Field-scale assessment of the unsaturated hydraulic properties of residual soils in southeastern Brazil.

Abstract

Field tests were carried out to estimate effective unsaturated soil hydraulic properties of layered residual soilsin Rio de Janeiro, southeastern Brazil. Data of this type are important for understanding the initiation of rainstorm-induced soil landslides, which often occur in the state of Rio de Janeiro as well as other areas having similar geologicsettings and climate conditions. Tests were carried out using a simplified field approach, referred to as the MonitoredInfiltration Test, which requires only a tensiometer to measure pressure heads below the wetting front, triggered by flowfrom a Mariotte bottle which maintains a constant pressure at the top edge of the soil profile. The data can then beanalyzed by numerical inversion using the HYDRUS-2D software package. The test is relatively fast since no steady-state flow conditions are needed, and versatile since the test can be carried out quickly on steep slopes with the help of amanual auger. Soil water retention and the unsaturated hydraulic conductivity functions were obtained for a range ofyoung, mature and saprolitic residual soils. The effective hydraulic properties of the distinct residual soil layers can bequite large, reflecting a need to provide a careful analysis of field-scale hydraulic heterogeneity in geotechnical analyses.