Long-term monitoring of the behavior of a talus-colluvium deposit

This article describes the behavior of a talus-colluvium deposit up to 70-m thick located in the Serra dos Orgaos, Rio de Janeiro/RJ, Brazil. The monitoring dataset of 13 years contributes to a better understanding of the mechanisms of movement associated with talus-colluvium deposits in Southeast Brazil. The study site has had a long history of slow movements, beginning in the 1970s. Field investigations, laboratory tests, monitoring, and theoretical analyses were carried out. In total, 446 groundwater-monitoring campaigns, 120 inclinometer readings, and full-time pluviometric monitoring were completed. Creep movement has been observed with a seasonality that is controlled by variations in the groundwater table (GWT). Due to the large strains, the soil residual shear strength may be mobilized at the failure surface. Analyses indicate that a better slope stability condition may be achieved by reducing the GWT level. The installation of deep horizontal drains (DHDs) led to reductions in the GWT and movement. The displacement rate decreased from 20 mm/month to 2.5 mm/year. Moreover, the monitoring data and analyses evidence the decrease of stability of the talus-colluvium mass with increased GWT and reactivation of movement with blocked drains. Good correlations (r(2) up to 0.90) were observed for rainfall, movements, GWT levels, and piezometer readings. Before the installation of the DHDs, there was a direct correlation between displacements and piezometric levels. After that, this correlation was not so evident. The response of the deposit to rainfall is no longer immediate but depends on a significant amount of accumulated rainfall.