On the evaluation of unsaturated flow in a natural slope in Rio de Janeiro, Brazil

Geotechnical engineers and geologists are frequently concerned with the complex phenomenon of rainstorm-induced landslides in mountainous tropical regions. This paper analyses the hydrological conditions that contributed to a huge deep-seated slide of a slope in Rio de Janeiro, Brazil. The slope failure occurred after a short period of intense rainfall, buried cars, and caused structural damages to an adjacent building. On the following day, the failure surface was completely saturated, despite the fact that no groundwater was observed within the slope. Understanding a rain-induced landslide is a considerable challenge. In the present case, there was an extra question to be answered: why were no slope movements observed a few months earlier, when a more intense rainfall reached the slope? Geological and geotechnical surveys were carried out aiming at determining complete soil profile characterization, piezometric levels and hydraulic parameters. Field investigations indicated a variable thickness unsaturated residual soil layer overlying a gneissic rock. The transition between the saprolitic soil and the sound rock consisted of a highly fractured and weathered rock. 3D flow modelling has shown that the amount of rain infiltration was insufficient to produce positive pore-water pressures. It was likely that development of preferential infiltration paths could explain the hydrological conditions of the slope during landslide. Water infiltration through the fractured soil–rock transition induced a rapid development of positive pore water within the soil mass. This favourable scenario might have been created by the lack of maintenance of the crest drainage channel, adjacent to the slide area. It is a common engineering practice to try to associate rainfall amounts to the risk of slope failure. The role that bedrock plays in generating pore pressure is seldom considered. The numerical analyses presented in this paper pointed out at the complexity of flow modeling of rain infiltration through unsaturated residual soil slopes. It essentially revealed that flow mechanisms other than rain infiltration over the slope surface might play a major role on hydrological pattern of slopes.