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Bukavu, on the south coast of Lake Kivu in the Democratic Republic of the Congo, suffers from slow but never ending landsliding. This leads to the continuous degradation and destruction of houses, buildings and of the roads, waterworks and sewerage infrastructure in several districts of the town. Research of mass-wasting processes is hampered by the limited geotechnical and hydrogeological information concerning the deeply weathered Tertiary and Quaternary lavas. There is also a disagreement about the exact location of active faults, believed to play a role in the mass wasting. Additionally, there is little information about the exact location and the rate of soil movements and whether ground instability is caused by tectonics and seismics or by increase in hydrostatic pressure. Based on aerial photographic interpretation, landslides cover more than 15% of the town of Bukavu. Thirty-one landslides occur outside the Bukavu “microrift” and do not contact active faults. Rather, they occur in actively incising river basins, ostensibly caused by neotectonic activity. Four from the six landslides within the “micrograben” are bigger and wider than the others, apparently governed by pre-existing tectonically induced landforms. They are adjacent to or crossed by active faults. They also fall far below the envelope of topographic thresholds for landslides established for North America and verified in Rwanda. Therefore, this threshold, a combination of slope at the head of the slide and surface drained to it, seems a promising tool to discriminate hydrologically from tectonically seismically induced landslides. Recent soil movements in Bukavu generally correspond to landslide distribution portrayed on the geomorphologic map. Most Bukavu soil movements occur on previous slides. Therefore, from an engineering–geologic standpoint, old landslides should be avoided, or, if economically feasible, be mitigated.

This study reveals that the Quaternary mantle of reworked Kalahari Sands undergoes a post-sedimentary composite process at the archaeological site of Gombe (Kinshasa, Zaire). There is a biological activity which consists of an upworking of soil particles, mainly by termites and worms. This causes a diminishing dry volume weight of the sediment. In response, the mantle reconsolidates. When the soil surface is horizontal, the particles which are not brought up will sink down in the soil profile as a result of the reconsolidation. When the soil surface is inclined, it is only the resettlement of the sediment which can give rise to creep, caused by wetting and drying. Creep of the dead mineralogical material alone seems to be very improbable once the structure and settlement of the sediment are in equilibrium with a given slope. Only a new phase of bioturbation can induce further creep movements. The experiments have shown that stone implements in reconsolidating Kalahari Sands do not exactly accompany the compaction movement of the sediment. Depending on their form, their size, their orientation and the water content of the sediment around them, stone implements can undergo differential movements during the reconsolidation process. This can result in a dispersion of the implements over levels of different ages. As creep is a phenomenon, going hand in hand with the resettlement of the sediment after biogenic upworking, partially analogous movements of stone implements can be expected in a creeping Kalahari Sands mantle. Partial or total reconcentration of the stone implements at the base of affective biogenic activity can be expected. This can lead to typical stone-line profiles which must be interpreted very carefully.