A preliminary study of the engineering properties of Dorbank

Abstract

Dorbank, a naturally indurated, pedogenic layer, is found over vast expanses of arid and semi-arid parts of southern Africa. Although considered a nuisance to local farmers, dorbank is relatively easy to rip from the soil profile to clear the way for agricultural developments. Its strategic position just beneath the topsoil in the soil profile, as well as its consistency which is often close to that of rock, raises the question of its engineering capabilities. In this study the properties of unbound, mechanically crushed dorbank gravels were investigated in the context of exploiting dorbank for road-building purposes. Moreover, the study investigated the pedogenic variance of dorbank for possible indicators of engineering quality. Dorbank samples were taken from three different regions in the Karoo – the vicinity of Vanrhynsdorp, Aggeneys and the Vaalputs radioactive waste disposal facility near Springbok. Samples were subjected to two stages of investigation. Firstly, the pedogenic nature of dorbank was determined. Different morphologies of dorbank and their relation to the soil type in which they were found were described, whereafter cementing agents were selectively dissolved from dorbank while noting the resulting effect on its structural strength. Polished dorbank sections were additionally analysed with scanning electron microscopy (SEM), coupled with energy-dispersive x-ray (EDX) spectroscopy to identify minerals that are deleterious to roadworks and to understand the chemical enrichment of dorbank by cementing agents. Secondly, the unbound granular qualities of dorbank were tested to establish its use as a material in the structure of a flexible pavement or as a gravel wearing coarse. Dorbank samples were crushed with a small jaw crusher from which the particle size distribution (PSD), the flakiness index (FI), pH, electrical conductivity (EC), Atterberg limits, linear shrinkage (LS), maximum dry density (MDD), optimum moisture content (OMC), soaked California Bearing Ratio (CBR) and direct shear strength (DST) of remoulded samples were determined. The strength and durability of particles that fell within the 13.2 to 19.0 mm size fraction were evaluated with the Treton impact test, the aggregate pliers test (APT) and the accelerated weathering test (AWT). Dorbank samples presented as platy and as massive morphologies in transported sandy soils. Platy dorbanks were found to occur in silty sands, and massive dorbanks in medium to coarse sands. Dorbanks were often found overlying calcrete horizons and containing calcerous veins, which cause adverse discontinuities in the macro- and micro-morphology of dorbank. From the selective dissolution test with NaOH and HCl, and with quantitative SEM-EDX analyses, samples were confirmed to be petroduric in nature. Dorbank fell short of the South African road-base material requirements, particularly due to its low CBR and low durability. Furthermore, dorbanks were found to be almost cohesionless from the DST results (c’ = 3 to 7 kN/m2) and non-plastic from the Atterberg limit test results. The aggregate strength and grading coefficient of dorbank from the Vanrhynsdorp and Vaalputs regions satisfy the requirements for use as a gravel wearing coarse, although possible corrugating, ravelling and a high re-gravelling frequency and dustiness can be expected. Finally, the study concluded that platy dorbank performs better in strength tests, but weather more quickly in the accelerated weathering test. The lack of cohesion and its non-plastic behaviour are likely due to the suppression of the activity of clay, which in turn is due to cementation and the aggregation of clay minerals. Further investigation of dorbank gravels, modified with the addition of fines, is suggested.