Applying a Generalized FMEA Framework to an Oil Sands Tailings Dam Closure Plan in Alberta, Canada

Abstract

Historically, tailings facilities have been designed primarily with consideration of the mine’s active life. This is problematic, as the lifespan of a tailings dam may far exceed the life of the mine. Over time, it is expected that these structures will transform into a mine waste structure and then eventually a landform. In Alberta, Canada, dam owners can submit a decommissioning, closure, and abandonment (DCA) plan and completion reports to apply for the facility to be de-registered as a dam. If successful, the structure would be considered a solid waste structure and may be reclassified and regulated as a mine waste dump. The Alberta Energy Regulator expects DCAs to be accompanied and supported by risk assessments that consider long-term physical failure modes, including failure modes that may not be applicable during operations, in accordance with Manual 019. To help support the process of de-registering a tailings dam, a risk management tool, referred to as a Generalized Failure Modes Effects (G-FMEA) framework, was developed and presented in the Journal of Minerals in the Special Issue Tailings Dams: Design, Characterization, Monitoring, and Risk Assessment. The G-FMEA was designed to be used for assessing risks of an external tailings facility in closure, with the goal of assessing the long-term risk of geotechnical failure to support the process of de-registration. In Alberta, a number of tailings dams are undergoing closure and reclamation activities. This paper applies the developed G-FMEA framework to an oil sands tailings dam in Alberta to demonstrate the application of the framework. The paper assesses two specific failure modes of two different elements, including clogging of the drains and surface erosion of the berm. The failure modes are assessed over different timescales to demonstrate how the consequence, likelihood, and risk rating may change over time. The results of this process are discussed in the context of the potential for the facility to be de-registered as a dam.

1. Introduction

Many tailings storage facilities (TSFs) in Alberta are in the process of being reclaimed and closed. The goal following closure is for the dam owner to submit a decommissioning, closure, and abandonment (DCA) plan and completion reports to apply for the facility to undergo de-registration as a dam [1]. Following this process, the structure would be reclassified and regulated as a mine waste dump under the Oil Sands Conservation Act or the Coal Conservation Act [1]. The Alberta Energy Regulator (AER), who is responsible for the regulation of energy dams, developed Manual 019: Decommissioning, Closure, and Abandonment of Dams at Energy Projects (Manual 019) to accompany the Alberta Dam and Canal Safety Directive (the Directive). Manual 019 specifies that this risk assessment must consider long-term physical failure modes, including failure modes that may not be applicable during operations [1]. It is important to note that Manual 019 indicates that “risk cannot be reduced to zero for oil sands tailings facilities, or any manmade structure for that matter” and that the ideal lowest consequence category would be comparable to natural analogues with an extremely low likelihood [1]. It is expected that active management should continue until the regulator accepts the completion report, at which point the operator is no longer required to actively manage and report on the structure as a dam [1]. In some cases, a tailings dam may require long-term active management if the residual risk is deemed to be too high [1].

To help support the process of de-registering a tailings dam as a dam in Alberta, a risk management tool (the Generalized Failure Modes Effects (G-FMEA) framework) was developed. The tool is summarized by Schafer et al. [2] in the Journal of Minerals within this Special Issue. Deregistration, defined here, is when the regulating governmental body assesses a dam structure to evaluate if it can be de-classified as a dam, so that it is no longer regulated as a dam. The term de-register is not a regulatory or legal term and has been commonly used in industry to describe this process. Throughout the literature, the terms de-license (i.e., Oil Sands Tailings Dam Committee (OSTDC) [3]) and deregulate (i.e., Oberle et al. [4]) have also been used to describe this process.

There are three different configurations of TSFs that may exist at an oil sands mine as defined by AER [1]:

“(1) External tailings facilities where perimeter containment is formed by perimeter dams and rising topography in some cases. The tailings are deposited entirely on natural topography above ground.

(2) Tailings stored in pit but a full height dam separates the tailings from the receiving environment.

(3) Tailings stored in pit but capacity of the pit container is increased by constructing a perimeter dam along a portion or the entire pit crest.”

The goal of the G-FMEA is to assess the long-term geotechnical risks of an external tailings facility (ETF) during the closure phase to support the process of de-registration [2]. However, the risk management tool should be able to be adapted for other configurations to account for additional risks that exist for those scenarios if needed. The G-FMEA should be used at the conceptual design stage, if possible, to assess the closure plan and evaluate the potential for de-registration [2]. It should be updated throughout the project life and can also be used for dams that have already been constructed or are orphaned [2]. This paper demonstrates how the G-FMEA can be used and applied in practice for a case study oil sands external tailings facility in Alberta, Canada. The G-FMEA charts from Schafer et al. [2] are used to evaluate the potential failure modes of an oil sands ETF closure plan using an element approach. This process demonstrates how the framework can be used to go from a generalized list of failure modes and applied directly to a case study. This process requires an evaluation of serviceability failure of each element. The paper then shows how to apply the risk matrix framework to two select failure modes (clogging of drains and surface erosion of the berm) and how the risk of these failure modes changes over time. The site-specific G-FMEA is used to show how the framework can be used to support the closure process at different points of time (conceptual design or de-registration). Overall, the paper serves to demonstrate how risk management can be integrated into the process of closure design to support de-registration. Ultimately, this supports long-term closure goals with a risk management focus.

2. Background

The G-FMEA serves as a screening tool for the closure phase of an ETF, where hazards that are assessed as acceptable will not require additional analyses, but hazards that are assessed to have a higher risk rating (or where there are a number of low-risk hazards) could require more detailed and/or quantitative risk assessments. This may include reliability analyses for critical risks. However, challenges with quantifying uncertainties in climatic conditions in the long-term would likely render uncertainty (particularly epistemic uncertainty) difficult to manage within reliability analyses.

The objective of the G-FMEA is: “How should geotechnical risks associated with an external tailings facility in Alberta be managed in the long-term to achieve an acceptable closure plan, such that the facility is able to be de-registered as a dam?” [2]. The G-FMEA includes charts (provided in Schafer et al. [2]) for the drainage system, foundation, dam body, and landform that provide failure modes for different elements present in each system. These charts form the backbone of the G-FMEA framework. Applying the G-FMEA framework to a site involves first defining the system, developing a block diagram, and breaking the system down into key elements. Each element should have the function defined, as well as what constitutes serviceability failure. Once this step has been completed, each element can be screened for applicable failure modes using the G-FMEA charts. A risk assessment is then conducted for every applicable failure mode, which involves determining the likelihood and consequence rating for each temporal scale and determining the risk rating. Appropriate risk mitigation options should be assessed and applied as appropriate.

The G-FMEA should be conducted for different assessment periods, including the immediate term, short term, medium term, and long term [2]. The goal with this is to account for time dependence, depreciation of system elements, and changing risk profiles, which are common critiques of failure modes effects analysis [2].

Schafer et al. [2] presented a likelihood rating table (

Table 1. Likelihood rating (Reprinted from Schafer et al. [2]).

Likelihood Rating	Qualitative Interpretation Guidance 1	Quantitative Interpretation Guidance	Annualized Probability of Occurrence
Almost certain	Almost certain that an incident will occur given the circumstances. Very high probability of one or more occurrences per year.	Higher than 10% probability in a year	p ≥ 0.1
Likely	High likelihood. Commonly observed at similar facilities.	Higher than 10% probability in 10 years	p ≥ 0.01
Possible	Has occurred a number of times within the industry and at least once at the site (or similar facilities in the region).	Higher than 1% probability in 10 years	p ≥ 0.001
Unlikely	Has occurred before within the industry, but not at the site.	Less than a 1% probability in 10 years	p < 0.001
Rare	Low likelihood of occurrence, but not impossible. Has not occurred at the site but has occurred in industry.	Less than a 1% probability in 100 years	p < 0.0001
Very rare	Very low likelihood of occurrence, but not impossible. Occurrence cannot be deemed non-credible	Less than a 1% probability in 1000 years	p < 0.00001
Close to non-credible	Extremely remote likelihood of occurrence. Although the mechanisms are technically plausible for the occurrence, it is seen as near non-credible.	Less than a 1% probability in 10,000 years	p < 0.000001

¹ Industry encompasses the mining industry as a whole.

), consequence rating table (

Table 2. Consequence rating (Reprinted from Schafer et al. [2]).

