A tailings dam is typically an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from the gangue. Tailings can be liquid, solid, or a slurry of fine particles, and are usually highly toxic and potentially radioactive. Solid tailings are often used as part of the structure itself.
Tailings dams rank among the largest engineered structures on earth. The Syncrude Mildred Lake Tailings Dyke in Alberta, Canada, is an embankment dam about 18 kilometres (11 mi) long and from 40 to 88 metres (131 to 289 ft) high. It is the largest dam structure on earth by volume, and as of 2001 it was believed to be the largest earth structure in the world by volume of fill.
There are key differences between tailings dams and the more familiar hydroelectric dams. Tailings dams are designed for permanent containment, meant to "remain there forever". Copper, gold, uranium and other mining operations produce varied kinds of waste, much of it toxic, which pose varied challenges for long-term containment.
An estimated 3,500 active tailings impoundments stand around the world, although there is no complete inventory, and the total number is disputed. As of 2000 these structures experience known "major" failures of about 2 to 5 annually, along with 35 "minor" failures. Assuming the 3,500 figure is correct, this failure rate is "more than two orders of magnitude higher than the failure rate of conventional water retention dams".
Unlike water retention dams, a tailings dam is raised in succession throughout the life of the particular mine. Typically, a base or starter dam is constructed, and as it fills with a mixture of tailings and water, it is raised. Material used to raise the dam can include the tailings (depending on their size) along with dirt.
There are three raised tailings dam designs, the upstream, downstream and centerline, named according to the movement of the crest during raising. The specific design used is dependent upon topography, geology, climate, the type of tailings, and cost. An upstream tailings dam consists of trapezoidal embankments being constructed on top but toe to crest of another, moving the crest further upstream. This creates a relatively flat downstream side and a jagged upstream side which is supported by tailings slurry in the impoundment. The downstream design refers to the successive raising of the embankment that positions the fill and crest further downstream. A centerlined dam has sequential embankment dams constructed directly on top of another while fill is placed on the downstream side for support and slurry supports the upstream side.
List of largest tailings dams
|Rank||Name||Country||Year completed||Structure height [m]||Structure volume [106 m3]||Reservoir volume [109 m3]||Installed capacity [MW]||Type|
|1||Syncrude Tailings Dam Mildred MLSB||Canada||1995||88||540/720||0.35||NA||TE|
|2||Syncrude Tailings Dam Mildred SWSS||Canada||2010||40-50||119||0.25||NA||TE|
|3||ASARCO Mission Mine Tailings Dam||United States||1973||30||40.1||0 ||NA||ER|
Type: TE - Earth; ER - Rock-fill; PG - Concrete gravity; CFRD - Concrete face rock fill
The standard of public reporting on tailings dam incidents is poor. A large number remain completely unreported, or lack basic facts when reported. There is no comprehensive database for historic failures. According to mining engineer David M Chambers of the Center for Science in Public Participation, 10,000 years is "a conservative estimate" of how long most tailings dams will need to maintain structural integrity. 
The lack of any comprehensive tailings dam database has prevented meaningful analysis, either gross comparisons (such as country to country comparisons, or tailings dam failures versus hydro dam failure rates) or technical failure analysis to help prevent future incidents. The records are very incomplete on crucial data elements: design height of dam, design footprint, construction type (upstream, downstream, center line), age, design life, construction status, ownership status, capacity, release volume, runout, etc.
An interdisciplinary research report from 2015 recompiled the official global record on tailings dam failures and major incidents and offered a framework for examining the severity and consequence of major incidents. That report shows a correlation between failure rates and the pace of copper ore production, and also establishes a relationship between the pursuit of lower grades of ore, which produces larger volumes of waste, and increasingly severe incidents. For this reason, several programs to make tailing dams more sustainable have been set in motion in countries like Chile, where there are more than 740 spread across the country.
The mining and processing byproducts collected in tailings dams are not part of the aerobic ecological systems, and are unstable. They may damage the environment by releasing toxic metals (arsenic and mercury among others), by acid drainage (usually by microbial action on sulfide ores), or by damaging aquatic wildlife that rely on clear water.
Tailings dam failures involving significant ecological damage include:
- the Brumadinho dam disaster, Brazil, January 25, 2019, where as many as 252 people are unaccounted for, and at least 134 are dead. The disaster released 12 million cubic meters of iron waste leading to the Paraopeba River.
- the Bento Rodrigues dam disaster, Brazil, November 5, 2015, considered the worst environmental disaster in Brazil's history, killed 19 people when an iron ore containment dam failed and released 60 million cubic meters of iron waste.
- the Mount Polley mine disaster, British Columbia, August 4, 2014, which released 10 million cubic metres of water and 4.5 million cubic metres of metals-laden tailings.
- the Ok Tedi environmental disaster in New Guinea, which destroyed the fishery of the Ok Tedi River, continuously from 1984 through 2013
- the Sotkamo metals mine, Finland, 4 November 2012, released "hundreds of thousands of cubic metres" of waste water which raised concentrations of uranium, nickel, and zinc in nearby Snow River, each to at least 10 times the harmful level.
- the Ajka alumina plant accident, Hungary, October 4, 2010, which released one million cubic metres of red mud, a waste product of aluminum refining, flooding the village of Kolontár and killing the Marcal River.
