Tailings Dam Monitoring: Instruments, InSAR, Inspections

Reference article · Updated 2026-11-06

Key facts

Pore-water pressure, deformation and seepage are the core quantities measured at a tailings dam.
A 2021 study using satellite radar found deformation inconsistent with consolidation settlement at Brumadinho in the 17 months before collapse.
Canadian guidance builds surveillance around an operation, maintenance and surveillance (OMS) manual and an emergency preparedness and response plan.
At Stava, case studies report that the dams had no serious stability checks or monitoring for more than twenty years.

What a surveillance programme is meant to do

Monitoring does not make a dam safe. It tests whether the dam is behaving as its designer expected. Surveillance usually has four parts:

instruments;
routine visual inspection;
periodic engineering review;
a documented response when readings or observations depart from expectations.

The aim is to detect adverse trends early enough to act. Action may mean lowering the pond, stabilising the embankment, or evacuating people downstream.

The case record shows why the programme matters. Reports on Stava describe dams that were never seriously checked for two decades. The review of Mount Polley showed that a weakness hidden in the foundation could escape inspection altogether. The mechanisms that surveillance targets are described on the page on causes of failure.

Instruments and what each one measures

Instrument	Measures	Why it matters
Piezometers (vibrating-wire, standpipe)	Pore-water pressure in the dam, foundation or tailings	Rising pressure reduces effective strength and can precede instability
Inclinometers	Lateral movement at depth along a borehole	Shows the depth and rate of shearing
Settlement plates and survey monuments	Vertical and horizontal movement of crest and slopes	Reveals deformation trends over months and years
Seepage weirs and flow meters	Flow and clarity of water emerging at toe and drains	Cloudy or increasing flow can indicate internal erosion
Pond-level and beach-length gauges	Water level, freeboard and position of the pond	Controls the risk of overtopping and keeps water away from the embankment
Rain gauges and weather stations	Rainfall inputs	Supports the water balance and storm response

Instruments are only as useful as the interpretation of their readings. Trigger levels, set in advance, define when a reading requires a response. A qualified engineer should review the data against the design assumptions, not simply log it.

Satellite and remote monitoring (InSAR)

Interferometric synthetic aperture radar (InSAR) compares repeated satellite radar images. It can detect millimetre-scale ground movement over wide areas. Research on the Brumadinho dam shows both the promise and the limits of the method.

Studies of the Brumadinho collapse

Grebby and colleagues applied a technique called ISBAS to radar data covering the 17 months before the January 2019 collapse. They reported deformation in parts of the dam wall and tailings that was inconsistent with consolidation settlement. The displacement rate also changed suddenly between the last two observations before failure.

Another group reported accelerating displacement shortly before the event. A later spectral analysis of similar time series, by Das, Priyadarshana and Grebby (2024), found a precursory indication of failure risk at least five months earlier.

Limits of the method

These are retrospective studies. Their authors present them as complements to in-situ monitoring, not replacements. Satellite data depend on the surface reflecting radar consistently, and vegetation and water can disturb this. Revisit times also limit how quickly a rapid change is seen. Drones, ground-based radar and laser scanning add further options for sites that can justify them.

Inspections, OMS manuals and independent review

Operator staff carry out routine inspections. They look for:

cracking and bulging;
wet areas and sinkholes;
changes in seepage;
damage to drains or spillways.

An operation, maintenance and surveillance (OMS) manual supports these inspections. It sets out procedures, responsibilities and trigger levels.

In Canada, two bodies describe this approach. The Canadian Dam Association issued a 2014 bulletin applying its dam safety guidelines to mining dams. The Mining Association of Canada issued an OMS guide, first published in 2003, and a tailings guide, first published in 1998. British Columbia's mining code guidance also has dedicated sections on OMS manuals and emergency preparedness.

An independent engineer carries out periodic dam safety reviews. In higher-risk cases an independent review board does so as well. These reviews check the design against actual performance. The Global Industry Standard on Tailings Management, launched in 2020, includes independent review and monitoring requirements. Its wider scope is covered on the page about tailings dam safety.

Dam breach analysis and emergency action plans

A dam breach analysis estimates three things: how much material could be released, how fast it would travel, and which areas would be inundated. For tailings, the flow can behave more like a liquefied mudflow than a water flood, so assumptions about rheology (how the material flows) matter.

The results feed inundation maps and the classification of consequences. They also inform an emergency action plan. Such a plan sets out warning procedures, evacuation routes and communication with downstream communities.

The size and speed of historical flows underline the need for planning. At Stava the flow is reported to have moved at about 90 km/h over roughly 4 km. It left no time for conventional evacuation. At Ajka, residents were evacuated afterwards because a further wall failure was feared, as described in the Ajka case study.

From data to decisions: trigger levels and response

A monitoring programme is effective only if its readings are linked to defined actions. Practice commonly uses graded trigger levels. An alert level calls for closer review. A higher action level calls for immediate engineering response. The levels are set from the design analysis and revised as the dam is raised and as experience accumulates.

Responses range from more frequent readings, through lowering the pond or stopping deposition, to evacuating downstream areas.

Responsibility has to be clear. The surveillance chain typically includes:

site operators, who read instruments and inspect;
a responsible engineer, who interprets the results;
an independent reviewer;
senior management, who are accountable for decisions to act.

When these roles are unclear, warnings can be lost between them. Records matter too. The long history of readings is what allows a change in trend to be recognised, and incomplete records weaken later investigation.

Limits of monitoring

Instruments sample only particular points, so a thin weak layer between them can be missed. Brittle failure in loose, saturated material can occur with little warning in the data. Monitoring also relies on maintenance: damaged, drowned or unread instruments are of no value.

Finally, a good dataset must lead to a decision. The chronology in tailings dam failures includes cases in which warning signs were noted or sealed but not resolved.

Frequently asked questions

What is a piezometer used for at a tailings dam?

It measures pore-water pressure in the dam, foundation or stored tailings. Rising pressure reduces effective strength and can be an early warning of instability.

Could satellite data have predicted the Brumadinho collapse?

Retrospective studies found deformation inconsistent with consolidation in the 17 months before failure. In one analysis, precursory signals appeared at least five months earlier. These are research findings and do not replace on-site monitoring.

What is an OMS manual?

An operation, maintenance and surveillance manual sets out procedures, responsibilities, monitoring requirements and trigger levels for a tailings or water facility.

What does a dam breach analysis provide?

Estimates of the released volume, flow speed and inundated area, which are used for consequence classification and emergency action plans.

Why does independent review matter?

An engineer outside the owner's team checks the design and performance against the data. Several investigations found design or ground-investigation weaknesses that routine inspection did not reveal.