Consequence Rating 1	Consequence of Failure of Element on the Rest of the System	Degree of Human Intervention Required	Environment	Community
Slight	Failure of element does not have cascading consequences.	Structural integrity maintained. No intervention or maintenance required.	No movement of tailings beyond the structure footprint.	No impact on local community.
Minor	Failure of element may have cascading consequences that do not result in global failure.	Structural integrity maintained. Minor or localized intervention or maintenance required.	Released tailings are not toxic 2, and/or minimal loss of habitat (<5%) of species of special interest 3, and/or acceptable restoration of water bodies and environment feasible in a short time frame (<5 years).	Impact 4 on local community for less than 1 year.
Moderate	Failure of element has cascading consequences that do not result in global failure.	Intervention or maintenance required to limit impact of cascading consequences.	Released tailings are not toxic 2, and/or moderate loss of habitat (5–20%) of species of special interest 3, and/or acceptable restoration of water bodies and environment feasible in a short time frame (<5 years).	Short-term (<5 years) impact 4 on local community.
Major	Global failure of tailings dam with minor release of tailings.	Intervention or maintenance required to maintain function of structure as a whole.	Released tailings are toxic 2, and/or significant loss of habitat (20–50%) of species of special interest 3, and/or acceptable restoration of water bodies and environment feasible in a moderate time frame (5–25 years).	Medium-term (5–25 years) impact 4 on local community.
Severe	Global failure of tailings dam with catastrophic release of tailings.	Structural repair not possible.	Released tailings are toxic 2, and/or very significant loss of habitat (>50%) of species of special interest 3, and/or acceptable restoration of water bodies and environment unlikely within an extended time frame (>25 years).	Long-term (>25 years) impact 4 on local community. Fatalities.

Notes: ¹ Assigned consequence should reflect the most likely outcome. If assigning consequence with consideration of the worst case or a combination of discrete outcomes, this must be declared. ² Toxicity assessment of tailings should consider an assessment of the fluids and solids (leaching potential, acidity, radioactivity). ³ Species of special interest is defined as a species that lives in the inundation area that would be greatly impacted by habitat loss (preferable to select a species that is provincially or federally listed). ⁴ Community impacts must be determined through meaningful engagement with stakeholders and may include a consideration of health, loss of access/destruction of traditional lands, housing, destruction/damage of farmland, harm to livestock, damage to water or soil resources, impacts to trapping and fishing, loss of animals, overall cultural impact, and employment. Reputation, legal aspects, and economics are not considered in this consequence table as they are considered site- and corporation-specific. It may be necessary to assess these aspects on a site-specific basis.

), and risk categories (

Table 3. Risk category (Reprinted from Schafer et al. [2]).

Risk Category	Description of Risk Category
Low	Risk minimal. Monitor risks. Acceptable closure plan.
Moderate	Risk tolerable with controls. Assess risk mitigation options and monitor these risks. Minor re-design of closure plan may be required to accommodate risk mitigation.
High	Risk undesirable. Risk mitigation should be employed to ALARP to reduce risk category. Closure plan may require alteration to accommodate risk mitigation.
Extreme	Risk intolerable. Risk mitigation required immediately to reduce risk category. Requires more detailed risk analysis. Closure plan requires alteration.

) to be used in conjunction with the G-FMEA. Schafer et al. [2] provides guidance on how a risk matrix can be color-coded and demonstrates the application through an example risk matrix. The example risk matrix, presented in Figure 1, will be used for the oil sands case study presented in this paper. The example risk matrix has a threshold line where hazards that have a risk rating higher than the threshold line may require additional analysis to adequately assess the risk. It should be noted that risk matrices used in practice should be developed with input from all relevant stakeholders.

3. Application of G-FMEA to a Case Study

The G-FMEA was used to evaluate an oil sands case study in Alberta, Canada. The purpose of this was to demonstrate the utility of the G-FMEA process.

3.1. Site Description (System Definition)

The first step of conducting the G-FMEA involves defining the system as described by Schafer et al. [2]. This section provides a base-level description of the site for information purposes. The ETF is a sand dam with a starter dyke constructed of overburden. The facility was constructed with a mixture of upstream and centerline construction. In upstream construction, tailings are discharged upstream from the crest of the dam (beginning first with the starter dyke) to form a beach, which becomes the foundation for subsequent raises [5]. In centerline construction, fill is placed on the downstream slope of the dam and tailings are discharged in the upstream direction to form a beach [5]. Consequently, the centerline of the dam remains at the same position relative to the starter dyke [5]. Analysis of the facility for this paper was conducted at a section of the ETF that had been constructed using upstream construction with hydraulic fill deposition and cell construction. Cell construction involves discharging tailings to a cell where the solids settle, and water and fines are decanted [6]. Dozers are used to maintain containment dykes and spread and compact the sand in the cell by using vibration [6,7]. The water and fines that do not settle flow to the end of the cell and through a weir structure to the beach [8].

The dam is composed of coarse sand tailings (CST) and beach deposits, including beach above water (BAW) and beach below water (BBW). CST is made up of the coarse fraction of whole tailings and is formed by cycloning the tailings [9]. The BAW and BBW deposits are formed when tailings are discharged into the pond. The coarser tailings will settle first on the beach, leading to the formation of these two different deposits. The BAW and BBW can have variable gradation, solids content, hydraulic conductivity, and density [6]. At the location of the analyzed section, the dam contains fluid fine tailings (FFT). The dam has a toe berm constructed of lean oil sands (fines and a modest amount of bitumen) [10], with benched slopes to provide stability to the facility. The foundation of the ETF at the analyzed section is typical for the Fort McMurray area and consists of the McMurray formation, Pleistocene sands, and McMurray formation tidal flat mud and mud flat. It should be noted that there is a weak tailings layer at the base of the tailings deposit and the upstream constructed lifts of the dam. The weak tailings layer was formed due to initial deposition of tailings into standing water, has a high bitumen and fines content, and is considered liquefiable. This layer creates stability issues, which led to the design and construction of the toe berm that is currently in place. The section used for analysis is extensively instrumented and monitored.

Drainage at the facility consists of two levels of 200 mm diameter perforated collector pipes surrounded by a woven geotextile sock drain that run parallel to the dam centreline. Smaller outtake pipes connect to the collector pipes at approximately 150 m intervals and discharge water to a perimeter ditch.

As the facility operations proceed towards closure, it is expected that the facility will be infilled with CST, partially displacing the FFT. It is expected that this process will result in a zone of mixed CST and FFT. This zone will be covered by approximately 4 m of CST that will be placed and compacted using methods similar to cell construction to create a trafficable surface. The compacted CST will then be capped with mine waste to enable construction of the landform. The vegetative cover for the reclamation surface is not yet defined. The reclamation surface is expected to have hummocks and a system of drainage channels that direct flow towards the outlet of the facility. The outlet is designed to have a base width of 100 m and side slopes of 15 horizontal:1 vertical.

The case study cross-section analyzed is provided in Figure 2.

3.2. Block Diagram, Function Definition, and Serviceability Failure Description

The next three steps of conducting the G-FMEA involve developing a block diagram, breaking the subsystem down into key elements and functions, and defining serviceability failure for each element. The block diagram is used to break the system down into functional subsystems that can be analyzed and serves to show the relationships between those subsystems. The block diagram for the case study is shown in Figure 3.

Table 4. Case study element functions and failure description.

System	Elements	Function (before Closure)	Function (after Closure)	Failure Description
Dam	Foundation	Supports capacity of the dam.	Supports capacity of the dam.	Instability associated with movements of the soil mass.
	Starter dyke	Allowed for initial containment of tailings. Provides stability.	Provides stability.	Instability.
	Drainage system	See drainage system.
	Cell sand	Containment, dyke stability, provides stability.	Provides stability.	Instability.
	Berm	Provides stabilization against liquefaction with weak tailings layer present at base of facility.	Provides stabilization against liquefaction, with weak tailings layer present at base of facility.	Instability.
	Beach above water	Containment, dyke stability.	Non-structural fill.	Instability.
	Beach below water	Containment of tailings and dam raises.	Non-structural fill. Potential source of instability.	Instability.
	Vegetative cover	Erosion stabilization and protection of slope.	Erosion stabilization and protection of slope.	Fails to provide erosion protection.
Drainage System	Level 1 drains	Control the phreatic surface in the dam.	Controls phreatic surface in the dam up to a certain point of time (must be determined with modelling). After this point, the drains are assumed to fail and will no longer serve a function in closure.	Drains fail to control phreatic surface.
	Level 2 drains	Control the phreatic surface in the dam.	Controls phreatic surface in the dam up to a certain point of time (must be determined with modelling). After this point, the drains are assumed to fail and will no longer serve a function in closure.	Drains fail to control phreatic surface.
	Outtakes—level 1	Transmit water from drains to perimeter ditch.	Controls phreatic surface in the dam up to a certain point of time (must be determined with modelling). After this point, the drains are assumed to fail and will no longer serve a function in closure.	Drains fail to control phreatic surface.
	Outtakes—level 2	Transmit water from drains to perimeter ditch.	Controls phreatic surface in the dam up to a certain point of time (must be determined with modelling). After this point, the drains are assumed to fail and will no longer serve a function in closure.	Drains fail to control phreatic surface.
	Perimeter ditch	Collect water from drains and intercept seepage and sediment.	Water and sediment collection.	Fails to collect, intercept, and transport seepage.
Landform	Cap (overburden and coarse sand tailings)	N/A	Trafficable surface, stability to landform, limits infiltration.	Landform instability.
	Infilled coarse sand tailings mixed with FFT	N/A	Non-structural fill. Potential source of instability.	Landform instability.
	Hummocks	N/A	Provide topographic variation and diversity. Designed to keep the water table low in upland areas.	Fails to provide topographic variation and diversity. Fails to help control the water table.
	Drainage channels	N/A	Directs flow to the outlet to be carried away from the facility.	Fails to direct flow to the outlet.
	Outlet	N/A	To ensure controlled discharge under exceptional inflow conditions.	Fails to control discharge.
	Vegetative cover	N/A	Stabilization of landform. Erosion protection. Ecosystem creation. Wildlife habitat.	Fails to provide erosion protection.

