Hydrothermal clays and their influence on slope stability in residual soils of Southern Brazil

Abstract

Clayey veins may develop in areas of hydrothermal activity by seepage of hot water through the soil and rock. In a slope located in Southern Brazil, such veins occur along the slip surface of some landslides. This paper aims to characterize this kind of clay and investigate its influence over the stability of that slope. An electro resistivity survey had been conducted to characterize the soil profile, and SPT boreholes were used to confirm geophysical data. An experimental laboratory campaign included the investigation of chemical, mineralogical, and geotechnical properties of these clay veins and the embedding residual soil. Drained shear strength of residual soil was measured through triaxial tests, and residual strength was measured for both materials using a ring shear apparatus. A series of limit equilibrium analyses were conducted to assess the role of clay veins on the slope stability. The geometrical discretization of veins is a difficult task, and an approach based on the assumption of overall shear strength is required to establish the operational shear strength parameters for this slope. A series of back analyses showed that failure surfaces consistent with those observed in the field were only reproduced when the adopted shear strength parameters of the soil mass were c’ = 5 kPa and φ’ = 20°. Considering the measured shear strength parameters of the hydrothermal clay c’ = (0 and φ’ = 8.5°) and the pattern in which the clay veins occur, it seems that they do not fully control the slope stability, but they significantly reduce the global cohesion and the average mass friction angle.