The collapsible nature of reworked residual granites in the Stellenbosch Municipality, South Africa

Abstract

In South Africa, residual granite is associated with the collapse phenomenon, a geotechnical hazard that was first identified in the 1950s. Residual granite is a product of chemical weathering (in-situ decomposition) or physical weathering of the parent rock (granite) whilst reworked residual granites is defined as residual granite reworked in-situ by termites. Residual granite usually consists of bulky-sized quartz particles, with silts, and bridged by colloidal matter. Extreme leaching of the colloidal matter creates a structure like a honeycomb. It, however, leaves voids within the structure, which affects the mechanical behaviour of the soil. The soil collapses upon wetting under additional loading. Construction on soils with a collapsible fabric has led to many documented problems. This paper, therefore, reports on the examination of mineral composition and particle shape examination and its effect on behavioural changes of potentially collapsible soil. The soil used for this study includes reworked residual granite obtained from the Stellenbosch area in the Western Cape of South Africa. A multi-element analysis and morphological studies were performed on mounts using the ZEISS EVO MA15 scanning electron microscope at the Centre for Analytical Facility at Stellenbosch University. Shape analysis confirmed that the severity of collapse increased with increasing angularity. In addition, the type and amount of clay present at particle contacts influenced the swell and collapse behaviour of reworked residual granite.