provides descriptions of the elements, the function before and after closure, and serviceability failure for each element based on the defined function.

3.3. Site-Specific FMEA

Each element discussed in Table 4 was assessed using the appropriate chart from the G-FMEA to assign failure modes to the elements. Appendix A shows the 128 failure modes that were identified for the dam elements. The next steps in the FMEA are to assess the applicability of the failure modes, determine the failure effects and consequences, determine the risk rating for each temporal scale, and determine corresponding risk mitigation as required. This step has not been completed in full but has been completed for demonstration purposes for failure mode 34 and 87 in

Table A2. Application of G-FMEA to a case study site.

Element	Failure Mode Identification	Failure Mode Description	Potential Trigger/Cause	Screening Assessment of Failure Mode	Failure Effects
Perimeter ditch	1	Blockage	Sedimentation	Is there erosion protection in place? What is the slope of the ditch? Is it sufficient to keep particles suspended? What is the strength of material? Have there been failures in this material before?	Flooding, discharge of process-affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	2	Blockage	Sloughing/slope failure of walls	What is the slope of the side slopes? What is the strength of material? Have there been failures in this material before?	Flooding, discharge of process-affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	3	Blockage	Beaver dams	Are there beavers in the area?	Flooding, discharge of process-affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	4	Blockage	Icing	Is the mine located in an area that could experience icing?	Flooding, discharge of process-affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	5	Reduction in cross-sectional area	Sloughing/slope failure of walls	What is the slope of the side slopes? What is the strength of material? Have there been failures in this material before?	Reduced capacity, erosion, potential flooding, discharge of process affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	6	Reduction in cross-sectional area	Excessive vegetation	Will the ditch regularly have water running through it or will it stay dry for a portion of the year? Are there deterrents in place to prevent the growth of vegetation?	Reduced capacity, erosion, potential flooding, discharge of process affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
	7	Change in slope	Erosion	Is there erosion protection in place?	Water discharge velocity change, local instability of dam
	8	Change in slope	Differential settlement	Does the material have the potential to consolidate or settle over time? Is it a cut into natural ground or is the material placed?	Creation of secondary channels, localized areas of erosion, local instability of dam
	9	Sand channel buoyancy	Freezing conditions in channels composed of sand	Are the drainage channels constructed of sand? Could the channel experience freezing?	Flooding, discharge of process affected water to the environment, blockage of level 1/level 2 drain outlet, rise in phreatic surface, increase in seepage, pond on reclamation surface, internal erosion, global instability, toe erosion
Outtakes-Level 1	10	Breakage of pipe	Break in pipe	Is the pipe capable of breaking?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	11	Breakage of pipe	Buckling	Is the pipe capable of buckling?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	12	Breakage of pipe	Physical degradation	Is the pipe capable of physically degrading over time?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	13	Clogging	Chemical or biological	Is chemical or biological clogging possible?	Failure to transmit water to perimeter ditch, clogging of level 1 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	14	Clogging	Particulate clogging (sediment from perimeter ditch or other)	Is sediment capable of clogging the pipe?	Failure to transmit water to perimeter ditch, clogging of level 1 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	15	Blockage	Perimeter channel (FM 1–6) or other (snow, debris, etc.)	Is the pipe outlet close enough to the base of the perimeter channel that it could become blocked?	Failure to transmit water to perimeter ditch, clogging of level 1 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability
Level 1 Drains	16	Breakage of pipe	Break in pipe	Is the pipe capable of breaking?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	17	Breakage of pipe	Buckling	Is the pipe capable of buckling?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	18	Breakage of pipe	Physical degradation	Is the pipe capable of physically degrading over time?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	19	Clogging	Chemical or biological	Is chemical or biological clogging possible?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	20	Clogging	Particulate clogging (sediment from perimeter ditch or other)	Is sediment capable of clogging the pipe?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	21	Clogging	Failure in level 1 outtakes (FM 13–15)	Is it possible for level 1 outtakes to fail?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	22	Clogging surround (woven sock or sand and gravel bed)	Biological, chemical, or particulate clogging	Is it possible for the material surrounding the pipe to become clogged?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	23	Breakage of connection between a drain and outlet pipe	Overloading, degradation of connection, poor installation	How are the drain and outtake connected?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
Outtakes-Level 2	24	Breakage of pipe	Break in pipe	Is the pipe capable of breaking?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	25	Breakage of pipe	Buckling	Is the pipe capable of buckling?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	26	Breakage of pipe	Physical degradation	Is the pipe capable of physically degrading over time?	Release of water into downstream shell, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, erosion on downstream slope
	27	Clogging	Chemical or biological	Is chemical or biological clogging possible?	Failure to transmit water to perimeter ditch, clogging of level 2 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	28	Clogging	Particulate clogging (sediment from perimeter ditch or other)	Is sediment capable of clogging the pipe?	Failure to transmit water to perimeter ditch, clogging of level 2 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	29	Blockage	Perimeter channel (FM 1–6) or other (snow, debris, etc.)	Is the pipe outlet close enough to the base of the perimeter channel that it could become blocked?	Failure to transmit water to perimeter ditch, clogging of level 2 drains, lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability
Level 2 Drains	30	Breakage of pipe	Break in pipe	Is the pipe capable of breaking?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	31	Breakage of pipe	Buckling	Is the pipe capable of buckling?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	32	Breakage of pipe	Physical degradation	Is the pipe capable of physically degrading over time?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	33	Clogging	Chemical or biological	Is chemical or biological clogging possible?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	34	Clogging	Particulate clogging (sediment from perimeter ditch or other)	Is sediment capable of clogging the pipe?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	35	Clogging	Failure in level 2 outtakes (FM 27–29)	Is it possible for the level 2 outtakes to fail?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	36	Clogging surround (woven sock or sand and gravel bed)	Biological, chemical, or particulate clogging	Is it possible for the material surrounding the pipe to become clogged?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
	37	Breakage of connection between a drain and outlet pipe	Overloading, degradation of connection, poor installation	How are the drain and outtake connected?	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
Foundation	38	Heave (seepage forces create zero effective stress condition)	Embankment loading, excessive rainfall, embankment seepage	What are the current hydraulic gradients and maximum possible due to geometry? What are the materials present? Are there cohesionless soils confined by an overlying lower permeability layer?	Global instability
	39	Vertical deformation from collapse of karst formation	Collapse of karst formation	Is there karst present in the foundation?	Cracking (transverse cracks—perpendicular to dam crest are larger problems than longitudinal cracks) in dam, internal erosion in dam, crest subsidence
	40	Vertical deformation caused by settlement of material	Consolidation	Will the materials in the foundation consolidate over time? How much consolidation has already occurred? Does the material have the potential to collapse?	Cracking (transverse cracks—perpendicular to dam crest are larger problems than longitudinal cracks) in dam, internal erosion in dam, crest subsidence
	41	Excessive/uncontrolled seepage through foundation or foundation/dam contact	Excessive rainfall	Is there potential for seepage through the foundation? What is the permeability of the materials?	Erosion of downstream toe, increase in porewater pressure in dam, global instability
	42	Shear failure along pre-existing shear plane from changing shear stress	Loading/unloading of foundation, earthquake, subsurface stress changes (geothermal development, in situ oil or gas production, wastewater injection, etc.)	Are there pre-existing shear planes? Is there the potential for anthropogenic loading or unloading events? Is the material erodible?	Slumping of downstream slope, translational slide, rotational slide, static liquefaction
	43	Shear failure along new shear plane from changing shear stress	Loading/unloading of foundation, earthquake, subsurface stress changes (geothermal development, in situ oil or gas production, wastewater injection, etc.)	Is there the potential for anthropogenic loading or unloading events? Is the material erodible?	Slumping of downstream slope, translational slide, rotational slide, static liquefaction
	44	Shear failure along pre-existing shear plane from changing shear strength	Degradation/weathering, porewater pressure change, progressive failure of strain-softening materials, brittle failure of contractive materials	Are there pre-existing shear planes? Is there the potential for degradation or weathering of the material? Is the material strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide, static liquefaction
