Safeguarding Embankment Dams
August 26, 2019 |
It has been just over 5 years since the Mount Polley tailings dam failure in British Columbia on August 5, 2014. However, variability in design and construction continues to challenge industry's efforts in managing tailings dam safety, as demonstrated by the two subsequent tailings dam failures (2015 and 2019) in Brazil. In this blog post, we will discuss what embankment dams are, what factors affect their safety, common failure causes of tailings dams, and what can be done to help safeguard them.
What are Embankment Dams?
Embankment dams are a versatile type of dam that can be built on various soil and rock foundations. An embankment dam is an earth and/or rockfill retaining structure, including concrete-faced rockfill structures and tailings dams.
Unlike concrete dams, embankment dams can be built on foundation other than bedrock. However, due to diverse geological origins of the foundation, it is important to anticipate and correctly identify potential problems associated with this type of dam, including its foundation.
Tailings dams are a special type of embankment dam. While embankment dams are generally used to store water for water-supply, irrigation and hydropower, tailings dams are used to store mine tailings, fly ash or other industrial waste. The storage of mine tailings introduces a potential environmental degradation issue related to acid-generation and/or metal leaching geochemical reactions. McLeod and Murray (2003) discussed some of the differences relative to the design of water-retention and tailings dams.
Natural Hazards Affecting Dam Safety
The main natural hazards that affect the safety of an embankment dam are floods and earthquakes. These hydrological and seismic hazards must be carefully evaluated and designed against. Appropriate design criteria must be established which are compatible with the project site setting and severity of consequences of dam failure to the downstream communities and environment. Other hazards, such as fire (for coal waste), landslide, debris flow and avalanche, may also need to be assessed during site selection and facility design, operation and maintenance.
Human Hazards and Factors Affecting Dam Safety
The design, construction and maintenance of a safe and properly functioning embankment dam requires an effectively implemented management system. The following human hazards and factors can undermine the safety of an embankment dam:
- Complacency: not recognizing and adequately addressing critical issues could lead to the deterioration of weak links in the dam system. Vigilance and critical review by experts may counter this.
- Constraints: internal and/or external constraints that could result in gradual reduction of margin of safety, loss of safety control and sudden failure. Such constraints include insufficient resources in time, funds and expertise.
- Ineffective change management: changes are inevitable over the life of an embankment dam, including changes in ownership, organization and personnel. Proper documentation, overlap of key personnel during transition periods and effective audits can enhance the effectiveness of change management.
- Ineffective quality management: built-in defence mechanisms are required to catch “errors” before serious incidents occur. Some of these defence mechanisms include a quality management system, peer review, risk assessment and management, and study of failure case histories. Errors and omissions can occur during design and operation/construction stages.
- Sabotage and terrorist activity: this includes all deliberate activities to damage or destroy a dam facility. Proactive security surveillance and vigilant control of site activities commensurate with prevalent site conditions are needed to guard against this threat.
Common Causes of Tailings Dam Failures
Embankment dams are vulnerable to the overtopping mode of failure. Once overtopped, an embankment dam is most often down cut by the erosion of released water, resulting in a dam breach and downstream flooding in rapid succession.
In 1994, the United States Committee on Large Dams (USCOLD) surveyed 185 cases of tailings dam failures and accidents. A dam failure is a breach of the dam structure resulting in the release of tailings; a dam accident refers to an event that did not result in the release of tailings. Klohn (1995) made the following observations of the USCOLD survey:
- The most common causes of failures/accidents are slope instability, earthquake and dam overtopping, while issues related to foundation, seepage and structural components are other important causes.
- Most failures and accidents (91% of failures and 82% of accidents) occur while tailings impoundments are in active use as opposed to being inactive. For inactive impoundments, adequate surveillance and maintenance of the facility should be rigorously carried out, and the water in the tailings pond should be reduced significantly.
- The upstream tailings dam is the most vulnerable type (45% of failures and 41% of accidents). The next vulnerable type is the water-retention dam (15% of failures and 14% of accidents). The likelihood of a tailings dam incident is relatively higher when water is retained within the impoundment (including inactive impoundments), especially if water is in direct contact with the dam without a significant tailings beach in between.
Similar survey results from the International Commission on Large Dams (ICOLD) review of 221 tailings dam incidents in 2001 corroborate the findings of USCOLD’s 1995 survey.
Besides running a normal and cost-effective dam operation, it is important for the operator to anticipate which component of the dam system could go wrong, what could be the potential consequences, and how to respond and minimize the consequences. Thus, it is critical to have a well-thought-out emergency response plan, detailing concrete actions to be taken by personnel at all levels, and having well-defined internal and external emergency communication protocols. Emergency response plans should also be continuously tested and improved.
Preventing Future Incidents
Two recent developments occurred after the 2019 Brumadinho upstream tailings dam failure in Brazil:
- The state of Minas Gerais, Brazil banned upstream tailings dams, following the example of Chile which banned such dams in 1970.
- The International Council of Mining and Metallurgy (ICMM) indicated that it would create an independent panel of experts in charge of developing a global standard for tailings facilities to be followed by its members.
All stakeholders, especially those who could be directly affected by a dam failure, should be aware of ongoing monitoring of dam performance, and be assured that design, operations and regulatory best practices are in place to reduce the risk to as low as possible. A higher level of governance, technical standards and regulatory consistency are required for protection of the public. Dam safety needs to have a much higher priority at all times so that it is prominent in the minds of all stakeholders over the long-term, even when other short-term urgent matters arise.
Klohn, E.J. 1995. Tailings Dams and Their Evolution to Major Hydraulic Structures and the Importance of Their Safety, Keynote Address to Canadian Dam Safety Conference, Banff, Alberta.
Lo, R.C. 2019. “Safeguard Embankment Dam Safety,” in Proceedings of the 87th Annual Meeting of International Commission on Large Dams (ICOLD), 9-14 June 2019. Ottawa, ON: Canadian Dam Association.
McLeod, H. and L. Murray. 2003. “Tailings Dam versus A Water Dam – What is the Design Difference?” in Proceedings of the 71st Annual Meeting of International Commission on Large Dams (ICOLD), 15 June 2003. Montreal, QC: Canadian Dam Association.
USCOLD 1994. Tailings Dam Incidents. United States Committee on Large Dams, 82 pps.