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As part of the planning and design process for expanding an upstream tailings facility in central Mexico was put on stand-by while options to improve its final stability were investigated. With tailings deposition on hold, passive drainage into the permeable foundation and a constructed underdrain gallery was shown to be lowering the saturation level. However, to reduce risk in the short term before the long-term stability could be assured by building a buttress, it was decided to install a dewatering system in the tailings to accelerate and maximize drawdown to increase stability during buttress planning, permitting, and construction.
To monitor the saturation level within this facility and use the data for the stability assessment, standpipes were installed along three cross-sections perpendicular to the dam. In addition, as part of a trial for the design of the dewatering system, vibrating wire piezometers (VWPs) were installed to measure changes in pore pressures during a pumping test. Five strings consisting of at least three sensors per location were strategically placed for this purpose. Analysis of pressure and water level data from both, the VWPs and standpipes before and during the pumping test, revealed a strong passive downward gradient within the tailings to the permeable foundation, and changes due to the pumping and precipitation events.
This paper highlights the usefulness of VWPs in understanding the flow regime within a heterogeneous tailings facility and the additional information they provide compared to standpipes. It was shown that VWPs can provide additional information about vertical hydraulic gradients and how different layers, or lenses respond to rainfall, re-wetting, and dewatering, both passive and active, that are not measured by the standpipes. These additional data were recognized to provide useful information on the behavior of the heterogeneous, layered tailings mass normally unavailable to the design engineers.
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