The geology of post-Chuniespoort deposits in the Verwoerdburg area south of Pretoria with specific reference to engineering properties of the red soils

Abstract

Dolomitic rocks of the Chuniespoort Group south of Pretoria contain karstic depressions filled with younger sediments. Constructions underlain by these sediments were recently damaged which necessitated a sedimentological and engineering geologica investigation of this material and a study of the bedrock geology. An area in Verwoerdburg was selected for this study. The bedrock comprises chert-poor dolomite,and its weathering product wad, of the Lyttleton Formation, overlain by chert-rich dolomite, chert breccia and chert residuum of the Eccles Formation. The succession dips at 20° towards the east. A syenite dyke is intruded parallel to the strike of the dolomites. Groundwater solution exploited lithological and structural weaknesses in the dolomites, producing a karst topography of sinkholes, bogazis and dolines. Ten outliers of consolidated sediments within the study area contain basal fluvioglacial diamictites, probably belonging to the Dwyka Formation. Overlying lacustrine mudstones, shales and fluvial sandstones, most likely belonging to the Ecca Group, reflect sedimentation in gradually shallowing karst depressions. A second depositional episode, probably in Late Tertiary to Middle Quaternary times, introduced quartz silt to the study area under loessic conditions. Later sedimentary reworking-mixed this silt with local weathering products and filled 16 karst features to a depth greater than 9m. ~ younger, thin and erratically distributed sandy silt layer was then deposited in the study area. Post-depositional weathering processes have modified the reworked loessic deposits, producing a red clayey silt with a collapsible grain structure. Under conditions of natural moisture content and loading, the clayey silt has an open grain structure but a relatively high strength and low erodibility because of the development of clay bridges between soil grains. However, a subsequent rapid increase in moisture content while the soil is under load, causes the clay bridges to break and collapse settlement to occur. An increase in subsurface water content usually results from leakage of water-bearing pipes or poor surface drainage. By monitoring all water-borne facilities and adhering to certain constructional procedures, the threat of soil collapse can be lessened. The red soils will then offer more favourable founding conditions than the surrounding dolomitic material.