Back analysis of an ancient rockslide at Lake Fundudzi, Limpopo Province, South Africa

A massive rockslide occurred in the Soutpansberg Mountains of the Limpopo Province ~20,000 years ago that blocked the course of the eastern flowing Mutale River forming Lake Fundudzi, which is the only known true inland lake in South Africa. The landslide occurred in the northwestward dipping and highly jointed Fundudzi Formation of the Soutpansberg Group. The failed slope was observed to be a homogenous jointed sandstone rock slope which is quartzitic in places. Cluster analysis, which uses fuzzy clustering in Rocscience Dips, revealed three major joint sets (JS1, S0 and JS3) in the rock mass with an average dip and dip direction of 85°/237°, 15°/283° and 73°/157°, respectively. The main failure plane best correlated with JS3, which dips towards the lake. The kinematic analysis also revealed that the mechanism of failure was planar which is in agreement with site observations. The strong rock material classification of the sandstone from a laboratory index and mechanical tests was one of the indicators that failure could have been mostly structurally controlled. The technique of back-analysis was employed using RocPlane in order to investigate the geotechnical conditions that may have resulted in failure. The results obtained from the sensitivity analysis in RocPlane showed that the joint roughness coefficient (JRC) followed by the residual basic friction angle (ϕ _r) were the main influential shear strength parameters on the factor of safety, F, as opposed to the joint wall compressive strength (JCS) which has very little impact on F for the range of values considered. Results from back-analysis show a range of possible combinations of JRC and ϕ _r values under different groundwater and seismic conditions. Whilst the groundwater shows more impact than the seismic coefficient, the field evidence points to seismic activity as a more plausible trigger than groundwater. This study shows the usefulness of back-analysis techniques in assessing various conditions of failure that may have resulted in ancient rockslides. Although it may not result in a unique set of geotechnical parameters that may have resulted in failure, it, however, gives an insight into probable causes of failure and, thus, can be useful in mitigation of future failures.