Geotechnical characterization and slope stability analyses of the Town Bush Valley, Pietermaritzburg, South Africa.
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Date
2018
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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
Description
Master of Science in Geological Sciences. University of Kwazulu-Natal, Durban, 2018.