	45	Shear failure along new shear plane from changing shear strength	Degradation/weathering, porewater pressure change, progressive failure of strain-softening materials, brittle failure of contractive materials	Is there the potential for degradation or weathering of the material? Is the material strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide, static liquefaction
	46	Internal erosion in foundation or dam/foundation contact from global backward erosion	Failure of soil above or around a backward erosion pipe to hold a roof, heave, high hydraulic gradients, design/construction defect, presence of non-plastic soils in the foundation	Are there non-plastic soils in the foundation?	Static liquefaction, global instability, unravelling/sloughing of downstream face, sub vertical cavities
	47	Internal erosion in foundation or dam/foundation contact from backward erosion piping	Heave, high hydraulic gradients, design/construction defect, presence of non-plastic soils that are capable of holding a roof	Are there non-plastic soils in the foundation and soils capable of ‘holding a roof’?	Enlargement of pipe, global instability, static liquefaction
	48	Internal erosion in foundation or dam/foundation contact from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defect	Is there a contact between a coarse-grained and a fine-grained soil? Is the geometrical and hydraulic condition for contact erosion met?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	49	Internal erosion in foundation or dam/foundation contact from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	50	Internal erosion in foundation or dam/foundation contact from concentrated leak	Fracture in foundation soil, hydraulic fracture, high hydraulic gradient, cracks at dam/foundation contact from vertical deformation in foundation or poor construction practices or differential settlement, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
	51	Thawing of foundation permafrost	Climate change	Is there permafrost in the foundation?	Cracking (transverse cracks—perpendicular to dam crest are larger problems than longitudinal cracks) in dam, piping in dam, crest subsidence
Starter Dyke	52	Vertical deformation (differential or otherwise) from consolidation	Consolidation/settlement	Does the material have the potential to consolidate? How much consolidation has occurred already? How much is expected to occur?	Release of pore water and loss of height (potential for pond to develop on reclamation surface), development of cracks above starter dyke, internal erosion, overtopping
	53	Seismic liquefaction	Earthquakes, induced seismicity, construction traffic, blasting	Seismic events in area? Induced seismicity? Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted?	Global instability
	54	Static liquefaction from changing shear stress	Loading/unloading; overloading, including increasing the load, construction activities at the crest, fill placement at toe; over steepening of downstream slope or toe, including erosion or excavation (slumping of downstream slope from shear failure); foundation shear (FM 42–45), surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41); slumping of downstream slope from shear failure (FM 42–45, 58, 59, 69, 70, 82, 83, 96, 97, 107, 108)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Is there a likelihood for anthropogenic contributions in the future (i.e., unexpected construction)? Is the site remote? What is the material of the downstream slope? Is there a nearby river?	Global instability
	55	Static liquefaction from changing mean effective stress	Change in pore pressures caused by a phreatic surface change (failure of level 1 and level 2 drains (FM 16–23 and 30–37))	Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted? Does control of the phreatic surface rely on drain function? Could drains become clogged or fail in the future?	Global instability
	56	Static liquefaction from long-term change in material properties resulting in changing shear strength	Changing shear strength caused by degradation/weathering, progressive failure, porewater pressure change, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? To what extent could weathering or degradation of the material occur? Will it result in an increase or decrease in strength? Will a change in the phreatic surface impact the strength of the material?	Global instability
	57	Static liquefaction from changing shear stress and mean effective stress	Lateral extrusion	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Weak layers interbedded in tailings? Could the tailings ‘squish’ out like toothpaste during loading?	Global instability
	58	Shear failure from changing shear stress	Loading/unloading crest, toe, upstream, downstream; surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41)	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping of downstream slope, translational slide, rotational slide
	59	Shear failure from changing shear strength	Degradation/weathering, porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Is there potential for weathering or degradation of materials? Does the porewater pressure rely on drain performance? Could drains fail over time? Are the materials strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide
	60	Internal erosion in dam from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defects	Is there a contact between a coarse and fine-grained soil? Is there a filter in place?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	61	Internal erosion in dam from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	62	Internal erosion in dam—concentrated leak	Cracks from foundation (FM 39–40, 51), tunnels created by burrowing animals, hydraulic fracture, high hydraulic gradient, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
Cell Sand	63	Vertical deformation (differential or otherwise) from consolidation	Consolidation/settlement	Does the material have the potential to consolidate? How much consolidation has occurred already? How much is expected to occur?	Release of porewater and loss of height (potential for pond to develop on reclamation surface), development of cracks, internal erosion
	64	Seismic liquefaction	Earthquakes, induced seismicity, construction traffic, blasting	Seismic events in area? Induced seismicity? Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted?	Global instability
	65	Static liquefaction from changing shear stress	Loading/unloading; overloading, including increasing the load, construction activities at the crest, fill placement at toe; over steepening of downstream slope or toe, including erosion or excavation (slumping of downstream slope from shear failure); foundation shear (FM 42–45); shear in the starter dyke (FM 58–59); surface erosion of cell sand or berm on downstream slope (FM 74–75. 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41); slumping of downstream slope from shear failure (FM 42–45, 58, 59, 69, 70, 82, 83, 96, 97, 107, 108)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Is there a likelihood for anthropogenic contributions in the future (i.e., unexpected construction)? Is the site remote? What is the material of the downstream slope? Is there a nearby river?	Global instability
	66	Static liquefaction from changing mean effective stress	Change in pore pressures caused phreatic surface change (i.e., failure of level 1 and level 2 drains (FM 16–23 and 30–37))	Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted? Does control of the phreatic surface rely on drain function? Could drains become clogged or fail in the future?	Global instability
	67	Static liquefaction from long-term change in material properties resulting in changing shear strength	Change in shear strength caused by degradation/weathering, progressive failure, porewater pressure change, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? To what extent could weathering or degradation of the material occur? Will it result in an increase or decrease in strength? Will a change in the phreatic surface impact the strength of the material?	Global instability
	68	Static liquefaction from changing shear stress and mean effective stress	Lateral extrusion	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Weak layers interbedded in tailings? Could the tailings ‘squish’ out like toothpaste during loading?	Global instability
	69	Shear failure from changing shear stress	Loading/unloading crest, toe, upstream, downstream; surface erosion of berm on downstream slope (FM 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41)	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping of downstream slope, translational slide, rotational slide
	70	Shear failure from changing shear strength	Degradation/weathering, porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Is there potential for weathering or degradation of materials? Does the porewater pressure rely on drain performance? Could drains fail over time? Are the materials strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide
	71	Internal erosion in dam from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defects	Is there a contact between a coarse and fine-grained soil? Is there a filter in place?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	72	Internal erosion in dam from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	73	Internal erosion in dam from concentrated leak	Cracks from vertical deformation in foundation (FM 39–40, 51), cracks from vertical deformation in starter dyke (FM 52), cracks from vertical deformation in BAW (FM 90), cracks from vertical deformation in BBW (FM 101), tunnels created by burrowing animals, hydraulic fracture, high hydraulic gradient, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
	74	Surface erosion from wind and overland flow resulting in rills, gullies, or sheet erosion	Destruction of vegetation (FM 112), rainfall, melting of snow, wind, increased seepage on downstream slope from failure of drainage system, increased seepage from internal erosion in embankment	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps
	75	Surface erosion—spring sapping (headward erosion of gullies due to concentration of seepage forces at the locus of the gully which accentuates erosion)	Destruction of vegetation (FM 112), increased seepage on downstream slope from failure of level 1 or 2 drains (FM 10–37), increased seepage from internal erosion of starter dyke, cell sand, BAW, BBW (FM 60–61, 71–72, 98–99, 109–110)	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection? Will seepage daylight on the downstream slope?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps
