1. TAILINGS AND MINE WASTE How to Avoid Tailings Failures October 2016
Jack Caldwell, Robertson Geoconsultants
D. Chad LePoudre, SNC-Lavalin

2. Overview
A Proposed Rule
Examples of Rule Violation
Our Personal Advice

3. Human Habitation
Rule: No tailings facilities upstream of human habitation or sensitive environments.
Violations re Human Habitation:
Stava
Merrispruit
Samarco
Violations re Sensitive Environments:
Mount Polley

5. No Slimes Near the Perimeter
Rule:
No slimes near the perimeter or in the zone of potential slope failure.
Violations:
Bafokeng – Piping through sand between slimes layers
Samarco
Promoted horizontal deformation leading to loose sands
Impede free drainage leading to saturated sands?

6. Bafokeng – at Failure
There are many theories about why the slope failed. Some ascribe failure to the bulldozer sent to raise the perimeter berm. Some say the pool cause increased seepage that induced piping of the sands between the clay layers that occurred in the outer perimeter slopes. Some believe the penstock that exited the dam at the point of failure cavitated and undermined the toe of the slope. We will never know, but the dam failed.

8. No Cascading Dams Rule: No cascading tailings dams. Violations:
Stava: one failed causing the other to fail
Merrispruit: flow from upper to lower exceeded storage capacity

11. No Clay Foundations Rule: Limit or avoid clay in the foundations.
Violations:
Bafokeng and other Rustenburg Dams: foundation failure when reached 30-m high
Mount Polley:
Failure to locate clay layer
Complex clay behavior not understood

12. MOUNT POLLEY Size of The Breach Polley Lake 370 m 40 m ?
Fonte: CBCNEWS, 2014.

14. No Water on Top Deck Rule: No or very small pool; or
Pool distance from crest = 5 x embankment height
Violations:
Bafokeng: pool at crest
Mount Polley:
No freeboard exacerbated failure consequences

15. An aerial view of the new tailings facility as it looked in the late 2005s

16. Low Slope Embankments Rule: Tailings embankments slopes of 5H:1V
Rockfill embankments as rockfill permits
Non-Violations:
UMTRA: all piles reconfigured to 5:1
Cannon Mine: good-quality, compacted rockfill at 1.73:1

17. Exhibit 33
The Weldon Spring Cell near St Charles, Missouri. The sideslope is a clean fill dike so that if gullies develop as observed at the Chahokia Mounds, there is no exposure of the encapsulated radioactive waste. Over the dike is an erosion resistant rock, which is also used on the top deck cover.

18. No Penstocks Rule: No penstocks through tailings or adjacent abutments
Violations:
Bafokeng: cavitation in penstock suspected as main or contributory cause
Padcal: Pipe break lead to large tailings loss

21. Risk Assessment FACTS: Mining Association of Canada Guidelines:
People make mistakes
Human systems fail to function
Mining Association of Canada Guidelines:
Apply Risk Management
Mine Manager responsible by law

22. Tolerable Risks Rule: If dam break consequences tolerable, proceed.
Other wise change design or do not construct

23. Lots of People Competent Responsible Engineer Intelligent ITRB
Engaged Staff
Committed Management
Educated Regulators
Informed Stakeholders

24. Tailings Stewardship
Identify potential design or operational issues or concerns;
Manage liabilities (reduce risk);
Identify opportunities to improve operational efficiencies, and reduce costs;
Provide input into design, construction, and operations throughout the life of the facility (including closure and post-closure);
Provide operators contemporary, “state of the practice” training;
Improve data management;
Provide a standardized review process; and
Prepare for upset conditions (emergency preparedness).

26. Higher Factor of Safety
Rule:
More than 1.5
Greater depending on:
Site complexity
Structure complexity
Site knowledge
Past performance of similar facilities

27. Cassagrande (1965)
Arthur Cassagrande (1965) The Terzaghi Lecture: Role of the calculated risk in earthwork and foundation engineering.
An embankment is to be built on a clay foundation.
From his investigations the designer concludes that the in situ shear strength of the clay may range between 1 and 2 tons per sq. ft.
If this project is an important dams whose failure would cause catastrophic losses, he might decide to use the very conservative design value of 0.6 tons per sq. ft.
Thus, he would protect himself against the wide range of uncertainty by an ample margin of safety.
i.e. with a factor of safety ranging between the limits of about 1.6 and 3.3.

28. Vick (2002)
Vick, S.G. (2002) Subjective Probability and Engineering Judgment.
There was once a day, now lost in the fog of time,
When some engineers got together—nobody really knows who—
And decided what the minimum factor of safety should be for various structures and analysis methods.
These minimum factors of safety are now embedded in codes and standards of practice.
The factors of safety in codes account for uncertainty and failure consequence , albeit in an unstructured way.

29. What Others are Doing
Many structural codes and many geotechnical codes dealing with foundations, retaining walls, and slope stability, have moved away from the use of a simple, one-size-fits-all factor of safety.
The most recent is the 2014 Canadian Highway Bridge Design Code. Their new equation is this:
φgR > or = ΣIiήiάiFi
Where
φg is a geotechnical resistance factor,
R is the characteristic geotechnical resistance (based on characteristic ground parameters),
Ii is a structural importance factor,
ήi is a load combination factor,
άi is the load factor, and
Fi is the characteristic load effect

30. Our personal preferences
Contentious and Debatable
Our personal preferences

31. From the BC MEM Mt Polley Report
The mine manager is responsible for the safety of the TSF
The mine manager gets input from
mine engineers
technical folk
consulting engineers.
But professional folk make mistakes
MAC guidelines say the mine manager must use risk management to overcome professional shortcomings
Hence in future the mine manager must include risk management as an integral part of tailings facility management.

32. A Lot Better Than No water on the tailings or in the voids of tailings
Competent Engineer(s) of Record
Independent Technical Review Board
Best Available Technology, Best Management Practice and best of everything else
Educated Regulators

33. Break these Rules, IF More conferences More papers More courses
More education
More wisdom