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~ iii iiniiiiiiiu iii STATE , OF COLORADO DIVISION OF MINERALS AND GEOLOGY DepartmeN of Natural Resources 131 i Sherman Si.. Ruom ? 15 Denver, Coloraria 80207 DIVISION OF Pnone: ao7) aeb-7sa~ MINERAL S FAX: 1303) A}2-8I Ob & GEOLOGY REC LA MA710N MINING•SAFETY DATE: September 6, 1999 Bill Owens TO: Gregg Squire Governor Greg E. Walther E.ecwrve Dvenor FROM: Allen Sorenson Michael B. Long Dimuon Director RE: Conditions for Approval, Connell Resources, Timnath Connell Pit, File No. M-99-050 Based on a review of the responses to the Division's adequacy review, the following is a discussion of the issues to be resolved through conditions placed on the application approval and a list of proposed stipulations. Ground Water Interception and Delivery The applicant has committed to the installation and operation of a ground water interception and deiivery system to maintain ambient water table elevations in the vicinity of [he pit. This plan, if properly installed, maintained, and operated, will be effective. However, the plan is qualitative and may be difficult to enforce. For example, the plan states that "The six monitoring wells installed on site will be used for monitoring ground water levels for the operation of the drain system," but provides no criteria for triggering the routing of ground water or water pumped from the pit to either the infiltration ponds or the river. It is my understanding that the Party to the Board's consideration of this application is satisfied with the plan, thus providing a sound basis for the Division's recommendation to approve. Slope Stability Issues The applicant has provided stability analyses and recommended mining setbacks for the roads and irrigation ditches at the pit. The applicant provides ajustification for assumptions used in the stability analyses that states, in part, that "The critical slope failure for a gravel highwall will occur in the gravel and not in the bedrock." While the Division does not agree with the applicant's rationale relative to the potential for bedrock failures, it has been determined that the proposed setbacks are adequate for stability. It should be noted that although the irrigation ditches should be stable against slope failure with the 25 foot setback, mining and dewatering within ZS feet may increase seepage losses from the ditch. It is also important to note that the 35-foot setbacks specified for protection of roads are to be measured to the right-of-way lines, not necessarily to the edge of the roadways. For future reference, a discussion of the potential for bedrock involvement in slope failures at gravel pits is provided here. When shale bedrock is unloaded by extraction of overlying material, stress relief