Effects of Cyclic Variations of Pore Pressure on the Behaviour of a Gneiss Residual Soil

Many slope failures in residual soils have been attributed to increased soil pore pressure. During the rainy season, water infiltration occurs, which can lead the slope to rupture by increasing the pore pressure. However, some landslides have been observed with pore pressures below those required for failure. Hypotheses for such ruptures include the occurrence of creep or fatigue due to cyclic variations in the piezometric level on the slope. This paper presents a study on the effect of pore pressure on the behaviour of the residual soil of the Soberbo hillside in Rio de Janeiro, Brazil, subjected to monotonic and cyclic pore pressure. Triaxial tests that simulate the increasing piezometric level (constant shear drained—CSD tests), drained creep (DC-CSD tests) and cyclic pore pressure variation (CPP tests) in slopes were carried out. The CSD test results showed that the failure due to increasing pore pressure is essentially dilatant and confirmed the peak strength obtained from conventional drained triaxial tests. Sudden failure was observed in all CSD tests at the end of the pore pressure increase phase. The results show that the failure of the slope may occur due to drained creep and the loss of soil shear strength under cycling pore pressure.