- the Baia Mare cyanide spill, Romania, January 30, 2000, called the worst environmental disaster in Europe since the Chernobyl disaster
- The Doñana disaster, southern Spain, 25 April 1998, which released 4-5 million cubic metres of acidic tailings containing heavy metals.
- the Church Rock uranium mill spill in New Mexico, July 16, 1979, the largest release of radioactive waste in U.S. history 
- three uranium tailings dams near the town of Ak-Tüz, present-day Kyrgyzstan, collapsed in a December 1964 earthquake, releasing 60% of their radioactive volume (600,000 cubic metres (21,000,000 cu ft)) into the Kichi-Kemin River and its agricultural valley
- an incident on April 7, 1961 released 700,000 cubic metres (25,000,000 cu ft) of uranium mine tailings from operations of the Soviet-era Wismut organization into the Zwickauer Mulde River in the village of Oberrothenbach
- the Mailuu-Suu tailings dam failure also in Soviet-era Kyrgyzstan on April 16, 1958 caused the uncontrolled release of 600,000 cubic metres (21,000,000 cu ft) of the radioactive uranium-mine tailings in to spill downstream into a portion of the densely populated Ferghana Valley
Tailings ponds can also be a source of acid drainage, leading to the need for permanent monitoring and treatment of water passing through the tailings dam. For instance in 1994 the operators of the Olympic Dam mine, Western Mining Corporation, admitted that their uranium tailings containment had released of up to 5 million m3 of contaminated water into the subsoil. The cost of mine cleanup has typically been 10 times that of mining industry estimates when acid drainage was involved.
The following table of the deadliest known tailings dam failures is not comprehensive, and the casualty figures are estimates.
|1962 Huogudu(火谷都), China tailing pond failure||September 26, 1962||Huogudu (火谷都), Gejiu， Yunan Province, China||171||Few details available. A tailings pond at a tin mine operated by Yunnan Tin Group collapsed. 368M m3 surged. One source reports 171 killed and another 92 injured; another has the date as September 26.|
|Mina Plakalnitsa||May 01, 1966||Vratsa, Bulgaria||480+||A tailings dam at Plakalnitsa copper mine near the city of Vratsa failed. A total 450,000 cu m of mud and water inundated Vratsa and the nearby village of Zgorigrad, which suffered widespread damage. The official death toll is 107, but the unofficial estimate was more than 480.|
|Certej dam failure||October 30, 1971||Certej Mine, Romania||89||A tailings dam built too tall collapsed, flooding Certeju de Sus with toxic tailings.|
|Buffalo Creek Flood||February 26, 1972||West Virginia, United States||125||Unstable loose constructed dam created by local coal mining company, collapsed in heavy rain. 1,121 injured, 507 houses destroyed, over 4,000 left homeless.|
|Val di Stava dam||July 18, 1985||Tesero, Italy||268||Poor maintenance and low margin for error in design; outlet pipes failed, leading to pressure on dam and sudden collapse. Ten people were ultimately convicted of manslaughter and other charges.|
|Mufulira||1970||Zambia||89||A tailings reservoir breached and collapsed into the copper mine below it, killing 89 night-shift workers.|
|Aberfan disaster||October 21, 1966||Wales||144||The collapse and landslide of a spoil tip accumulated above the mining town on geologically unstable ground killed 28 adults and 116 children (not an engineered structure)|
|Hpakant jade mine disaster||October 25, 2015||Myanmar||113||A slag heap reportedly used by multiple operators in this jade-mining region became unstable and flooded into nearby residences (not an engineered structure) |
|El Cobre landslide||March 28, 1965||Chile||300||Shaking from a magnitude 7.1 earthquake caused failure of two tailings dams at the El Soldado copper mine. The resulting flow destroyed the town of El Cobre.|
|Merriespruit Tailings Dam Failure||February 22, 1994||Virginia, Free State, South Africa||17||On the 22nd February 1994 the Merriespruit tailings dam failed by overtopping as a consequence of heavy rains causing a flowslide (static liquefaction) of part of the embankment (Davies 2002). Water mismanagement was to blame that caused 600,000 m3 of tailings (1.2 Million tonnes) to mobilize out of the impoundment where the flow eventually stopped 2 km away in the town of Merriespruit (Penman 1998; Davies 2001). 17 people were killed and scores of houses were demolished (Fourie 2003). Figure 1 shows the extent of the damage to the town and the scale of the breached embankment.|
|Taoshi landslide||September 08, 2008||Linfen, Shanxi Province, China||254+||Iron mine tailings, formerly administered by the state and then put into private hands, collapsed into a village at 8 a.m.|
|Bento Rodrigues dam disaster||November 05, 2015||Mariana, Minas Gerais, Brazil||19||A tailings dam at an iron ore mine jointly owned by Vale S.A. and BHP and suffered a catastrophic failure releasing around 60 million cubic meters of iron waste into the Doce River which reached the Atlantic Ocean.|
|Brumadinho dam disaster||January 25, 2019||Brumadinho, Minas Gerais, Brazil||186+||A tailings dam at an iron ore mine operated by Vale S.A. suffered a catastrophic failure.|
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|last=(help)CS1 maint: Extra text: authors list (link)
- Talk:List of largest dams in the world#Phantom Dams
- Talk:List of largest dams in the world#Structure Volume
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- Estimate based on height, dimensions from Google Earth and, where available, cross section. Accuracy ±15%
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- Estimate based on structure volume and dimensions from Google Earth
- Zero reservoir size because full of tailings
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