Berm	76	Vertical deformation (differential or otherwise) from consolidation	Consolidation/settlement	Does the material have the potential to consolidate? How much consolidation has occurred already? How much is expected to occur?	Release of pore water and loss of height (potential for pond to develop on reclamation surface), development of cracks, internal erosion
	77	Seismic liquefaction	Earthquakes, induced seismicity, construction traffic, blasting	Seismic events in area? Induced seismicity? Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted?	Global instability
	78	Static liquefaction from changing shear stress	Loading/unloading; overloading, including increasing the load, construction activities at the crest, fill placement at toe; over steepening of downstream slope or toe, including erosion or excavation (slumping of downstream slope from shear failure); foundation shear (FM 42–45); shear in the starter dyke (FM 58–59); shear in cell sand (FM 69–70); surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41); slumping of downstream slope from shear failure (FM 42–45, 58, 59, 69, 70, 82, 83, 96, 97, 107, 108)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Is there a likelihood for anthropogenic contributions in the future (i.e., unexpected construction)? Is the site remote? What is the material of the downstream slope? Is there a nearby river?	Global instability
	79	Static liquefaction from changing mean effective stress	Change in pore pressures caused by a phreatic surface change (failure of level 1 and level 2 drains (FM 16–23 and 30–37))	Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted? Does control of the phreatic surface rely on drain function? Could drains become clogged or fail in the future?	Global instability
	80	Static liquefaction from long-term change in material properties resulting in changing shear strength	Changing shear strength caused by degradation/weathering, progressive failure, porewater pressure change, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? To what extent could weathering or degradation of the material occur? Will it result in an increase or decrease in strength? Will a change in the phreatic surface impact the strength of the material?	Global instability
	81	Static liquefaction from changing shear stress and mean effective stress	Lateral extrusion	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Weak layers interbedded in tailings? Could the tailings ‘squish’ out like toothpaste during loading?	Global instability
	82	Shear failure from changing shear stress	Loading/unloading crest, toe, upstream, downstream; surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41)	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping of downstream slope, translational slide, rotational slide
	83	Shear failure from changing shear strength	Degradation/weathering; porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Is there potential for weathering or degradation of materials? Does the porewater pressure rely on drain performance? Could drains fail over time? Are the materials strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide
	84	Internal erosion in dam from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defects	Is there a contact between a coarse and fine-grained soil? Is there a filter in place?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	85	Internal erosion in dam from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	86	Internal erosion in dam from concentrated leak	Cracks from vertical deformation in foundation (FM 39–40, 51), cracks from vertical deformation in starter dyke (FM 52), cracks from vertical deformation in cell sand (FM 63)), tunnels created by burrowing animals, hydraulic fracture, high hydraulic gradient, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
	87	Surface erosion from wind and overland flow resulting in rills, gullies, or sheet erosion	Destruction of vegetation (FM 112), rainfall, melting of snow, wind, increased seepage on downstream slope from failure of drainage system, increased seepage from internal erosion in embankment	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps
	88	Surface erosion from spring sapping (headward erosion of gullies due to concentration of seepage forces at the locus of the gully which accentuates erosion)	Destruction of vegetation (FM 112); increased seepage on downstream slope from failure of level 1 or 2 drain system (FM 10–37); excessive erosion from internal erosion of berm, starter dyke, cell sand, BAW, BBW (FM 60–61, 71–72, 98–99, 109–110)	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection? Will seepage daylight on the downstream slope?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps
	89	Toe erosion	Flow action from perimeter ditch or nearby river, release of a dam from a beaver, flood event, river changing course over time, destruction of vegetation (FM 110), excessive seepage and toe erosion (FM 41), excessive erosion from internal erosion (FM 84–85)	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection? Is there a nearby perimeter ditch or river? Is there known animal activity in the area?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps, beaver bafflers
Beach above water	90	Vertical deformation (differential or otherwise) from consolidation	Consolidation/settlement	Does the material have the potential to consolidate? How much consolidation has occurred already? How much is expected to occur?	Release of pore water and loss of height (potential for pond to develop on reclamation surface), development of cracks, internal erosion
	91	Seismic liquefaction	Earthquakes, induced seismicity, construction traffic, blasting	Seismic events in area? Induced seismicity? Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted?	Global instability
	92	Static liquefaction from changing shear stress	Loading/unloading; overloading, including increasing the load, construction activities at the crest, fill placement at toe; over steepening of downstream slope or toe, including erosion or excavation (slumping of downstream slope from shear failure); foundation shear (FM 42–45); shear in the BBW (FM 107–108); surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41); slumping of downstream slope from shear failure (FM 42–45, 58, 59, 69, 70, 82, 83, 96, 97, 107, 108)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Is there a likelihood for anthropogenic contributions in the future (i.e., unexpected construction)? Is the site remote? What is the material of the downstream slope? Is there a nearby river?	Global instability
	93	Static liquefaction from changing mean effective stress	Change in pore pressures caused by a phreatic surface change (failure of level 1 and level 2 drains (FM 16–23 and 30–37))	Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted? Does control of the phreatic surface rely on drain function? Could drains become clogged or fail in the future?	Global instability
	94	Static liquefaction from long-term change in material properties resulting in changing shear strength	Changing shear strength caused by degradation/weathering, progressive failure, porewater pressure change, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? To what extent could weathering or degradation of the material occur? Will it result in an increase or decrease in strength? Will a change in the phreatic surface impact the strength of the material?	Global instability
	95	Static liquefaction from changing shear stress and mean effective stress	Lateral extrusion	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Weak layers interbedded in tailings? Could the tailings ‘squish’ out like toothpaste during loading?	Global instability
	96	Shear failure from changing shear stress	Loading/unloading crest, toe, upstream, downstream; surface erosion of cell sand on downstream slope (FM 74–75), surface erosion of berm on downstream slope (FM 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41)	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping of downstream slope, translational slide, rotational slide
	97	Shear failure from changing shear strength	Degradation/weathering, porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Is there potential for weathering or degradation of materials? Does the porewater pressure rely on drain performance? Could drains fail over time? Are the materials strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide
	98	Internal erosion in dam from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defects	Is there a contact between a coarse and fine-grained soil? Is there a filter in place?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	99	Internal erosion in dam from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	100	Internal erosion in dam—concentrated leak	Cracks from vertical deformation in BBW (FM 101), cracks from vertical cracks in foundation (FM 39–40, 50), tunnels created by burrowing animals, hydraulic fracture, high hydraulic gradient, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
Beach below water	101	Vertical deformation (differential or otherwise) from consolidation	Consolidation/settlement	Does the material have the potential to consolidate? How much consolidation has occurred already? How much is expected to occur?	Release of pore water and loss of height (potential for pond to develop on reclamation surface), development of cracks, internal erosion
	102	Seismic liquefaction	Earthquakes, induced seismicity, construction traffic, blasting	Seismic events in area? Induced seismicity? Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted?	Global instability
	103	Static liquefaction from changing shear stress	Loading/unloading; overloading, including increasing the load, construction activities at the crest, fill placement at toe; over-steepening of downstream slope or toe, including erosion or excavation (slumping of downstream slope from shear failure); foundation shear (FM 42–45); surface erosion of cell sand or berm on downstream slope (FM 74–75, 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41); slumping of downstream slope from shear failure (FM 42–45, 58, 59, 69, 70, 82, 83, 96, 97, 107, 108)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Is there a likelihood for anthropogenic contributions in the future (i.e., unexpected construction)? Is the site remote? What is the material of the downstream slope? Is there a nearby river?	Global instability
