Unstable ground conditions associated with Karoo outliers in the dolomitic environment of the Far West Rand

Abstract

Areas of the economically important Far West Rand, which are underlain by dolomite, have a latent potential for the development of unstable ground conditions ranging in intensity from catastrophic sinkhole occurrences to differential surface movements measureable only in millimetres. Karoo outliers, occurring within the dolomitic environment, have special significance not only as a source of brickmaking raw materials but also as areas which have been traditionally considered more stable and safer for development than their surrounds. The occurrences of major instability and the severe structural damage to the Driefontein Brickworks subsequent to dewatering has motivated this study, which specifically examines the causes of instability in Karoo outliers, so that the human and capital resources deployed in such areas can be protected. The study shows that post-Transvaal weathering and erosion has caused not only the development of a harsh, karstlike topography in the Malmani Subgroup dolomitic bedrock, but also the creation of a covering layer of unstable residual soils. The subsequent deposition of Karoo sedimentary rocks, which may contain indiscriminately disseminated, expansive clay minerals, adds further complexities to the conditions which promote ground instability. Areas of the economically important Far West Rand, which are underlain by dolomite, have a latent potential for the development of unstable ground conditions ranging in intensity from catastrophic sinkhole occurrences to differential surface movements measureable only in millimetres. Karoo outliers, occurring within the dolomitic environment, have special significance not only as a source of brickmaking raw materials but also as areas which have been traditionally considered more stable and safer for development than their surrounds. The occurrences of major instability and the severe structural damage to the Driefontein Brickworks subsequent to dewatering has motivated this study, which specifically examines the causes of instability in Karoo outliers, so that the human and capital resources deployed in such areas can be protected. The study shows that post-Transvaal weathering and erosion has caused not only the development of a harsh, karstlike topography in the Malmani Subgroup dolomitic bedrock, but also the creation of a covering layer of unstable residual soils. The subsequent deposition of Karoo sedimentary rocks, which may contain indiscriminately disseminated, expansive clay minerals, adds further complexities to the conditions which promote ground instability.