Infiltration Study in a Thickened Tailings Deposit

A4   |   Letter

SRK News | Issue 55: Mine Water Management

Beatriz Labarca, Principal Geologist 

In 2007, SRK Chile was involved in designing the first large-scale facility for thickened tailings in Chile. The project included a pilot plant to provide data on the beach angle of the tailings, and tests to estimate infiltration from the tailings into the ground. The soil water characteristic curve was determined in the laboratory and the water content was measured during the operation of the pilot plant. Based on the collected data, a 1D model of unsaturated flow was built to simulate, layer by layer, the deposition of tailing over time and predict infiltration. The results of that model predicted that infiltration would occur only as a non-saturated front, 2m below the contact between tailings and soil.

Mine operation and tailing deposition started in 2011 without being able to achieve the design thickening, i.e. a solids content of 67%. In January 2015, SRK ran a complete field program to evaluate infiltration from the tailing deposit and compare it with the predictive model.

Geophysical exploration, test pits, sonic drilling for sampling tailings and the foundation soil were undertaken within the tailings deposit and underlying soil. The water content within the deposited tailings was found to be around 17%, whereas in the soil, was around 5%. Both tailings and soil are low permeability materials, a condition that minimised the extent of flow between both materials. If tailings are saturated, water is moving to the soil.

Due to the high evaporation rate, field data showed that five days is enough to reduce the average water content in the tailings when deposited in layers of 20cm to 60cm. This is supported by results of the water balance analysis indicating that infiltration into the underlying soil would be negligible, consequently tailings water should not affect the groundwater of the area.

The field data were used to update the 1D unsaturated model (Feflow). Modelling using the updated SWCC for both soil and tailing reproduced the observed water content for both materials, with simulated water content of tailings at 18.6% and the soil at 4.9% below 2m of depth.

After validation, new scenarios were run to simulate future operating conditions. Results indicate little variation on the current soil water content as the tailings layer works as an impermeable barrier, and due to the high evaporation rate.

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