2021-12-07_REVISION - M2012032 (25)

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only performed during the summer and fall (i.e., June through November). Haul trucks will transport tailings or waste rock to the Atlas TSF during the winter months. The TSFs will be regraded into the necessary lifts each summer. Monitoring wells are placed on the north(downhill side) of each tailings pile as shown on Map G-1. These groundwater wells are sampled quarterly for field parameters and a water chemistry suite analysis. More detail about groundwater sampling may be found in Exhibit G. The results of these tests and their evaluation will be incorporated in the DRMS Annual Report. Also, sample test data from any sediment pond discharges associated with the Atlas TSF will be evaluated in the Stormwater Annual Report. Dozers will compact the tailings/waste rock in 12"lifts to a 94%maximum dry density. A secondary compactor will be used,if needed to achieve the proper compaction. Quality control density samples of the compacted materials will be conducted using a field density instrument every 2000 tons of tailings that are placed. As described in the Mine Plan and the Reclamation Plan(Exhibit E),the piles are graded to slopes that are no steeper than 3H:IV. A slope stability analysis of the worst-case tailings pile was performed as part of the Amendment 1 application. A copy of this analysis also was provided as Attachment 3 to TR-15. Based on the apparent stability of the existing un-reclaimed waste rock pile slopes in the area,a detailed slope stability analysis was not performed on this material under the original design. Many of these older mines have slopes as steep as 1.2H:1V compared to the 3H:1V proposed minimum slope of the Revenue and Atlas TSFs. The existing Revenue TSF has maintained slopes of approximately 1.1H:1.OV for over 120 years with no signs of slope instability. This is due to the blasted rock having extremely angular pieces,which are prone to being stable at steep angles due to the inherent angle of friction of this material. The internal angle of friction of this material is approximately 45 degrees. Since the waste rock has very little fine material,the water seepage through is expected will have no effect on the stability of the pile since there is no cohesion in this waste rock, only internal angle of friction. A slope stability analysis was submitted as Exhibit T to Amendment 1,which presents a slope stability analysis using commonly accepted circular failure equations and material parameters for both the Revenue TSF and the Atlas TSF. The analysis indicates that the waste embankments have a calculated safety factor above 1.3, using conservative strength parameters,which constitutes an adequate design. Geotech on slope stability on tailings produced by the Revenue Mill will be redone by a private consulting firm once the mill is producing tailings. Map C-lb shows the avalanche path that covers a large portion of the Revenue TSF. The Atlas TSF is located in an area that is lower risk from avalanches whereas the Revenue TSF area is in a high risk area for avalanche activity. A plan has been incorporated to safely trigger these avalanches using an experienced contractor. 5.2.1 Revenue TSF The Revenue TSF has a total capacity of approximately 811,000 tons. The Revenue TSF will be constructed in one-foot lifts that slope towards the north at a roughly 2%grade. This will protect the embankment from damage from avalanches during the life of the mine. The Revenue TSF will be constructed to have maximum slopes of 3H:1V. At its full construction,the waste pile is expected to be 60 feet tall from the toe along Sneffels Creek to the highest point on top. The average height of the waste pile will be around 55 feet. However,most of this height will be provided by the existing material,not the new Revenue waste. The footprint of the Revenue TSF is shown on Map C-la,with the full pile build out shown on the cross sections in Map C-3. Revenue Mine Amendment 02 D-15 December 2021