The influence of weathering on the engineering soil profile : a study of low relief basalts in South Africa

Abstract

The high relief or mountainous basalt in and around the regions of Lesotho has extensive information on the nature and properties of the basalts, as a result of several developments on these basalts such as the Lesotho Highlands Water Project. This resulted in a wide variety of research conducted and published. In contrast, less published research is conducted on the lower relief or flat lying basalts leading to fewer available information on the nature and properties of low relief basalt deposits. Five flat lying basalt sites were investigated in order to gather data on low relief/flat lying basalts. The two main sites of this investigation were researched in the Kruger National Park (KNP) on the southern basalts (Nhlowa) a high rainfall area, and on the northern basalts (Mooiplaas) a low rainfall area. These are two of four research supersites created in the Kruger National Park in order to create cross-disciplinary data rich sites. A mineralogical and chemical analysis for comparison of the parent rock and soil profiles between these two sites were undertaken. The Sibasa Formation basalts at Siloam and the Springbok Flats basalts at Roedtan and Codrington are three additional low relief basalt sites, in different rainfall areas. These sites were investigated to compare to each other and to the KNP sites. XRF and XRD data were used together with profile development in their respective different climatic environments and basalt origins to determine the weathering progression and characteristic of low relief basalts. This was done by studying the depletion and enrichment of major and trace elements (Fe, Ca, Mg, Na, K, Ti, Y, Zr) through the profile from basaltic rock to residual soil. Furthermore, identifying the breakdown of primary minerals, such as Augite and Plagioclase, to secondary Smectite minerals further supports the identification of weathering progression in low relief basaltic rock. This can potentially aid in pre-planning of engineering geological investigations on residual basaltic soils. Additional information on the Atterberg Limits will also be presented in order to start to determine the effect low relief basalt weathering has on the engineering properties of the residual basalt soils. In setting out to determine the influence different climatic conditions contribute to the weathering process and soil profile development of low relief basalts, it became clear that research of a specific rock type in a single setting is not sufficient. In order to truly understand the behaviour of basalt weathering, a combination of low relief, mountainous other settings in variable climatic conditions are necessary. This study shows the significant effect a moderate shift in climatic conditions such as rainfall and evaporation intensity have on the development of residual soils on low relief basalts, such as pedocrete and secondary mineral formations.