Variability of the undrained strength of the plastic tailings from the Germano dam in Mariana, Brazil

Abstract

Mining tailings may be divided in non-plastic (sand, silty sand, or sandy silt tailings) and plastic (fine tailings, usually mixtures of silt and clay-sized particles). The understanding of the behavior of these materials is a great geotechnical challenge because of their wide variations in mineralogy, physical–chemical, and geotechnical characteristics. Sand tailings have been widely studied in the last decades because they are often susceptible to liquefaction, and they usually constitute the beach of the tailings dam which is the most important zone for stability analysis in upstream dams. However, the study of the characteristics and the strength of the plastic tailings is also important since layers of plastic tailings are sometimes found in the midst of the sand tailings. The aim of this article is to study the normalized undrained strength ratio (su/σ′v) of plastic tailings of the Germano dam in Mariana, Brazil, and to present a criterion developed for their identification. The method consisted of analyzing and scrutinizing Piezocone (CPTu), Field Vane (FV) tests, and the water levels. An extensive review of the iron ore plastic tailings properties was also performed. A total of approximately 900 occurrences of plastic layers were analyzed. The results included the discovery of perched water tables and significant vertical downward flow in some locations. The conclusion is that su/σ′v histograms are well fitted by lognormal distributions and the layer thickness influences the undrained strength. Moreover, the strengths are relatively low (for example, su/σ′v = 0.11 on average for plastic layers with thickness of 1.0 m or more).