5.3 Discussion of Key Risks
5.3.1 Dust
Potential dust emissions associated with the construction of TSF Cell 1/2 stage 8 will be most likely to occur during:
Preparation of the existing Cell 1/2 embankment crest by removing the wearing course, scarifying, wetting and re-compacting the Zone A fill;
Raising the TSF embankment Zone A to the design levels and grades; and
Placing Zone E erosion protection material on the eastern margin.
To minimise the potential of dust emitting beyond the TSF perimeter, saline water will be administered via watercart. Northern Star recognises that Saline water used for dust suppression has the potential to adversely affect vegetation if plants are exposed to contained salt through over-spraying or runoff into vegetated areas.
The following management practices will be implemented to minimise the impact on surrounding vegetation:
Dusting is likely to occur during the handling of embankment materials, vehicular movement and the profiling of embankment walls. Material stockpiles will watered down during the rehandle phase to ensure that dusting is adequately managed. If local wind speeds are conducive to elevated dusting, construction works will be terminated until conditions improve.
Windrows will be constructed along roads and hardstand areas to prevent saline water from draining into the surrounding environment.
Watercarts will administer water via a roof mounted cannon to the Cell 1/2 embankment during the compaction phase. Wind direction and speed will be observed daily by the contract shift supervisor. If the prevailing wind direction poses a risk of overspray emitting beyond the facility perimeter watercart operations will be terminated until that risk abates. Wetting down of roads/access tracks will be undertaken with dribble bars, which is a Northern Star requirement for all watercarts working outside mining areas (i.e. Haul roads).
Water truck operators are instructed to avoid over spraying/watering and report any damaged bunding or saline runoff containment infrastructure.
5.3.2 Damage to vegetation through direct contact with hypersaline water or tailings from pipeline leaks, over topping or dam break
All pipelines will be double skinned PE100 and constructed and installed to Australian Standards AS4130 and AS413 and Plastics Industry Pipe Association of Australia Limited (PIPA) Guideline POP003. Only qualified poly welders will be employed to construct the pipelines and welding certificates will be retained on site to be presented upon request. Bunding has been constructed to contain spills and leaks for the tailings pipeline in accordance with Licence condition 1.2.1. All tailings and dewatering/process water pipelines are connected to Citect Process Monitoring System which monitors water levels and pressure in tanks, dams and pipelines. The pipelines and TSF will be inspected twice per 12 hour shift as a part of daily processing plant operations. Inspections undertaken at the frequency stated above either meet or exceed DWER licence condition 1.2.6. All these management actions are aimed at preventing spillage of hypersaline water and/or tailings from the pipeline.
In the event of a hypersaline water or tailings spill, telemetry will automatically shut off pumps and isolation valves will be closed by site personnel. Bunding will assist in containing the spill. If bunding fails to contain the spill as a result of a major pipeline break and/or the failure of the above management actions, the tailings flow or return water flow would be stopped. Earthmoving machinery will be mobilised to contain the spill. Environmental personnel will assess the extent of the contamination and, if necessary, contaminants removed using earthmoving equipment and remediation of the area completed (ripping, seeding) if required.
Any spills will be reported to DWER within 24 hours as per Operating Licence condition 4.3.1. A detailed written report describing remedial actions and any other information requested by DWER will
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be prepared and forwarded to DWER within 7 days of the incident. Monitoring of the spill site would continue quarterly for a minimum of twelve months, and all monitoring results will be reported in the Annual Environmental Report.
TSF Cells 1/2 Stage 8 has been designed with a minimum top of embankment freeboard of 300mm to prevent over topping by tailings or incident rainfall. The level of tailings will be monitored visually on a daily basis. Cells 1/2 Stage 8 will be used in conjunction with the existing Cell 3 if emergency events are encountered.
The TSF has been designed in accordance with DMIRS and ANCOLD guidelines to minimise the risk of dam break (wall failure). Construction will be supervised by a suitably qualified engineer to ensure it is constructed according to design. Operational, inspection and monitoring conditions are in place to minimise risk of dam break.
Knight Piésold conducted an initial dam break study as part of the design of Cell 3. Modelling was conducted on the final embankment height of 22m when the TSF will be at its maximum storage volume to determine worst case scenario. The assessment was reviewed to specifically model Stage 3 capacity (Knight-Pièsold, 2016). Modelling indicates the following:
A dam breach could occur on either the northern or southern embankment of Cell 3;
A tailings storage volume at Stage 2 capacity (worst case scenario) is estimated at 7.7Mm3 at an embankment height of 15.5m;
An estimated outflow volume as a result of embankment failure of 2.56Mm3 based on the Rico 2007 regression;
The localised detailed topography around the immediate vicinity of the TSF (Cell 3) was assessed for the case of a dry condition embankment slump type failure. An outflow volume of approximately 2.56Mm3 with a failure breach slope 1V:180H is shown below in (Figure 9).
For the breach occurring on the northern embankment, the predicted impacted area is 251ha and the run out distance is approximately 3km. For a breach on the southern embankment the area of impact is approximately 65ha and the run out distance is 1km.
Embankment failure modelling indicates that the ANCOLD Consequence Category remains unchanged as a ‘High C’ due to the risk of at least one life being lost in the event of a failure.
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Figure 9: Modelled outflow of tailings in a dry slump type embankment failure at Cell 3 (Knight Pièsold, 2016)
Golder Associates Pty Ltd conducted an updated dam break study as part of the design of Cell 1/2 Stage 7 in August 2019. Modelling was conducted on the final embankment height of 378mRL being filled to capacity to determine worst case scenario. A review was completed on the adequacy of the dam break safety bund which exists between the Whirling Dervish Pit (and operational underground mine) and TSF Cell 1 (as this is the only cell from which a dam break scenario could affect the underground mine). Modelling indicated the following:
A dam breach could occur from the Cell 1 area which could affect the underground mine in Whirling Dervish;
A tailings storage volume at Stage 7 capacity (worst case scenario) is estimated at 12.3Mm3.
An estimated outflow volume as a result of embankment failure of 2.6Mm3 has been assumed;
A dam break bund has been constructed between the Whirling Dervish Pit and the TSF to account for a dam break scenario;
The dam break safety bund has adequate Factors of Safety to withstand dynamic forces for sliding to a factor ~6;
The bund has been constructed to ensure there is 0.5m freeboard in the event an embankment event occurs.
To ensure that operation of Cell 1/2 Stage 8 is safe and meets best practise, monitoring of groundwater at existing downstream monitoring bores will continue at the prescribed licenced frequency. Existing piezometers will continue to be monitored monthly to assess embankment stability. New crest settlement survey pins will be installed prior to operation to track movement of the embankment during deposition. Modelling is currently in progress for impacts associated with the Stage 8 raise of Cells 1/2, which may result in the dam break bund being raised again. This will be submitted through an associated Mining Proposal and Closure Plan to DMIRS for assessment and approval.
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Figure 10: Modelled outflow of tailings in a dry slump type embankment failure at Cell 1 (Golder, 2019).
An Emergency Action Plan has been developed in the unlikely event that a dam break event occurs.
The Plan is incorporated in the Tailings Storage Facility Operating Manual which will be revised in parallel with the construction of the Stage 8 lift of Cells 1 & 2.