Geo-seismic environmental aspects affecting tailings dams failures.

Author:Mayoral, Juan M.
Position:Technical report
 
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INTRODUCTION

Mining activity and other industrial processes generate large quantities of solid waste that annually need to be disposed of and stored in the so-called tailings dams in different parts of the world. The search for an economically, technically sound and environmentally admissible alternative to deal with this type of materials is a multidisciplinary activity where the geotechnical engineer plays a key role. Among the different aspects to consider in tailings dam design, seismic dam stability analysis is the most relevant in projects located in earthquake prone areas to ensure that catastrophic failures will not occur. In practice, these evaluations very often become challenging, due to presence of very fine nonplastic uniform granular soils, both in the tailings dam body and foundation. In addition, these materials can be totally or partially saturated and exhibit a very loose structure if the hydraulic fill method is used to transport and dispose of them. Hydraulic filling is the preferred method by miners for construction of tailings dams due to its low cost, especially considering that tailings dams do not have a direct economic value. Thus, detailed geotechnical investigations are not carried out in most cases and tailings dams are built based on simplified empirical recommendations, usually given by mine owners that increase dam height as a function of the storage volume required.

In order to reduce the environmental hazard associated with contaminant release, dam seismic stability analyses are necessary to ensure that slope instability, freeboard loss, or a larger catastrophic failure, such as those reported in technical literature (1,-4) will not occur. Seismic analysis of tailings dam should account for the degree of internal drainage and the characteristics of the design earthquake, to properly estimate the amount of pore pressure built-up during the seismic event, as well as the pertinent impact on the reduction of shear strength and the modification of dynamic soil properties (i.e., shear stiffness and damping). Based on these evaluations, proper assessment of the volume of material mobilized during a potential failure and the pattern and speed of deformation can be achieved, thereby providing guidance to estimate the extension of the affected zones. Thus, maps of geotechnical hazard and quantification of specific risks can be developed. Pseudo-static methods, based on limit equilibrium and others formulated considering the sliding block mechanism are sufficient to define geotechnical hazard zones but insufficient to quantify the seismic risk, which requires a complete understanding of the kinematics of the slope failure process (5). A complete representation of the failure mechanism usually requires using constitutive models to simulate the dynamic response of the soil, coupled with numerical techniques, such as finite differences or finite element methods. Such models have evolved over time, from the classical hyperbolic Masing-type models (6) to more sophisticated models based on bounding surface hypoplasticity theory (7).

A fully nonlinear dynamic analysis of tailings dams requires using a numerical model able to account for the effect of generation and dissipation of pore pressure within the dam body and foundation and its impact on the variation of shear strength which, in turn, will lead to permanent displacements. Perhaps, the best representation of this phenomenon can be achieved with a fully coupled constitutive model, where the equation of motion is solved simultaneously with the diffusion equation during an effective stress analysis (7). However, in general, the more sophisticated a constitutive model is, the more cumbersome it becomes to use in engineering practice.

This study reviews some of the key geo-seismic environmental aspects to consider during seismic evaluations of tailings dams and presents a practice-oriented numerical scheme which, implemented on a lagragian platform, appears to capture the overall behavior of ground response in geotechnical problems well in which large deformations are likely to occur, such as liquefaction or cyclic mobility. This approach enhances the formulation proposed by Dawson et al. (8). The method allows for incorporating the degree of internal distortion of the mobilized soil mass, drainage conditions, localization of failure planes near the slope surface and the plastic yield at depth. The methodology is illustrated through its application to the analysis of a case history and the model predictions are compared with field measurements, both qualitatively and quantitatively. This approach, along with a risk analysis framework (9), can be used in dam safety evaluation assessments.

ANALYTICAL FRAMEWORK

Seismic parameters: The parameters required to characterize the seismic loads to use in the evaluation of the seismic response of an earth dam, such as a tailings dam, depend on the method of analysis considered. Nevertheless, in general, the following parameters should be accounted for: the earthquake magnitude, the acceleration response spectrum for design and the maximum displacement expected in the dam foundation. In particular for tailings dams...

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