Caracterização geotécnica de sedimentos aluvionares do Rio Açu-RN

Abstract

The second great construction accident of DNOCS (National Department of Constructions Against Drought) occurred at the Eng. Armando Ribeiro Gonçalves dam, which holds waters from the Piranhas – Açu River, in the State of Rio Grande do Norte. This construction suffered an upstream slope rupture while in the building phase. The rupture’s surface developed almost completely over a plastic argillaceous material, native of alluvial sediments from Piranhas-Açu River. The main objective of this work is to characterize and evaluate the resistance characteristics of the clay, on which it was designed in the original and modified section of the dam, with the intention of obtaining relevant information that helps to comprehend the phenomenon of the dam’s rupture. This study was realized through traditional characterization and permeability tests, and mechanical tests (densification and triaxial compression). The characterization tests made possible to categorize the studied soil as a silt clay of low compressibility (CL-ML). For the studied soil, it was found a permeability coefficient of 8,81x10-8 cm/s, showing that the material has low permeability. The densification tests resulted on a density coefficient of the order of 2,87x10-3cm/s, also it was possible to verify a small tendency of expansion of the material when flooded. As from the triaxial tests of UU type (unconsolidated-undrained), it was observed that at unsaturated condition the soil behavior is better represented by a bilinear resistance envelope, while at the saturated condition, the behavior is better described as an inclined envelope. It was also noted a significant drop of the resistance, caused by the increase of moist content in the soil.