1996-02-06_REVISION - M1977493 (11)

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Mr. Allen Sorenson Division of Minerals and Geology February 1, 1996 Page 5 Phase 1: A formal evaluation by the Division of CMC's report on the magnitude of the seismic event. Data for the Climax Mine are hereby submitted as an attachment to this letter. Please note that the evaluation references the Tenmile Dam (3- Dam) yet is applicable to the entire Climax Mine site. Phase 2: Once the magnitude and return probability analysis has been formally reviewed, CMC and the Division can discuss additional information needs. Regarding instrumentation and static stability evaluation on the tailing impoundments, CMC recognizes that piezometric level can be indicative of dam stability. Tailing facilities at Climax were designed with static stability as a design parameter. CMC views the request for static stability demonstration to be part of Rule 6.5 and will address this issue apart form the EPP review. In general, static stability analyses show that with a high phreatic the tailing dams at Climax maintain a reasonable factor of safety. If the phreatic surface were to approach high levels in the impoundment, however, other operational problems such as localized slumping would ensue. Climax operationally maintains water levels on the ponds to affect control of the phreatic surface. Because of the above discussion, Climax disagrees that piezometer levels on the Climax tailing impoundments should be an enforceable component of the EPP. CMC proposes that summary piezometer data be submitted to the Division on an annual basis. (d) Eagle Park Reservoir, Robinson Lake, Chalk Mountain Reservoir, and Buffehr's Lake are under the jurisdiction of the Office of the State Engineer. The Retention Pond, a small settling pond presently filled with sediment, was used for settling suspended solids in previous water treatment at Climax. It is not regulated by the State Engineer. 10. (a) CMC believes that this request is outside of the intent and purview if the EPP. (b) All three tailing impoundments at Climax will pass the Probable Maximum Precipitation (PMP) event under hydologic criteria used by the Office of the State Engineer. Our evaluation shows that the Robinson and Tenmile Tailing Impoundments are capable of passing 100% of the Probable Maximum Flood (PMF) under a worse case scenario. The worse case scenario considers both the snowmelt and thunderstorm PMF at pond storage limit with all interceptors washed out. The evaluation of Mayflower Tailing Impoundment shows passage of the PMF at 88% with the interceptors intact and 77% under the worse case storm conditions described above. The rules for Class I dams regulated by the Office of the State Engineer define a dam capable of passing 75% of the PMF with one foot of residual freeboard meets appropriate design criteria. Based on our evaluation using more conservative freeboard criteria, all tailing facilities can safely contain the inflow design flood. Static stability evaluations outlined under item 9 (c) of this letter may result in increased minimum freeboards.