	104	Static liquefaction from changing mean effective stress	Change in pore pressures caused by a phreatic surface change (failure of level 1 and level 2 drains (FM 16–23 and 30–37))	Density of material (contractive or dilative)? Saturated or unsaturated? Hydraulically placed or compacted? Does control of the phreatic surface rely on drain function? Could drains become clogged or fail in the future?	Global instability
	105	Static liquefaction from long-term change in material properties resulting in changing shear strength	Changing shear strength caused by degradation/weathering, progressive failure, porewater pressure change, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? To what extent could weathering or degradation of the material occur? Will it result in an increase or decrease in strength? Will a change in the phreatic surface impact the strength of the material?	Global instability
	106	Static liquefaction from changing shear stress and mean effective stress	Lateral extrusion	Density of material (contractive or dilative)? Hydraulically placed or compacted? Saturated or unsaturated? Weak layers interbedded in tailings? Could the tailings ‘squish’ out like toothpaste during loading?	Global instability
	107	Shear failure from changing shear stress	Loading/unloading crest, toe, upstream, downstream; surface erosion of cell sand on downstream slope (FM 74–75); surface erosion of berm on downstream slope (FM 87–89); excessive and uncontrolled seepage through foundation resulting in erosion of toe (FM 41)	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping of downstream slope, translational slide, rotational slide
	108	Shear failure from changing shear strength	Degradation/weathering, porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials, failure of level 1 and level 2 drains (FM 16–23 and 30–37)	Is there potential for weathering or degradation of materials? Does the porewater pressure rely on drain performance? Could drains fail over time? Are the materials strain-softening or brittle?	Slumping of downstream slope, translational slide, rotational slide
	109	Internal erosion in dam from contact erosion	Parallel flow in coarser layer to the interface between the coarse-grained and fine-grained soil, high hydraulic gradients, design/construction defects	Is there a contact between a coarse and fine-grained soil? Is there a filter in place?	Global instability, static liquefaction, settlement of the crest, loss of stability or unravelling, eroded material can clog the permeable layer and increase the porewater pressure (could result in hydraulic fracture and uplift of the downstream toe or a rise in the phreatic surface), development of a pipe
	110	Internal erosion in dam from suffusion	High hydraulic gradients, design/construction defect, presence of widely gap-graded or non-plastic gap-graded soils	Is the material widely gap-graded or gap-graded non plastic?	Global instability, seepage on the downstream slope, settlement of the crest, permeability may increase as erosion progresses or decrease if clogging occurs
	111	Internal erosion in dam—concentrated leak	Cracks from vertical deformation in foundation (FM 39–40, 51), tunnels created by burrowing animals, hydraulic fracture, high hydraulic gradient, design/construction defects	Is there a crack or gap that could allow for a concentrated leak to develop?	Global instability, development of a pipe
Vegetative cover	112	Destruction of vegetation	Suffocation by eroded material, forest fires, pests and disease, climate change, large storm event, anthropogenic contributions, surface erosion in cell sand or berm (FM 74–75, FM 87–89), evolution of vegetation over time due to climate change	Does the resistance to erosion rely on the vegetation? Is the area susceptible to forest fires? Are there pests/disease that could lead to vegetation destruction? What is the downstream slope? Is the area remote? Are there surrounding communities that could lead to destruction of vegetation (i.e., recreational vehicles)?	Increase in surface erosion, instability of downstream slope, global instability, change in overall evapotranspiration and water balance impacts (infiltration versus runoff)
Cap (overburden and coarse sand tailings)	113	Excessive settlement	Consolidation	Does the material have the potential to settle over time?	Formation of ponds on reclamation surface, overtopping, piping (increase in seepage forces and gradients)
	114	Differential settlement	Consolidation, poor construction practices	Does the material have the potential to settle over time? Are there areas that have the potential to settle more than others?	Formation of ponds on reclamation surface, failure to direct surface water runoff towards drainage channels, development of cracks, formation of preferential flow paths, localized depressions, increased infiltration, increased phreatic surface
Infilled coarse sand tailings mixed with FFT	115	Excessive settlement	Consolidation	Does the material have the potential to settle over time?	Formation of ponds on reclamation surface, overtopping, piping (increase in seepage forces and gradients)
	116	Differential settlement	Consolidation, poor construction practices	Are there areas that have the potential to settle more than others?	Formation of ponds on reclamation surface, overtopping, piping (increase in seepage forces and gradients), failure of drainage channels to behave as intended, localized depressions
Hummocks	117	Shear failure from changing shear stress	Loading/unloading crest, toe, surface erosion on slopes, failure of underlying material to support hummock	Is there potential for anthropogenic contributions (i.e., excavations or construction)? Erodibility of material?	Slumping, translational slide, rotational slide, blockage of drainage channels
	118	Shear failure from changing shear strength	Degradation/weathering, porewater pressure change, change in permeability over time, failure of drains, progressive failure of strain-softening materials, brittle failure of contractive materials	Is there potential for weathering or degradation of materials? Are the materials strain-softening or brittle?	Slumping of slopes, translational slide, rotational slide, blockage of drainage channels
	119	Surface erosion from wind and overland flow resulting in rills, gullies, or sheet erosion	Destruction of vegetation, rainfall, melting of snow, wind	Is the material susceptible to erosion? Is there a vegetative cover or erosion protection?	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in drainage channel with sediment, development of negative drainage, development of large erosion scarps
Drainage channels	120	Washout of erosion protection (riprap)	Precipitation event larger than design events (including extreme or repeat events)	What precipitation events are the channels designed for? What is the chance of exceedance over 1000 years? How susceptible are the underlying materials to erosion?	Excessive erosion (erosion gullies, etc.), change in slope of drainage channels, erosion and release of materials underlying drainage channels
	121	Blockage (complete or partial)	Debris, beaver dam, icing, sedimentation, slumping from slope failure, ingress of vegetation, slope failure/excessive erosion from nearby hummock	Are there beavers in the area? Is there a chance for a slope failure? Could debris be carried downstream and deposited in the channels, resulting in a complete or partial blockage? Is the mine located in an area that could experience icing?	Formation of a pond upstream of the drainage channel, blockage breakthrough resulting in flooding, overtopping from pond formation, reversion back to a pond, piping through dam (increase in seepage forces and gradient)
	122	Sand channel buoyancy	Freezing conditions in channels composed of sand	Are the drainage channels constructed of sand? Could the channel experience freezing?	Flooding
	123	Erosion control failure	Improper design/construction, differential settlement	Do the drainage channels rely on erosion control for stability? Is there the chance for differential settlement of the channel?	Excessive erosion (erosion gullies, etc.), change in slope of drainage channel, erosion and release of materials underlying drainage channel, formation of secondary channel
Outlet	124	Washout of erosion protection (riprap)	Precipitation event larger than design event (including extreme or repeat events), flood following sand channel buoyancy event in drainage channel	What precipitation event is the outlet designed for? What is the chance of exceedance over 1000 years? How susceptible are the underlying materials to erosion?	Excessive erosion (erosion gullies, etc.), change in slope of outlet, erosion and release of materials underlying outlet
	125	Blockage (complete or partial)	Debris, beaver dam, icing, sedimentation, slumping from slope failure, ingress of vegetation, increase in depositional material due to failure of erosion protection in drainage channels upstream	Are there beavers in the area? Is there a chance for a slope failure? Could debris be carried downstream and deposited in the outlet, resulting in a complete or partial blockage? Is the mine located in an area that could experience icing?	Formation of a pond upstream of the outlet, blockage breakthrough resulting in flooding, overtopping from pond formation, reversion back to a pond, piping through dam (increase in seepage forces and gradient)
	126	Sand channel buoyancy	Freezing conditions in channels composed of sand	Is the outlet constructed of sand? Could the channel experience freezing?	Flooding
	127	Erosion control failure	Improper design/construction, differential settlement	Does the outlet rely on erosion control for stability? Is there the chance for differential settlement of the channel?	Excessive erosion (erosion gullies, etc.), change in slope of outlet, erosion and release of materials underlying outlet, formation of secondary channel
Vegetative cover	128	Destruction of vegetation	Suffocation by eroded material, forest fires, pests and disease, climate change, large storm event, anthropogenic contributions	Does the resistance to erosion rely on the vegetation? Is the area susceptible to forest fires? Are there pests/disease that could lead to vegetation destruction? Is the area remote? Are there surrounding communities that could lead to destruction of vegetation (i.e., recreational vehicles)?	Increase in surface erosion, deposition of material in drainage channels that could lead to a blockage via sedimentation, development of negative drainage on reclamation surface, ponding of water near dam crest, internal erosion (increase in seepage forces and hydraulic gradient)

