Seepage estimates from tailings impoundments

A4   |   Letter

SRK News | Issue 55: Mine Water Management

Simon Lorentz, Principal Hydrologist

Tailings Impoundments Seepage EstimatesSRK has a long history of evaluating tailings dam hydraulic and hydrological processes. These studies inform water balance estimates, flood control design, seepage predictions, and stability analyses.

Recently, the Department of Water and Sanitation in South Africa issued a Proposed Amendment to the Regulations Regarding the Planning and Management of Residue Stockpiles and Residue Deposits, enabling the use of risk-based design of pollution control barrier systems in mine tailings. In order to address a risk-based design, detailed analysis of the infiltration, evaporation and redistribution of rain and slurry water within the tailings impoundment is required.

This has become particularly necessary with cyclone deposition, where coarse, underflow tailings are deposited on the perimeter of the impoundment and the overflow fines are deposited onto the beach. In addition, the level of the phreatic surface throughout the deposit needs to be accurately established so that stability analyses can be performed. The drawdown due to drains and the potential seepage through barrier systems at the base of the tailings also require detailed analysis.

To address the requirements for risk-based design, HYDRUS-2D, a 2-dimensional, finite element soil-water physics model has been used.

HYDRUS-2D was used initially to simulate the behaviour of saturated and unsaturated zones in an existing tailings impoundment. The hydraulic characteristics of the impoundment were measured in-situ and derived from piezocone analyses. Simulated fluxes from drains were compared with monitoring records; the simulated phreatic surfaces were compared with observed piezometer water levels, and hydraulic pressure gradients extracted from the 2-D model were compared with those observed in piezocone tests.

The successful representation of the existing hydraulics in the model has allowed for the evaluation of barrier systems for a new tailings storage facility with similar tailings material. Here, specific boundary conditions have been invoked to represent the non-Darcy characteristics of seepage losses through a geomembrane barrier system. Theoretical and experimental flux estimates for geomembrane performance under various conditions, reported in the literature, were used to develop an envelope of possible boundary responses. These were simulated to derive potential seepage fluxes into the footprint.

The simulations were also effective in the positioning and design of the drains, as well as deriving conditions for stability analyses.

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