Fourteen-year slow moving landslide monitoring and benchmark velocity parameters of natural slopes at ‘Serra do Mar’, Brazil

This paper presents the analysis and results of a 14-year monitoring of slow-moving landslide behavior along a 100 m high slope at Serra do Mar, Brazil. The slope is located near a roadway and an industrial area and was suffering from creep movements triggered by an excavation at its foot. Movements were especially severe during the rainy periods due to water table fluctuation. Inclinometer readings from 2009 to 2011 showed that the sliding involved a soil mass of 15 to 20 m thick and was in the so-called tertiary phase, with relatively high acceleration. Prediction models showed that the slope failure would probably occur in another two to three years, which required immediate implementation of mitigation actions. By the end of 2011, several deep horizontal drains were installed along the slope to reduce the water table level. Since then, the inclinometers showed that acceleration was eliminated and velocity was substantially reduced, bringing the slope back to primary and secondary, stable movements. Monitoring results of deep horizontal drains shows that flow volumes increase substantially during the rainy seasons, showing that the solution efficiently stabilizes the slope. With monitoring results for both secondary and tertiary creep phases, and comparisons to other monitored slopes in the region, benchmark parameters related to slope velocity and acceleration for Serra do Mar slopes are discussed and presented. This constitutes the first organized study on slope movement velocities at Serra do Mar and presents an important contribution to researchers and designers.