Influence of Rainfall Patterns on the Slope Stability of the Lumwana (the Malundwe) Open Pit

Malundwe open pit has experienced slope failures in the pit’s upper stack during rain-season since mining started in 2008. Laboratory and numerical model investigation methods were performed to investigate the influence of the rainfall pattern on the stability of the upper stack having saprolite domains. The saprolite soil was classified using the British soil classification system as slightly clayey, silty, sand with fine gravel which are of low plasticity. The water retention test using pressure test conducted on saprolite, reveals that the saprolite soil could not dry up completely as it contains about 9% volumetric content of water with the matric suction of 300 kPa during the most dehydrated month. The rainfall patterns at the mine site increase the groundwater level on the saprolite material. The rising of the groundwater table leads to the reduced matric suction and increased volumetric water content in saprolite domain. The reduced matric suction in the saprolite soils reduce the saprolite soil’s shear strength effectively leading to slope instability. Therefore, rainfall infiltration is the facilitator for initiating the failure of the upper slope at Malundwe open pit.