Geotechnical characterization and slope stability analyses of the Town Bush Valley, Pietermaritzburg, South Africa.

Abstract

The construction of settlements over zones of instability is increasing the impact of disasters across the world both in developed and developing nations. Many areas in the greater Pietermaritzburg region in South Africa, such as the Town Bush Valley, are prone to slope instability due to the terrain morphology and high intensity rainfall. This study has investigated the geotechnical conditions at the Town Bush Valley, in Pietermaritzburg. A geotechnical characterization of the Town Bush Valley has been undertaken in order to understand the geotechnical conditions prevailing on site. Furthermore, two critical slopes were selected for slope stability analyses to investigate the conditions under which failure would occur. The method of analyses chosen was the Morgenstern and Price method using the Rocscience, SLIDE software. The analyses involved a deterministic approach and a probabilistic approach. In the deterministic approach, all the input variables were considered as constant values. In the case of the probabilistic approach, the effective shear strength parameters were chosen as the random variables in order to account for their uncertainty. Prior to the analyses, sensitivity analysis was conducted in order to see the effect of the effective shear strength parameters, c´ and φ´, on the factor of safety. Various scenarios, including groundwater conditions and surcharge load, were considered during the analyses. Results from the site characterization show that the site is characterized by heterogeneous talus material, which is underlain at depth by shales of the Pietermaritzburg Formation and sandstones of the Vryheid Formation. Particle size analysis, Atterberg Limits Determination and consolidated-drained triaxial tests were undertaken on the talus material. The slope stability analyses show that the probabilistic approach presents a better insight into the assessment of the slope than a deterministic approach in accounting for the uncertainty in the geotechnical parameters. The random behaviour of the geotechnical parameters was quantified through various probabilistic functions. The various functions derived during probabilistic slope stability analyses, allowed for an assessment of the reliability of the data sets. Keywords/Phrases: Deterministic slope stability analysis; Phreatic surface; Probabilistic slope stability analysis; Random variables; Town Bush Valley