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~. DIVISION OF MINERALS AND GEOLOGY Depanmenl of N'~wral Resources 1313 Sherman 5t., Room 215 Drneer. Culorado 80'03 Phune U031866~35G7 FT\: ~303i 83'-8106 DATE: June 21, 1999 T0: Gregg Squire FROM: Allen Sorenson•r~~ STATFr OF COLORADO DIVISION OF MINERALS GEOLOGY P.EC LA%AiION MIN ING•SAFETY Guvemur C~rg E. Wdlchcr E,ecmme D~~caur Michael B. Long D~rls~nn D~rcclo~ RE: Reservoir Lining Plan, Connell Resources, Timnath Connell Pit, File (\'o. iVI-99.00 The following issues were identified during a review of the permit application for the Timnath Connell Pit and remain to be addressed for the Division to recommend approval of the application. This review was limited to assessing exhibits to the application intended to assure that reclamation would achieve the post mining land use of developed water resources as a lined pit, to assessing the stability of structures within 200 feet of the pit, assessing impacts to the water table, and long term stability of the reclaimed pit. The Division can stipulate to the stability of the reclaimed 3:1 reservoir slopes both when the resen~oir is filled and during rapid draw down. However, the applicant should rescind the statement in Exhibit E that slopes more than 10 feet below or 5 feet above the water line may be constructed at 2:1. This would not appear to be a hardship in that the pit is planned to be 16 feet deep. The applicant must provide a demonstration that the depth to bedrock will not significantly exceed 16 feet in any areas of the pit's perimeter. This demonstration is necessary to evaluate pit slope stability and backfill volumes for bonding purposes. 2. The analyses provided in the application do not consider the stability of highwalls created during mining and prior to backfillin~, nor do the analyses examine the potential for adverse impacts on structures within 200 feet of the limits of mining. The applicant should provide stability analyses for the critical pit cross-section that would occur during mining or obtain compensation agreements with the owners of structures within 200 feet. If a stability analysis were provided absent shear strength testing of the soils and bedrock, the Division would have an issue with the soil properties listed in Table I to the Slope Stability Analysis section of the application. The alluvium friction angle of 32 degrees would be acceptable, and the bedrock strength should be analyzed considering the residual strength of the material, which would be much less than the strength listed in Table 1. Analyses considering the residual strength in the bedrock should be fur[her analyzed for non-circular failure surfaces. 3. The 5126/c.y. unit cost in Exhibit L for installation of the compacted low permeability reservoir slope backfill may be low. This cost should be evaluated considering the need to dewater the work area, rip and excavate a keyway. rip and excavate a percentage of the backfill material from the pit floor, to moisture condition and place the backfill in thin lifts. and to provide for