For the past few years, KCB has collaborated on several geomembrane research projects with Prof. R. Kerry Rowe and his team at the Geo-Engineering Centre at Queen’s University in Kingston, Ontario. The research is assessing the effectiveness of geomembrane liners in tailings storage facilities (TSFs). In a recent blog post, we gave an overview on geomembranes and touched on KCB-Queen’s collaborative research projects on the use of geomembranes at TSFs. In this blog we are going to discuss some of the recent KCB-Queen’s research findings.
Geomembranes have been used in various geotechnical/geoenvironmental engineering projects since the 1960s, and in the 1980s, geomembranes became popular in mining applications. Research on geomembranes since then has shown that their effectiveness is directly linked to the number of defects (holes) they have. See Figure 1 below for an example of geomembrane use at a TSF.
Key aspects of the geomembrane research at Queen’s include quantifying leakage through geomembrane holes in a typical TSF configuration, and assessing the potential for piping of tailings through holes in geomembranes installed on a coarse-grained foundation (e.g. gravel). In this context, piping is a phenomenon where the transport of tailings occurs through geomembrane holes caused by seepage forces – see Figure 2 below.
Dr. Prabeen Joshi, a Geotechnical/Geoenvironmental Engineer at KCB, was one of the researchers on this project. His research involved placing thickened tailings over a geomembrane with holes less than 1 to 20 mm in diameter, with a soil underliner of varying permeabilities. He performed several tests to measure the impact of the following factors on leakage rates:
- geomembrane type (linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE))
- size of holes
- wrinkles (or folds)
- effective stress
- tailings and underliner permeability
- potential piping conditions
The most important takeaway from his work is that for the same defect size, leakage from geomembranes at TSFs is much smaller than leakage from geomembranes at landfills and heap leach facilities (Joshi and McLeod, 2018). The leakage at TSFs is reduced due to the presence of the less permeable tailings placed directly over a geomembrane with defects. Also, the geomembrane type and presence of wrinkles had no observable effect on long-term leakage rates (Joshi et. al., 2017).
During his research, Dr. Joshi noted that piping is induced when a coarse foundation is present below a geomembrane (with holes) and the tailings are subjected to very high pore pressure. He demonstrated that failure due to piping can be prevented by installing a geotextile with an appropriate mass/unit area adjacent (either on top or below) to the geomembrane (Rowe et. al., 2016). KCB and Queen’s decided to further investigate the piping phenomenon. The research work was carried forward by Alan Chou, one of Dr. Joshi’s research colleagues at Queen’s.
The study found that piping was more likely to occur under low effective stress conditions, such as when a thin layer of tailings lies below a few metres of water (e.g., during the development of a reclaim pond) (Chou, 2017; Chou et al., 2018). While piping appeared to stop at higher stresses, the flow rate through the defect was up to 2-3 times higher than predicted using traditional leakage equations, if the tailings initially experienced some erosion at lower stress.
This research shows that geomembrane liners are very effective at reducing seepage in TSFs, provided they are placed with the correct underliner that meets filter compatibility with the tailings. It has advanced the state of practice for geomembrane design at TSFs and allowed KCB engineers to make effective use of geomembranes at the facilities we design. Several KCB staff members helped develop the research topic and gave feedback during the project. In addition, KCB’s geotechnical laboratory was involved in the collection and preparation of tailings for testing at Queen’s.
Alan Chou wrote the following in his master’s thesis:
“Research into landfill applications has shown that defects govern leakage through geomembranes. Because the presence of some defects is common even in well-designed and constructed facilities, an allowable number of defects is often factored into the design (e.g., Giroud and Bonaparte 1989; Giroud, 1997; Rowe et al., 2004; Giroud, 2016; and many more). The leakage resulting from these defects in landfill facilities can then be estimated using equations presented in Rowe (1998; 2012). However, the presence of tailings results in a different configuration than landfill applications, making the equation not applicable.” (Chou 2017)
As there are differences in the configuration of TSFs and landfills, the design fundamentals also need to be different. Therefore, there needs to be more discussion among tailings design engineers about moving away from the current approach of using knowledge from landfill research when designing TSFs.
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Chou, Y. C. (2017). Leakage Phenomena of Tailings Overlying a 1 cm Diameter Circular Geomembrane defect (master’s thesis, Queen’s University). Accessed 08 27, 2018. https://qspace.library.queensu.ca/handle/1974/22769
Joshi, P. (2016). Hydraulic Performance of Geosynthetic Liners in Landfills and Tailings Storage Facilities (Doctoral dissertation, Queen’s University). Accessed 08 27, 2018. https://qspace.library.queensu.ca/bitstream/handle/1974/14352/Joshi_Prabeen_201604_PhD.pdf
Joshi, P. and H. McLeod. 2018. “Effectiveness of Geomembrane Liners in Minimizing Seepage in Tailings Storage Facilities: New Knowledge,” in Proceedings of the 26th International Congress on Large Dams, July 4-6, 2018. Vienna, Austria: International Commission on Large Dams.
Joshi, P., R.K. Rowe and R.W.I. Brachman. 2016. “Physical and Hydraulic Response of Geomembrane Wrinkles Underlying Saturated Fine Tailings”. Geosynthetics International 08/2016; 24(1):1-13., DOI:10.1680/jgein.16.00017.
Rowe, R.K., P. Joshi, R.W.I. Brachman and H. McLeod. 2016. “Leakage through Holes in Geomembranes below Saturated Tailings”. Journal of Geotechnical and Geoenvironmental Engineering. 143(2), DOI: 10.1061/(ASCE)GT.1943-5606.0001606.
Yung-Chin (Alan) Chou, R. Kerry Rowe, Richard W.I. Brachman. 2018. “Erosion of Silty Sand Tailings Through a Geomembrane Defect Under Filter Incompatible Conditions”. Canadian Geotechnical Journal. Published on the web 21 February 2018. https://doi.org/10.1139/cgj-2017-0602.