PLATINUM GROUP METALS LTD

Figure 18-12: Dry Stack TSF Layout with Potential Extension Area

18.9.6 Water Management

The basin lining system of each phase will include the following components.

• 1.5 mm HDPE mono-textured geomembrane.

• Geosynthetic Clay Liner (GCL).

• Leakage detection system, comprising a granular drainage medium and slotted pipes to intercept and collect leakage.

An underdrainage system will be positioned on top of the HDPE liner along the upstream toe of the containment embankment and across the basin of each phase of the facility, collecting rainfall and runoff during the initial period of each phase, and seepage as each phase is developed.

Runoff from the side slopes will be collected on the step-in benches. The benches actively utilized by the haul trucks will be drained via bench penstock drains which will connect to the lined perimeter catchment paddocks. The runoff collected from the benches that do not function as roadways can be used for run-off attenuation and drained to the paddocks as required or left to evaporate.

Rainfall and runoff on the crest of the Dry Stack TSF will accumulate at a low point and be allowed to evaporate or infiltrate. Any excess water collected can also be pumped to the lower paddocks. The water collected in the paddocks and from the underdrainage system will be drained to the concrete-lined solution trenches which connect to the silt trap and low-lying stormwater pond.

Waterberg Technical Report

Clean stormwater runoff from the external upstream catchments will be diverted around the Dry Stack TSF by earth-lined diversion channels and berms.

As the phases are completed, the sidewalls will be rehabilitated with soil and vegetation cover. Dust will be continuously suppressed with water mobilized through the water bowser, which has a 13,000 L water capacity.

18.9.7 Water Balance on Dry Stack TSF

The daily stochastic water balance of the Dry Stack TSF was assessed throughout its development phases to determine the size of each stormwater pond. The assessment aimed to ensure that each pond would not spill more than once in 50 years under normal operating conditions as is legislated in South Africa.

The inflows and outflows considered for the water balance included the following conditions.

• Inflows: Entrained moisture in the tailings; rain falling directly onto the Dry Stack TSF, surrounding paddocks, and the stormwater pond.

• Outflows: Evaporation from all water surfaces, evapotranspiration of the Dry Stack TSF, infiltration into the tailings body and interstitial lockup, water returned to the processing plant, and water used for dust suppression.

The water balance indicated the following required stormwater pond capacities.

• Phase 1 stormwater pond: 45,000 m3.

• Phase 2-4 stormwater ponds: 300,000 m3.

The volume of water returned to the process plant will be influenced by the size of the catchment areas which will change with the development of the Dry Stack TSF phases and vary with the seasonal variations of rainfall and evaporation.

18.9.7.1 Historic Class C Liner

A Class C liner comprises the following items.

• 1.5 mm HDPE Geomembrane.

• 300 mm of compacted clay liner (CCL) or GCL.

• A leakage detection system.

A GCL was selected to replace the compacted clay as no available clay source nearby has been identified. A cost to supply and install the GCL and the HDPE liners was obtained.

18.9.8 Geotechnical Investigation

A geotechnical investigation of the TSF site was completed by Inroads Consulting Close Corporation in 2017. This included excavation, drilling, profiling of test pits and drill holes, sampling of soils, and the laboratory test work performed on the samples.

Waterberg Technical Report

The soils encountered at the TSF were characterized by transported soil of mixed origin, but mainly of aeolian provenance comprising silty sands of loose to medium dense and occasionally dense to very dense consistencies. The soil generally exhibits a pinhole structure, suggesting that it has the potential to undergo additional collapse settlement if loaded and subsequently wetted. The sand overlies talus and nodular ferricrete and occasional calcrete nodules or, where the latter are absent, it extends to the bottom of the pits at an average depth of 2.8 m in the range of 0.4 m to 5.8 m. No groundwater was noted in any of the drill holes or test pits; however, the investigation was completed at the end of the dry season in 2017. More test pits are recommended before construction commences and during the wet season to understand the potential for groundwater in the area.

18.9.9 Seepage and Stability Assessment

The stability of the TSF was assessed under various seepage conditions. The stability assessment was conducted as per the framework in the draft version of the South African National Standard Code of Practice for Mine Residue (SANS 10286:2022), which is based on the Global Industry Standard on Tailings Management (GISTM).

The results show that all Factor of Safety (FoS) values and phreatic surfaces derived from seepage analyses conform to the minimum requirements set by SANS 10286:2022. Based on the slope stability results, the proposed Dry Stack TSF design slope geometry is feasible as the stability FoS exceeds minimum requirements. Pseudo-static and residual undrained slope stability results conform with minimum requirements.

The strain in the liner was also investigated and it was shown that the peak tensile strains do not exceed 1.2% at the final TSF height. This is less than the maximum allowable tensile strain of 3%.

18.9.10 Recommendations for the Dry Stack TSF

The following recommendations are provided for the Dry Stack TSF detailed design phase.

• Confirmation of design criteria and site selection.

• Further optimization of the capital and operating cost estimate, where possible, by completing the following tasks:

• Developing a tender inquiry on the detailed design to acquire final construction and deposition rates.

• Further optimizing earth and civil works, where possible.

• Finalizing the responsibilities of the operator by incorporating input from all parties (contractor, client, and consultants).

• Further evaluation and possible optimization of geochemical risk in terms of liner requirements / details.

• Further geotechnical assessments of the collapsible soils, including impact roller testing to determine its effectiveness. Also, groundwater potential under the facility needs to be tested.

• Recommendation for continued monitoring of the risks relating to the following items.

• Collapsible soils.

• Severe desiccation cracking.

Waterberg Technical Report

• Advanced transient seepage analysis and the resultant slope stability analysis.

• Additional triaxial test work on the foundation soils.

• Test work to determine the interface shear strength between the HDPE liner and tailings and HDPE liner and in situ soils.

• The state of the placed tailings should be investigated with cone penetration testing to evaluate the tendency of the material to contract during shearing.