1. Tailings Dam Stability
Environment Theme –
Tailings Dam Stability
May 2016

2. Purpose of Tailings Management
Understand the characteristics of your tailings, protection of wildlife, groundwater, uncontrolled releases, management of process fluids and closure and rehabilitation of TSF’s.
Link to protocol 9
The purpose of tailings management is to Understand the characteristics of your tailings, protection of wildlife, groundwater, uncontrolled releases, management of process fluids and closure and rehabilitation of TSF’s. These themes are further defined in the Evolution Environmental Protocol 9 available on QHSE. Each site has a management plan which links back to the Protocol along with site based procedures and inspection forms to ensure we are acting to ensure tailings dam stability and prevent environmental harm.
Link to protocol 9 in the slide.
The ANCOLD Guidelines are published by the Australian National Committee On Large Dams. State guidelines are developed from the ANCOLD and would be referenced by each site TSF Specialist engineer. The ANCOLD set of guidelines provides for risk assessment, environmental management, monitoring instrumentation to name a few.
ANCOLD – National Guidelines Point of Reference

3. Hazards and Consequences
Tailings embankment stability hazards can present immediate and long term risks which require a considered management approach
Potential Impact – Environmental, Social and Economic
Air, soil, surface and groundwater contamination
Inundation Impacts to regional communities
Cessation of processing due to lack of tailings storage
Global failure examples exist including large companies operating in contemporary jurisdictions getting it wrong
Tailings embankments are risk assessed slightly different to conventional dam embankments due to their construction generally being non continuous (ie subsequent lifts/augmentations) and the content of the storage being hazardous to the receiving environment as well as subject to wetting and drying cycles proving the storage to be dynamic in embankment and contents. The storages present immediate and long term risks for example an immediate risk could present itself as dust blowing off the TSF during a drying cycle which contains metals later settling in the receiving environment, long term risks are generally unseen for some time without monitoring instrumentation or other specialist dam reviews. For example embankment piping where water slowly seeps and saturates the embankment and finally the toe of the storage or groundwater.
Though the construction and operation of TSF’s is highly regulated there are still notable examples of TSF failures across the globe in highly regulated jurisdictions. Some recent failures include Mt Polly in British Columbia, The Brazilian dam failure of 55M m3 just last year and in the first week of January this year a storage in the Hunter Valley of NSW of which all had embankment failures for various reasons. The Mount Polley video attached provides a good example of how these events affect the receiving environment and community both socially and economically. Please watch. 3min video.
Link to Video – Mount Polley Mine Tailings Breach – click here

4. When should we consider Stability?
Siting, Design, Construction, Operation and Closure and Rehabilitation Phases
Understand your Risks – Teams involved in TSF Management include Planning (multi-disciplinary), Processing, Mining (Tech Services), Geotechnical, Environmental and Safety
Inspections/Audits - Daily, Weekly, Monthly, Quarterly Annually
Risk Example: Upstream raising is cost efficient however has a greater reliance on tailings to provide stability
The last slide considered why it is important to observe dam stability and this should be carried out at all stages of the dam life from cradle to grave.
To understand the risks a team of specialist knowledge is required as with processing, mining, exploration or any other aspect of our business there are many areas of risk to consider.
The undertaking of inspections is an administrative control which aims to ensure that higher controls such as engineering and prevention are implemented for the safety of the structure and prevention of environmental harm.
The image on screen is a simplified example of three types of construction for earth dams. As you can see the base example is of a downstream lift, this embankment type offers the greatest stability as it is the largest at base and not subject to displacement as readily as the centreline or upstream lifts can be. When considering which embankment type is suitable for a site the topography, embankment material, climatic conditions, potential for seismic activity among other factors are all taken into consideration. The upstream lift is given the highest risk on the scale as it relies the most on tailings for stability. As mentioned earlier tailings are subject to wetting and drying and this is required for consolidation. When tailings consolidate they shrink as water is removed, the way in which tailings shrink is dependant on the composition of the tailings for example how much clay is in the tails and how does the clay act? Is it a swelling, cracking clay? Imagine the tailings under the embankment shifting and how this impacts on the upstream embankment stability and we begin to understand at a conceptual level why the risk is increased with this type of storage.

