A slope in southern Brazil has suffered persistent instability problems. Geotechnical investigation indicates the occurrence of two residual saprolitic soils under different levels of alteration and weathering. These soils are Yellow Residual Soil of Granite (YGrRS) and Biotite-rich Residual Soil of Granite (BGrRS). A third material, Filling Clay (FC) also occurs in the area. It has a hydrothermal origin and fills veins in the residual soil mass. Conventional CD triaxial tests revealed ϕ’ = 30° and c′ = 13.5 kPa for YGrRS and ϕ′ = 26.4° and c′ = 20 kPa for BGrRS. For FC, ring shear tests indicated ϕ′r = 8°. Slope stability analysis where clay veins are absent allows reproduction of a failure that happened in 2010 and validation of parameters obtained for residual soils. Conversely failures where clay veins frequently occur can only be explained by considering that the shear strength of the soil mass is lower than the one measured in laboratory tests. This hypothesis is justified by the presence of clayey veins, which reduce the overall strength of the soil mass by providing weaker paths for the development of failure surfaces.