Tailings dam monitoring: a practical guide

What is a tailings dam?

A tailings dam is an embankment built to store tailings — the ground rock, process water and residual chemicals left over once the valuable mineral has been extracted from ore. Because tailings are produced continuously for the life of a mine and cannot economically be moved elsewhere, they are stored on site, permanently, in what is properly called a tailings storage facility (TSF).

This makes a tailings dam unlike a conventional water dam in three ways that matter for safety. It is not built once and finished — the embankment is raised in stages over decades as the facility fills. It is often constructed from the tailings themselves rather than from engineered fill. And it is never decommissioned and drained at the end of its life: the material stays there after the mine closes, and so does the responsibility for it.

Why tailings dams fail

Tailings dam failures are rare, but their consequences are severe and often fatal, because the stored material can liquefy and travel far downstream. Investigations into major failures repeatedly identify a small number of contributing mechanisms: overtopping when the water level rises above the crest, slope instability, internal erosion through the embankment, foundation failure, and seismic liquefaction.

Several of these share a common precursor — water where it should not be, or more of it than the design allows. A rising pond reduces the margin between the water surface and the crest, raises the phreatic surface within the embankment, and increases the risk of both overtopping and structural instability. Water management is therefore central to tailings dam safety, and water level is one of the few precursors that can be measured directly, continuously and cheaply.

Freeboard: the metric that matters

Freeboard is the vertical distance between the surface of the water in the facility and the lowest point of the dam crest. It is the buffer that absorbs a storm, an unexpected inflow, or a period when decant is not keeping up with deposition. Every tailings storage facility operates to a required minimum freeboard set by its design and its operating permit.

Freeboard is a moving number. It falls as tailings are deposited and the pond rises, and it can drop sharply during heavy rainfall — precisely when getting a person onto the catwalk to take a reading is most difficult and least safe. In South Africa GN 704 sets the minimum dry freeboard at 0.8 metres above full supply level. A facility can be comfortably compliant at the morning inspection and below its required freeboard by nightfall, with nobody aware until the next scheduled reading.

How tailings dams are monitored

Monitoring a tailings storage facility means watching several things at once. Piezometers track pore water pressure inside the embankment. Survey monuments, inclinometers and satellite radar detect movement and deformation. Inspections check for seepage, cracking and erosion. Water level and freeboard measurement tracks the pond itself.

Historically the water level part has been the most manual. Someone walks the facility, reads a staff gauge or a marked pole, and records the value — typically once or twice a day, in daylight, in conditions safe enough to be out on the structure. That produces a handful of data points per day, and no data at all during the storms and after-hours periods when conditions change fastest.

Continuous instrumentation changes the shape of that record. A sensor mounted on the existing catwalk measures the distance down to the water surface at a set interval and transmits each reading off site, producing a continuous series rather than a series of snapshots — including overnight, during storms, and when the facility cannot safely be accessed.

What continuous monitoring changes

The value is not that it removes the site inspection — it does not, and it should not be presented as a substitute for an inspection regime. The value is what is known in between inspections, and what is available at the moment somebody has to make a decision.

When a threshold is crossed at 17:30 and the next manual reading is due at 06:00, continuous monitoring turns a twelve-hour blind window into an alert. When the mine calls after a storm and nobody can get onto the catwalk, it turns a guess into a current number. And when a regulator or an auditor asks what the water was doing on a particular night, it turns a logbook entry into a defensible record.

Standards and regulatory context

Following the Brumadinho failure in Brazil in January 2019, the Global Tailings Review was co-convened by UNEP, the Principles for Responsible Investment and the International Council on Mining and Metals. It produced the Global Industry Standard on Tailings Management (GISTM), launched in August 2020, which sets expectations across the full facility lifecycle — including monitoring, surveillance, and the ability to demonstrate a facility’s performance over time.

South Africa has no single statute governing tailings facilities. Several Acts apply at once — among them the Mine Health and Safety Act, the National Water Act, the Mineral and Petroleum Resources Development Act, and the National Environmental Management Act — and SANCOLD notes that their requirements can at times appear contradictory. SANS 10286 (“Code of Practice: Mine Residue”, originally SABS 0286 of 1998) provides the guidance on siting, design, operation, surveillance and closure, but it is not itself legally binding. Since 2001 the Department of Mineral Resources has required the owner of a mine residue deposit to maintain a Mandatory Code of Practice for the facility, updated on a continual basis.

Freeboard is where the requirement becomes a number. Government Notice R704 of the National Water Act (June 1999) requires that any dam or tailings dam forming part of a dirty water system be operated with a minimum dry freeboard of 0.8 metres above full supply level. Government Notice R577 under the MPRDA (April 2004) instead requires 0.5 metres above the expected maximum water level for a 1:100 year, 24-hour event; because that is usually the less onerous of the two, freeboard on South African tailings dams is normally managed to GN 704.

Requirements differ by facility and change over time, so specific obligations should always be confirmed against the permit conditions and the current regulations for the site in question. What is consistent across regimes is the direction of travel: more frequent monitoring, better records, and a demonstrable ability to show what was known and when.