5. Subsurface Hazards *Can also be Pit deposition related
Embankment Piping – Common due to embankment saturation, weakened foundation and saturated tailings
Anthropogenic Springs – surface dampening and flows which surface after dam operation outside of the dam surface area
*Can also be Pit deposition related
Beneath the surface of the tailings water is separating from tailings. Managing the viscosity of the tailings prior to deposition is important to reduce the effects which water can have in the storage. This can be achieved by maximising the effectiveness of a thickener, spigotting for maximum evaporation potential or placement and utilisation of decant waters among a few.
What the image is demonstrating is water finding its way through the foundation of the storage so, under natural ground where it will express itself in groundwater bores, or quite often show as dampness or flow at the toe of a storage. Springs have also been reported as developing close to TSF’s which contain TSF relevant waters. Waters can soak the embankment also and cause stability issues. Any dampness or seeps, flows on the downstream batters must be noted and remediated immediately.
The issues shown here can also present in Pit type storages, Pits are the lowest risk storage as they are built into natural ground providing greater compaction and stabilty than is possible to achieve in an earthen dam however, water can find its way into conduits below the surface as seen in working Pit walls and underground where seepage is noted. The hydraulic conductivity in the vicinity of the Pit and earth dams is important to understand prior to tailings deposition as moisture contained in natural ground and in the TSF can act as a barrier to flows this due to the consolidation of talilngs preventing water movement.
As mentioned up front evaporation and tailings void of as much water as possible are the best prevention methods to seepage issues – remove the water prior to it becoming a risk for management.

6. Surface Visible Hazards
Longitudinal cracking – Maintenance required can lead to piping and ultimately wall failure
Lateral cracking - ½ crest width Rule triggers reporting to regulator (Qld)
Erosion – 30cm Rule; any erosion greater than 30cm deep or wide requires guided maintenance
Water ponding on embankments – increases hydraulic pressure
Longitudinal
It is always advisable to remediate a seen hazard prior to it increasing in risk magnitude. Visible hazards on TSF embankments include erosion and cracking as well as seepage and unintended moisture (not from rainfall).
Longitudinal cracking is cracking that runs parallel to the crest of the embankment. This provides a conduit for water/rainfall runoff which will deepen and widen over time. The conduit allows for soakage of the embankment as seen in the earlier slide. Longitudinal cracking is an indicator of water ponding and the potential for greater issues because of this. Any cracks should be taken back to solid ground and re filled to an engineered standard. If crests are constructed with a camber or graded slope and maintained the likelihood of cracking can be reduced. Cracks can also be an indicator of poor engineering sub surface of which technical advice should be sought.
Lateral cracking is cracking from the upstream to the downstream batters via the crest. In Queensland this type of failure indicator must be reported to the regulator once it extends greater than half way across the crest. Remediate and regular maintenance is always preferred.
Tailings beaching must be achieved to maintain a dry upstream batter on the embankment. The beach essentially provides another layer of embankment and as shown with upstream lifts should be kept at a consistent moisture to ensure stability i.e. lack of movement of engineered embankment over the tailings bed due to lack of movement in the tailings bed.
Any leak detected outside of the storage is a concern. Seepage systems which are built into the design/embankment and embankment toe direct seepage in a controlled manner toward seepage ponds for management. Seepage which does not conform to the engineered seepage system design requires immediate attention. History has shown seep sites progress to total dam failure within hours as a worst case scenario.
Leaks – not related to engineered seepage systems
Lateral

7. Monitoring Monitoring is guided by the site TSF Operating Manual
Regular surface survey to maintain wet and dry cycling
Processing personnel required to inspect and report daily on visible hazards
Regular (Monthly) detailed inspections as per site specific forms performed by suitably qualified persons
Annual Inspections to be complete by Registered Professional Engineer Queensland (RPEQ) Qld Only , other states as per guidelines
As per slide
Sub surface water table monitoring – embankment and toe, monitoring bores local and regional also
Third Party Audit – Group to Coordinate in FY17

8. Prevention – Maintenance
Any remedial earthworks must be performed as identified
Guided deposition – for example spigotting, central decant, thickened tailings deposition must be undertaken that the storage is able to perform as per design to:
Keep water off embankments
Allow tailings to consolidate
Reduce unintended seepage/piping effects
Consideration of any Audits, Inspections and the Annual Engineering Report Recommendations is essential – the latter noted as a leading cause of the Brazil Tailings Disaster where 55M m3 of tailings entered the receiving environment
As per slide

9. Rehabilitation
The purpose of rehabilitation is to return the site to an agreed land use after mining.
The rehabilitation of tailings dams is required to be safe, non-polluting, self-sustaining and geotechnical and geochemical stable.
Progressive Rehabilitation commences at construction – eg: establishment of ground cover (grasses) on embankments
*Wooded vegetation not recommended as in time roots can create a hazard to stability – the voids left by rotted roots can provide a conduit to seepage and piping.
As per slide

10. Questions PJO Processing Plant TSF Janet A PIt Scott lode pit
Image of the north west portion of the Pajingo Mine lease. Shows the Pajingo Mine three tailings storages. From the foreground Scott Lode Pit (13ha) has been used as a tailings storage since 2000 followed up gradient by Janet A Pit (2.5ha) which ceased mining in 2012 and commenced thickened tailings deposition in April The TSF was the original storage for PJO and has been in use since 1989 – the storage is at Stage 7 (18ha) and has approval for another 5 x 3m upstream lifts. PJO plan to cycle between the three storages for the current life of Mine.
Scott lode pit