2019-08-30_REVISION - C1981018 (2)

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Colorado Division of Reclamation, Mining and Safety To: Clayton wein From: Zach Trujillo 0 Jason Musick Date: August 30, 2019 De: Colowyo TD-72 Review s requested, I have reviewed Deser do Mine's Deserado TR-72 in regards to the engineering report associated to the proposed refuse pile DP-A. As discussed, this review pertains only to the engineering designs and principles provided for the proposed DP- and not the Rules and Regulations associated with waste banks. n behalf of Deser do, Huddleston-Berry Engineering &Testing, LLC H provided and engineering report (Deport) regarding the design of waste bank 1 P-A proposed by Deserado. As part of the Deport, a subsurface investigation was conducted which included a total of 14 borings as shown on Figure 2 in the Deport. Borings were drilled between a depth of 7 and 40 feet below the surface and were used to identify subsurface conditions and material properties of the soils. The result of these drillings showed no signs of groundwater and a soil profile consistent with the area as documented in Deserado's PAP. With this information, HB performed a series of slope stability analyses providing a combination of scenarios. This included steady state, circular and block failure planes, conservative material property values, and a saturation zone within the refuse pile as primary scenarios in the analyses. out of the six scenarios provided, only one result was below a safety factor SF of 1.5 which was an increased groundwater level within the refuse pile. The result of this scenario was a SF of 1. . while below the generally accepted FS of 1. , this scenario is highly unlikely given the site characteristics and still results in a stable refuse pile. HB in the Deport also states this. To ensure the stated values and variables were used in the analyses, I performed three stability analyses (attached Analysis 1, 2 and 3) as a check using the software GALENA. Using proposed Map 162, the critical profile (cross section : 1 +00 through 3 +00) was determined and used to construct the stability analyses along with the values stated in the Deport. The critical profile matches that which was used in the Deport analyses as well as the FS results for the similar scenarios (Analysis l and Analysis . Analysis 3 was an additional scenario added that included elevated water table within the refuse and seismic loading. The result of this was FS of 1.1 . Analysis 3 is considered very unlikely and was used as an extreme case scenario as an additional check. while very unlikely, this scenario remained stable.