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Geotechnical Characterization of Old Tailings for Use as Sand Stacking Foundation

Author: Solange Paihua, Orlando Román, Rubén Vargas
Conference: Tailings and Mine Waste 2016
Date: October 2-5, 2016

ABSTRACT
Several mining projects are evaluating the use of existing tailings facilities for disposal of fresh tailings due to a trend of reducing new areas impacted by mining activities. New tailings can be placed in old tailings facilities using alternative methods such as sand stacking or filtered tailings. Placement of new tailings in an old tailings facility could lead to potential slope stability problems during and/or at the end of operations and so the understanding of the behavior of the old tailings as a foundation material for the new facility should be carefully
studied.

This paper presents the results of a geotechnical site investigation program of piezocone penetration tests (CPTu) and standard penetration tests (SPT) that was carried out in an old tailings facility to evaluate its feasibility as a foundation material. The old tailings (approximately 20 to 30 years old) need to be geotechnically characterized to assess how they will behave after being loaded by the fresh tailings. The information collected during the geotechnical site investigation allowed the definition of the stratigraphic profile of the existing tailings facility. The old tailings were composed of interbedded coarse tailings (loose sand) with thicknesses between 10 and 20 meters and fine tailings (soft clay/silt) with thicknesses between 10 and 18 meters.

It was necessary to assess the behavior of the tailings during shearing, with variable pore water pressures, and with varying strength parameters in order to evaluate the feasibility of this disposal method because the physical stability of the future tailings storage facility will primarily depend on the characteristics of the existing tailings foundation. The soft fine tailings are of particular interest for the stability assessment of the new tailings facility. Geotechnical recommendations are presented for the fresh tailings disposal using the sand stacking method and for future stages of the project.

INTRODUCTION
Several mining projects are evaluating the use of existing tailings facilities for disposal of fresh tailings due to a trend of reducing new areas impacted by mining activities. This paper presents the results of a geotechnical site investigation program developed for a feasibility study in old tailings storage facility located in the South American Amazon Region, on top of which new tailings will be placed through sand stacking.

The sand stacking strategy has been assessed as a medium-term alternative for tailings disposal. Sand stacking basically consists of disposing the tailings using cells contained by nonstructural berms within the tailings storage facility, so the tailings can be disposed as a slope above the maximum elevation of the existing dams, thus creating additional storage capacity of tailings with no need to raise the perimeter containment dams. Tailings will be hydraulically discharged into the cells until the latter are filled up; this process will be repeated after each of the cells has been filled, drained and dried up naturally.

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