Authors: Sudhir Tripathi, Jordan Scheremeta, Jeff Coffin, Jason Reiva
Conference: Tailings and Mine Waste 2020
Date: November 15-18, 2020

ABSTRACT
Suitable locations for safe and economic disposal of mine tailings that pose low long-term liability are limited at many mine sites. In recent years, backfilling of existing mine pits with tailings has garnered attention as one viable alternative. The case presented herein, despite having some fundamental resemblance to typical in-pit filling practices, is unique due to the hydrogeology of the area and envisioned complexities of the tailings deposit development within the open pit over the life cycle of the facility. Consolidation modeling was conducted to assess the life-cycle of the facility and the anticipated performance of a proposed intermediate separation layer and overlying geomembrane liner system constructed over soft tailings that are needed to mitigate local groundwater contamination.

Owing to the mine’s plan to develop the in-pit Tailings Storage Facility (TSF) as two separate deposits – 1) Lower Deposit, and 2) Upper Deposit – separated by a free-draining separation layer, the consolidation modeling approach consisted of several three-dimensional (3D) and one dimensional (1D) models using the finite-difference CONDES consolidation software developed at the University of Colorado at Boulder. The first step included development of a 3D consolidation model to assess the anticipated void ratio distribution of the unlined Lower Deposit at the end of deposition within that portion of the TSF. The Lower Deposit is planned to be unlined because the hydrogeology of the area provides a hydraulic sink toward the pit and the groundwater is maintained below a certain level by continuous pumping, which mitigates the potential for groundwater contamination. The Upper Deposit is located above the hydraulic sink elevation and will require a geomembrane liner system to prevent contamination of groundwater sink. This required the design of a rockfill Separation Layer on the surface of the Lower Deposit to account for continued consolidation-settlement of the Lower Deposit and the Separation Layer due to self-weight and filling of the Upper Deposit.

To model this deformation and design the necessary camber on the Separation Layer, a 3D consolidation model was completed on the TSF Upper Deposit to assess the void ratio profile and average dry density of the material driving the consolidation of the Lower Deposit. The predicted average dry density was used to estimate the combined uniform surcharge pressure exerted on the Lower Deposit by the overlying rockfill Separation Layer and the Upper Deposit. Next, several 1D consolidation models were completed under the anticipated combined uniform surcharge pressure to predict differential settlements of the geomembrane liner system due to the compressibility of the unlined Lower Deposit. These analyses were used to design the necessary camber on the Separation Layer to mitigate damage to the geomembrane liner system of the Upper Deposit and to maintain gravity flow across the Separation Layer (below the hydraulic sink elevation) to perimeter collection sumps. Finally, the modeling results were combined to estimate the anticipated dry density of the tailings at the end of filling of the overall TSF.

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