Effects of Weathering on some Physical and Mechanical Properties of Ewekoro Limestone, South-western Nigeria

Ogunsola N. O., Olaleye B.M., Saliu M. A.

The study is on the investigation of the effects of weathering on physical and mechanical properties of Ewekoro limestone, south-western Nigeria 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 area 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.  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 in unweathered samples to 2.42 g/cm³ in highly weathered samples. The average porosity values increases from 7.60 % in unweathered samples to 11.27 % in highly weathered samples. 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 decrease respectively from 45.13 MPa and 2.30 MPa in unweathered samples to 16.30 MPa and 1.50 MPa in highly weathered samples. 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. 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. The regression analysis reveals strong relationships between some of the correlated parameters incorporating weathering states. 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.