A waste dam (tailings dam) is generally an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from its gangue. Tailings stored in these dams can be liquid, solid, or a slurry of fine particles, and are usually highly toxic and potentially radioactive which are hazardous to environment. The main difference of these dams from any other earth-fill dam is, these are made permanently to store the waste from the ores. The leakage of these dams can be very dangerous for environment, people in the vicinity. The leakage is prevented by using geomembranes, clay cores and checked continuously. However, the construction steps of these dams are very similar to earth-fill dams that are built for other purposes such as hydropower, irrigation and etc.
As mentioned above, as the stored ore wastes can be very hazardous to environment, the stability of a mine waste dam (tailings dam) is considered to be very important in recent years, especially after failures of some of them. A chronological list of failures of mine waste dams can be seen in https://www.wise-uranium.org/mdaf.html (reached on March 30th, 2021). According to this list, the rate of tailings dam failures is increasing: about 65% of the total failures in the last 60 years occurred between 1990 to 2021.The failures are mainly due to heavy rain (Rico et al., 2008; Azam and Li, 2010), poor drainage conditions, poor detailing in the construction phases, overloading the dam with excessive amount of waste (higher than the designed level), static and seismic liquefaction (Ormann et al., 2013) and etc. The dam failures not only cause casualties but also the downstream side of the dam is covered with waste mud, which is toxic and environmentally hazardous. These failures are a big threat in the cities, forests, lakes and etc. in the vicinity of the dam (Rico et al., 2008).
Azam and Li (2010) has reviewed the tailing dam failures for the last 100 years. According to them, the most important cause of failure is unusual weather. Foundation failures were common in the past, but in recent years failures due to foundation have been reduced. Zandarin et al. (2009) has performed a detailed numerical analysis on a tailing dam from a nickel industry in Cuba and concluded that, the guidelines and codes in practice are valid and the pore pressures are very much important in stability of tailings, and must be operated carefully.
In addition to the causes of failures above, the topography of the site is also important in the stability of the dams. The slopes in the upstream and downstream sides of the dam can create various problems for the dam safety. However, most of the studies in literature (Ozcan et al., 2013; Cho and Song, 2014; Das and Hedge, 2020; Jin et al. 2020) deal with the stability of the dam body instead of the side slopes in the downstream and upstream. The main problem for the case in this study was the instability in the right-slope at the downstream side of the main dam body. A slope failure has occurred on the side slopes and there was a probability of this failure to affect the main dam body. For this reason, the purpose of this study is to assess the effects of instability in the downstream right - slopes of a tailing dam in the main dam body. The 3D numerical analyses were performed in that unstable area to understand if those instabilities affect the dam body or not. These 3D numerical analyses were performed using Midas GTX NX 3D. The whole area was covered in the analyses including the dam body, upstream and downstream sides so that the effects of the instabilities can be estimated accurately.
The deformations in the dam body are calculated as a result of these numerical analysis. According to the FEMA2005 (Federal Guidelines for Dam Safety Earthquake Analysis and Design of Dams), the expected performance of the dam is judged according to the severity of the deformation, such as loss of freeboard, potential of cracking leading to failure of the embankment or foundation. Similarly, the performance of the dam following a movement can be measured by considering the (i) the use of the reservoir; and (ii) the ability or lack thereof to quickly repair a damaged structure. It suggests a limiting deformation of 2 ft (~ 0.6m). Similarly, Cetin (2014) has summarized the allowable deformations of a dam body from various standards and says that 0 to 1.5m permanent deformation is acceptable if the settlement of the dam is less than one tenth of the dam height according to Hawaiian Dam Safety Guide and Division of Safety of Dams (DSOD) California, however, according to FHWA-SA-97-076, the acceptable deformation is 0.3m.
According to U.S. Department of the Interior Bureau of Reclamation Design Standards No.13 Chap. 9 (Static Deformation Analyses), the magnitudes of horizontal deformations (into and down valley) are relatively small compared to the vertical settlement and this depends on the geometry, material and dam zone properties. For the dams that are built properly, the settlements at the crest after the construction are generally range between 0.2 and 0.4 percent and rarely exceed 0.5 percent of the embankment height. Keeping this in mind, the “1 percent rule” is used to make a conservative design. According to Chap. 13 (Seismic Analyses and Design), a predicted deformation of less than 1 foot (~ 0.3m) would not be a threat to the dam (unless a critical part of the dam is damaged). Similarly, it is also mentioned that, for any embankment dam, estimated deformations exceeding 3 feet would raise concern about cracking and loss of freeboard.