocks. The samples were also subjected to weathering cycles test using both slightly acidic rain water and saturated magnesium sulphate (MgSO4) salt solution as the slaking fluid to determine physical or mechanical weathering effects. Thin sections were prepared for textural and mineralogical studies of the unweathered and weathered samples of the two rock types. The chemical composition analyses of the samples were done using X-ray Fluorescence spectrometry (XRF). The physico-mechanical variations with progressive weathering grades were statistically evaluated using Excel and Statistical Package for Social Sciences (SPSS) and mathematical models were generated. Quantitative test results show the dry and bulk densities, dry unit weight and specific gravity decreases with an increase in weathering state while the water content, water absorption and porosity increases with an increase in weathering state. The average values of dry density decreases from 2.69 g/cm³ and 2.74 g/cm³ in unweathered samples to 2.42 g/cm³ and 2.13 g/cm³ in highly weathered samples for limestone and marble respectively. The average porosity values increases from 7.60 % and 0.89 % in unweathered samples to 11.27 % and 3.00 % in highly weathered samples for limestone and marble respectively. The range of mechanical properties tests results decreases with an increase in weathering state with overlap of values across the different weathering grades. The average uniaxial compressive strength and point load strength values decreases respectively from 45.13 MPa and 2.30 MPa in unweathered samples to 16.30 MPa and 1.50 MPa in highly weathered samples for limestone and from 137.6 MPa and 2.69 MPa in unweathered samples to 1.11 MPa in marble samples. The results of weathering cycles test show an increase in percentage weight loss with increasing number of cycles and weathering, with slightly acidic rain water having a reduced percentage weight loss compared with saturated MgSO4 salt solution. Textural, mineralogical and chemical studies conducted on the unweathered and weathered samples in both rock types show that noticeable textural, mineralogical and chemical changes occur in the rock fabric as a result of increase in weathering state. The results of geochemical studies show that calcium oxide (CaO) and potassium oxide (K2O) values decrease with increasing degree of weathering respectively, with 82.32 % and 0.82 % in unweathered sample to 45.21 % and 0.45 % in highly weathered samples for limestone and 45.02 % and 0.08 % to 39.60 %, and 0.05% in marble samples. Silica (SiO2) and iron oxide (Fe2O3) increase as the weathering progresses respectively with 8.43 % and 2.35 % in unweathered sample to 19.93 % and 4.30 % in highly weathered sample for limestone and from 8.38 % and 2.55 % in unweathered sample to 10.25 % and 4.18 % in highly weathered sample for marble. The regression analysis reveals strong relationships between some of the correlated parameters incorporating weathering states. The coefficients of determination R2 values of close to 1.0 were obtained in most of the curves of the correlated parameters. The regression analysis shows that dry unit weight is a poor predictor of both point load strength and tensile strength in limestone and marble. Point load strength is a better predictor of tensile strength in both samples while porosity, water content and water absorption are good predictors of uniaxial compressive strength. 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