, which are summarized in

Table 5. Demonstration failure modes.

Element	Failure Mode Identification	Failure Mode Description	Potential Trigger/Cause	Failure Effects
Level 2 Drains	34	Clogging	Particulate clogging (sediment from perimeter ditch or other)	Lack of control of phreatic surface (potential rise in phreatic surface), increase in seepage, pond on reclamation surface, internal erosion, global instability, release of water into downstream shell, erosion on downstream slope
Berm	87	Surface erosion from wind and overland flow resulting in rills, gullies, or sheet erosion	Destruction of vegetation (FM 112), rainfall, melting of snow, wind, increased seepage on downstream slope from failure of drainage system, increased seepage from internal erosion in embankment	Slope failures (shallow surficial movement, slumps), change in downstream slope angle, blockage in perimeter channel with sediment, development of negative drainage, development of large erosion scarps

. These failure modes were selected due to the way that they may change and develop over time. A detailed description of the clogging of the level 2 drains due to particulate clogging (failure mode 34) is provided in Section 3.3.1, and a description of the surface erosion failure of the berm is provided in Section 3.3.2. It should be noted that this work is not accompanied by a level of confidence assessment for each risk rating (as recommended in the G-FMEA steps), as this work is currently underway by a researcher at the University of Alberta.

3.3.1. Clogging of the Level 2 Drains

The level 2 drains consist of a 200 mm perforated collector pipe surrounded by a woven geotextile sock placed on a bed of sand and gravel, and they are located in the cell sand. The function of the level 2 drains before closure is to control the phreatic surface of the dam. Following closure, the function of the drains changes over time, and they are required to contribute to the control of the phreatic surface for a period of time. At this mine site, it is assumed that the drains will fail in the long term, and the facility has been designed to support this assumption. However, the question of how long the drains will need to be operational to support the lowering of the phreatic surface is critical. Seepage modelling to support this question is currently underway.

Schafer et al. [11] interviewed industry professionals on tailings dam closure. Notably, interview participants had conflicting comments regarding the progression of the phreatic surface over time [11]. Some participants indicated that it is expected that the phreatic surface will go down in the long term, while others indicated that there are a number of conditions under which this may not be true, such as the drains clogging [11]. This shows the importance of assessing drain performance over time, as it may have a critical role in how the phreatic surface changes over time, which ultimately may impact the stability of the structure.

Drains may fail in a number of ways, as shown in Table A2, which may include a breakage of the pipe due to buckling or physical degradation, clogging (chemical, biological, particulate, failure from the outtakes), clogging of the surround (biological, chemical, particulate), or a breakage of the connection between the drain and outlet pipe. This section focuses on the clogging of the level 2 drains due to particulate clogging.

As per the G-FMEA framework, the analysis should take place over four temporal scales: immediate-term, short-term, medium-term, and long-term. For this tailings facility, it is expected that the immediate-term and short-term period will fall within the adaptive management period, as the mine site will still be mining following closure of the facility. The medium-term period will fall within the reactive management period, and the long-term occurs at about 1000 years. For the purposes of this analysis, it was assumed that the drains were required to control the phreatic surface for the immediate-term, short-term, and medium-term periods. As such, failure occurs when the drain is no longer able to control the phreatic surface. For the long-term period, the drains were assumed to fail and are no longer considered to be a controlling feature of stability. Consequently, long-term risk assessment was not conducted for this failure mode.

The completed risk assessments for the immediate-term, short-term, and medium-term temporal scales for particulate clogging of the level 2 drains are provided in

Table 6. Immediate-term, short-term, and medium-term risk assessments for particulate clogging of level 2 drains.

Temporal Scale	Likelihood	Consequences				Risk Rating				Controls
		Element Failure Consequence	Human Intervention	Environment	Community	Element Failure Consequence	Human Intervention	Environment	Community
Immediate term	Very rare	Minor	Minor	Slight	Slight	L	L	L	L	No control needed
Short term	Rare	Minor	Minor	Slight	Slight	M	M	L	L	No control needed
Medium term	Possible	Moderate	Moderate	Slight	Minor	H	H	L	M	Modelling to predict drain performance and location of phreatic surface over time, monitoring of pore water pressures

. As the temporal scale increases risk assessment, there is an increase in likelihood of the drains failing due to particulate clogging, from very rare in the immediate term to possible in the medium term.

The consequences also change as the temporal scale increases. The phreatic surface has continued to drop in the facility. For the purposes of this work, it is assumed that the phreatic surface may be below the drain location at some spots throughout the facility at the time of closure. This hypothesis should be supported by modelling and onsite monitoring, which are beyond the scope of this work. Using these assumptions, it is expected that the failure of the level 2 drains may result in a minor rise in the phreatic surface, but this is not expected to result in global failure. In the immediate term and short term, it is expected that failure of the element may have cascading consequences. However, these cascading consequences are not expected to result in global failure due to other controls in place during the adaptive management phase, resulting in a minor consequence rating. This is coupled with the potential for minor or localized intervention, which is possible due to the presence of staff on site. Due to the expected consequences, a rating of slight was assigned in the environment and community categories.

In the medium term, if the facility requires the level 2 drains to function for stability, it is expected that failure could result in a rise in the phreatic surface, an increase in seepage, and formation of ponds on the reclamation surface, but these consequences are not expected to result in global failure, resulting in a moderate consequence rating. However, this failure may require human intervention or maintenance to limit the extent of the cascading consequences (i.e., if extensive ponds formed on the reclamation surface), resulting in a moderate consequence rating. A slight consequence rating was assigned to the environment category as no tailings are expected to move beyond the footprint from this failure. It is expected that there may be a short-term impact on the local community if ponds form on the reclamation surface, depending on the end land use, resulting in a minor consequence rating. Modelling must be conducted to determine the influence of drain failure on the phreatic surface and if drain function is required to maintain stability in the medium term (i.e., to keep the phreatic surface low such that the potentially liquefiable units de-saturate). Modelling is an effort to reduce the uncertainty associated with the risk assessment, not necessarily reducing the likelihood or consequences. However, understanding the failure mode may result in the likelihood or consequence rating being reduced. Following the completion of the modelling, more targeted controls to reduce the likelihood and consequence can be developed, if needed.

As shown in Table 6, this results in risk ratings that increase as the temporal scale increases for the failure consequences, human intervention, community, and end land use consequence categories. The risk rating for the environment consequence category remains low for the different temporal scales as tailings are not expected to go beyond the facility footprint due to a failure of the level 2 drains.

3.3.2. Surface Erosion of the Berm

The berm consists of lean oil sands with benched slopes to provide stability. The function of the berm before and after closure is to provide stabilization against liquefaction due to a weak tailings layer present at the base of the facility. Failure of the berm (instability) may occur due to a number of different failure modes, as shown in Table A2, including vertical deformation, seismic liquefaction, static liquefaction, shear failure, internal erosion, surface erosion, and toe erosion. This section focuses on surface erosion from wind and overland flow.

Unlike the particulate clogging of the level 2 drains, the risk assessment for the surface erosion of the berm was completed over all four temporal scales. The completed risk assessments for all four temporal scales for surface erosion of the berm are provided in

Table 7. Immediate-term, short-term, medium-term, and long-term risk assessments for surface erosion of the berm.

Temporal Scale	Likelihood	Consequences				Risk Rating				Controls
		Element Failure Consequence	Human Intervention	Environment	Community	Element Failure Consequence	Human Intervention	Environment	Community
Immediate term	Possible	Minor	Minor	Minor	Slight	M	M	M	L	N/A
Short term	Possible	Minor	Minor	Minor	Slight	M	M	M	L	N/A
Medium term	Likely	Moderate	Minor	Minor	Minor	H	M	M	M	LEM to evaluate geomorphic evolution, monitoring, maintenance thresholds
Long term	Likely	Moderate	Moderate	Moderate	Moderate	H	H	H	H	LEM to evaluate geomorphic evolution, monitoring, maintenance thresholds

. It is common practice during operations for erosion gullies to be repaired on an ongoing basis as they develop in the downstream, bare sand slopes. Similarly, erosion events may be less critical in the immediate term and short term due to the opportunity for maintenance.

Slingerland et al. [12,13] conducted a study to investigate erosion of a sand dam in the Alberta oil sands using a Landscape Evolution Model (LEM) to aid in assessing the long-term geomorphic stability. The results of the study for a 200-year simulation period indicated that the morphology of the structure was well established within 60 years, with gullies most frequently forming in areas of concentrated flow (i.e., horizontally concave dam sections) and retrogressively travelling inward towards the pond [12,13]. The development of erosional features on a dam may be impacted by the dam height (including the length of the slope), the grain size distribution of the material, and the climate [13]. The results showed that some potential areas of concern in the long term may be substantial sediment discharge off-site, the formation of deep gullies (greater than 15 m deep), and the filling of drainage channels [13]. The results also suggested that it would be advantageous to grade dams such that surface water is collected and directed toward armored swales and transported to the perimeter channel, as opposed to having a uniform slope that allows sheet flow [13].

Slingerland et al. [12,13] also showed that vegetation played a huge role in reducing erosion. Overall, major gullies started to form before the vegetation matured, with no major gullies forming following vegetation maturity [12]. Erosion gullies began to form near the bottom of the slope as the soil typically has a higher water content, resulting in the flow transitioning from dispersive flow to centralized, efficient flow paths [12]. This centralized flow allows the water to gain speed and increases its erosive power [12]. Erosion gullies also formed from surface water being directed and concentrated in one location and then finding the easiest route to travel [12]. Overall, the results of the study indicated that erosion in the long term may result in the structure not functioning as designed, resulting in failure of the structure to retain tailings. This indicates a potential need for ongoing maintenance of the structure.

The section being analyzed (and taken to be representative of the facility) has mature vegetation on the downstream slope. Consequently, it is expected that erosion events will occur with a ‘possible’ likelihood in the immediate term and the short term. As time progresses, it is expected that the likelihood will change from ‘possible’ to ‘likely’ as climate change occurs, leading to more extreme events and a transition of vegetation. Assessing the likelihood of surface erosion in the long term is further complicated by the potential for anthropogenic activities, which may alter the projected landscape and amplify the likelihood of surface erosion (i.e., clearcutting all trees on the downstream slope).

