Resumo em Inglês:

Abstract A proper evaluation of the mechanics of tailings is a crucial step toward the geotechnical analyses required throughout the project lifecycle of tailings storage facilities. In particular, an essential aspect of behavior is the stress level dependency of the stress-deformation constitutive relationships, which needs to be assessed when a new dam is raised or when a new deposition of tailings occurs. For that purpose, the present paper addresses the response of zinc tailings under triaxial compression tests with confinement pressures ranging from 20 kPa to 4000 kPa. Particle breakage was assessed by determining the grain size distribution in some specimens sheared at specific confining pressures. The results indicate the existence of multiple-strength envelopes in which the critical stress ratio (M) varied from 1.58 for the lowest confinement levels to 1.20 for the highest pressures. This was accompanied by a substantial increment in the acceptable content of the material, resulting from grain crushing occurring at higher pressures.

Resumo em Inglês:

Abstract The disposal of tailings has become a challenge for the mining sector, environmental agencies, and society. This research aims to analyze the geomechanical and environmental behavior of cemented bauxite tailings (BT) for use in cemented dry tailings piles, a new trend for tailing disposal. The experimental program consisted of a series of unconfined compression tests and splitting tensile tests in BT with addition of 2%, 4%, and 6% of Portland Cement (Ci), and compacted at 18 kN/m3, 19 kN/m3, and 20 kN/m3. Leachability and solubilization tests were done with specimens with 20 kN/m3 and Ci of 0%, 4%, and 6%. All samples were done with water content of 18% and cured for 7 days. Increasing in Portland cement content and dry units of weight increases unconfined compressive strength (qu) and splitting tensile strength (qt); therefore, the increase in porosity decreases these mechanical properties. The importance of those parameters was confirmed by ANOVA and Tukey test analysis. The porosity/Portland cement content index allowed the prediction of qu and qt of BT-Portland cement blends; however, the qt/qu remained constant (0.14) and independent of this index. The environmental analysis showed that the addition of Portland cement to BT reduced the concentration of manganese in the leachate and the concentrations of chromium, iron, and sodium in the solubilized extract, complying with the toxicity and drinking water standards recommended by the USEPA. However, it also increased the concentration of aluminum in both extracts, exceeding the limit set by CONAMA 460 (Brazilian standard).

Resumo em Inglês:

Abstract Past research has focused on spatial variability or the influence of randomly distributed strength on slope stability. This research explores stratigraphic variability, specifically the influence of positional changes in the boundary between two materials of very different strengths on stability. Specifically, the boundary between drained and undrained material commonly present in upstream tailings dams. Two-dimensional (2D) and three-dimensional (3D) slope stability analyses are compared. The study showed that when a single random 2D cross-section was selected, this had an 11% probability of being unsafe (i.e., the resulting 2D factor of safety was higher than the 3D factor of safety calculated considering stratigraphic variability), while extrapolating a randomly selected 2D section to represent the entire 3D slope (i.e., ignoring stratigraphic variability) increased this risk to 46%. The reason why an extrapolated 3D slope stability analysis was unconservative is that, in many instances it failed to account for weak sides, toe, and back scarp conditions that existed when stratigraphic variability along the length of the slope was taken into consideration. These features can be overlooked when an analysis assumes homogeneity and highlights the importance of investigating conditions along the length of a slope particularly if a 3D analysis is to be performed.

Resumo em Inglês:

Abstract This paper presents a review on the main simplified methods to estimate the horizontal seismic coefficient for 2D pseudo-static slope stability in earth and tailings dams. The pseudo-static method is conceptually simple, required for stability of soil slopes by seismic codes and regulatory agencies, but from a geotechnical point of view is a complicated approach, given the difficulty to select appropriate values of the seismic coefficient and the shear strength parameters. Most of the simplified methods are based on a decoupled approach where the numerical or analytical solution is obtained in two independent steps: firstly, the dynamic response of the dam, followed by evaluation of the permanent displacement of the sliding mass. The seismic forces in a decoupled approach are not adequately considered in the first step since the displacements along the sliding surface are neglected. These forces may exceed the dynamic strength of the soil, leading to an overestimation of the response, especially when the predominant frequency of the earthquake is close to the fundamental frequency of the sliding mass. On the other hand, coupled analyses capture the interaction between the dynamic response and the sliding surface response to compute permanent displacements directly, thus enabling a correlation between the horizontal seismic coefficient and allowable permanent displacements. The seismic response of the slope is represented by the fundamental period of the potential sliding mass, and the site-dependent seismic demand is characterized by the 5% damped elastic design spectral acceleration at a degraded period of the potential sliding mass.