Table 7 shows how the risk rating increases as the temporal scale increases in the different consequence categories. In all cases, it is not expected that erosion of the berm would result in a global failure of the facility. For the element failure consequence, there is an increase from a moderate risk rating to a high risk rating due to a change in the consequence category rating. Reviewing Table 2, it can be seen that a consequence rating of minor and moderate have the same definition for the element failure consequence category. The moderate consequence category was assigned for the medium term and long term due to the unknowns associated with these time frames. For the human intervention category, there is a change from a moderate risk rating in the immediate term, short term, and medium term to a high risk rating in the long-term. This is due to a moderate consequence rating assigned in the human intervention category for the long-term temporal scale, due to the potential need to employ intervention or maintenance to limit cascading consequences and prevent global failure. In the environment category, the risk rating changes from moderate in the immediate. term, short term, and medium term to a high risk rating in the long term. This is due to a change in the consequence rating from minor to moderate. It should be noted that this assumed that the tailings are not toxic. For the community, the risk rating increases from low in the immediate term and short term to moderate in the medium term to high in the long term. It is assumed that no fatalities will occur due to a failure of the element, but that there may be short-term impacts to the community as a failure may impact the post-mining land use, require intervention and maintenance, and result in tailings being carried away from the footprint, which has the potential to impact fishing and so on.

For controls, this failure mode should be analyzed with an LEM to evaluate geomorphic evolution, monitoring to evaluate erosion progression in the vegetated slope and confirm if it aligns with the LEM, and defined maintenance thresholds, if necessary.

4. Discussion

Risk management consists of two main components, including risk assessment and risk control, and is a systematic process where risks are recognized and assessed, and appropriate control measures are implemented [14]. The G-FMEA framework outlines the steps for conducting both risk assessment and risk control. Step 8 of the process involves assessing appropriate risk mitigation options based on the risk rating for each temporal scale and then repeating the risk assessment process as needed. Controls are briefly discussed for the two different failure modes that had risk assessments completed using the risk matrix. These controls focus on two key elements, including detailed modelling and monitoring and maintenance. The monitoring and maintenance requirements must be clearly outlined and must be in line with the long-term closure plan. This means that if the long-term closure goals include walk away closure with no maintenance, then the use of monitoring and maintenance as a control is not appropriate, in contrast to a closure plan that includes targeted monitoring and maintenance. Once the risk mitigation/controls are outlined and completed (in the case of modelling), the risk assessment can be repeated to re-assess the risk. This process may result in a need to alter the closure plan to adequately reduce the risk. The success of risk mitigation methods may depend on where the ETF is at in its life cycle. The earlier risk management practices are employed in the closure planning process, the easier it is to make meaningful changes that will positively impact the long-term risk. For example, Slingerland [15] investigated the impact of different dam slope shapes on surface erosion using LEM and found that the shape of the downstream slope had a substantial impact on the erodibility of the slope, where uniform slopes contribute to the development of focused drainage patterns and thus an increase in erosion. Slingerland [15] noted that implementing these dam shapes into the initial dyke design is an important element to success. Once the dam is farther into its life cycle, these types of design changes to aid in mitigating the risks associated with erosion would be more difficult to employ, and maintenance controls may need to be relied on more heavily.

While the G-FMEA was not completed in full for the case study dam, the application of the framework was demonstrated. Completion of the G-FMEA may indicate that some failure modes require additional risk assessment to adequately quantify the risk. The framework and example risk matrix outlined by Schafer et al. [2] states that a threshold can be used with the risk matrix where hazards that fall above the threshold may require further risk assessment methods. This is due to the known pitfalls of risk matrices. If a threshold value is adopted, careful consideration should be given to hazards that fall above the threshold, particularly those with a high risk rating, where as low as reasonably practical (ALARP) principles may be employed. Assessment of risk using risk matrices should always be conducted with awareness of the shortfalls of risk matrices.

Once the G-FMEA has been completed for all of the failure modes, this can be used to assess the success of the closure plan to support de-registration. If this is in the earlier stages of design, this process can be highly effective at making meaningful changes to the design of the facility and the closure plan to reduce long-term risks. This process would be highly effective if integrated into the process of landform design, which is defined as “the integrated, multidisciplinary design and construction of mining landforms and landscapes, directed by a dedicated team working with different mine operations groups and others over the life of the mine and beyond” [16]. Landform design has a focus on integration—including integration across time scales and disciplines [16]. The process of integrating risk management of the closure plan into the overall design of the facility and designing with the end in mind is the best tool for successful de-registration. The G-FMEA can then be re-visited and updated, as necessary, over the life of the facility. In all cases, an essential element is that the risk mitigation options are assessed and implemented with consideration of the long-term custodial transfer plans.

Once a facility has reached the stage of reclamation and closure, as with the oil sands dam presented in this case study, the completed G-FMEA can be used to help support the de-registration process. Manual 019 released by the AER in 2020 requires dam owners to complete risk assessment to support DCA plans and completion reports [1]. A requirement of the DCA plan is to assess risk using FMEA or a similar method, and this should include identification of hazards and potential failure modes, analyzing the hazards, assessment of the consequences and likelihoods, characterizing the overall acceptability of the residual risk, and redesigning to reduce the residual risk if needed [1]. The G-FMEA can be used for this purpose to aid in assessing and managing the risks associated with closure.

5. Summary and Conclusions

Globally, mining companies and regulators are working towards how to close tailings facilities in a responsible and sustainable way to achieve the goal of physical, chemical, ecological, and social stability, with no further risks to life or the environment, as outlined by ICOLD [17]. Getting to this point requires collaboration between industry, regulators, and academia. The Alberta Energy Regulator (AER) has developed new guidance for closure of tailings dams, referred to as Manual 019, which requires risk assessments that consider long-term physical failure modes [1]. To help support the effort of de-registering a tailings dam in Alberta, Schafer et al. [2] developed the G-FMEA Framework to evaluate the geotechnical risks associated with closure of external tailings facilities and assess if the closure plan is sufficient to support de-registration. The framework was developed through collaboration between industry, the regulator, and academia.

This paper shows how the G-FMEA framework developed by Schafer et al. [2] can be applied to a case study site. The case study site used for this work is an external tailings facility located at an oil sands site in northern Alberta. For the oil sands facility, the system definition and block diagram were developed and used to develop failure modes for the individual dam elements using the generalized charts from the G-FMEA. From there, select failure modes (clogging of drains and surface erosion of the berm) were used to demonstrate how the risk matrix can be used to assign risk ratings. The failure modes were assessed over different temporal scales to demonstrate how the risk may change over time. In both cases, the risk rating increases as the temporal scale increases (from immediate-term to long-term). In part, uncertainty contributes to the increasing likelihood, and modelling may be a useful exercise to aid in informing risk decisions. Full completion of the G-FMEA can be used to assess a closure plan based on the residual risk and if this closure plan will support the de-registration, which is a vital step to fulfilling long-term closure goals and satisfying the social license to operate. A level of confidence rating was not applied to the individual risk ratings as this work is currently being developed at the University of Alberta. The G-FMEA is intended to be used and applied throughout the life of a structure—from the planning phase to assessing risks associated with closure.

Limitations of the G-FMEA process lie in the tremendous amount of information required to do a comprehensive evaluation of a tailings facility for de-registration, especially at the conceptual planning stage when data is more limited. The G-FMEA also uses a risk matrix for evaluation of risk. Risk matrices have a number of pitfalls that risk matrix developers and users must be aware of that are outlined comprehensively in Schafer et al. [2].

The G-FMEA framework takes an element approach to evaluating the risk of a tailings facility and encourages risk assessors to evaluate the interactions between individual components. This is different from many FMEAs being conducted in industry that look at the tailings facility as an entire structure and evaluate global stability (ultimate failure), rather than breaking the structure down into individual elements and evaluating function failure of each element. While the overall process may be time intensive, it is necessary to evaluate the long-term risk of a tailings facility. Specifically, incorporating the G-FMEA into the early processes of design may allow for closure-based decision making to be used in the overall design process (i.e., if erosion is identified as being critical to long-term stability, the downstream slopes could be modified at the onset to minimize long-term erosion).

The overall G-FMEA framework may be adapted for other structures and jurisdictions, as needed. This would require an adaptation of the G-FMEA generalized charts, the likelihood table, consequence table, and risk rating presented in Schafer et al. [2]. Regardless, the overall methodology is transferable across the mining industry.