2011-03-14_REVISION - M1977493 (43)

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CM0001787 • Mr. B. Romig January 22, 1997 Construction soil quantities presented in Table 4 were incorrect. A corrected Table 4 is included with this letter. The construction soil quantities are largely dependent on foundation excavation quantity and the percentage of the excavated material that can be reused in the Dam construction as discussed in Section 4.1.2. If the excavation quantity increases beyond the Dam footprint, or if the percentage of unusable material decreases, than additional construction soils may be needed from the established borrow areas or new borrow areas. Options for additional borrow areas are: increasing the depth and extent of the Tenmile Tunnel Portal Borrow Area; developing the Mosquito Fault area if the material has acceptable chemical properties; developing the area west of Robinson Lake in the vicinity of T?-D8;.or establishing a new borrow outside of the immediate area of construction. The Report assumed that sufficient quantity of riprap could be ripped from fractured bedrock at the bottom of the island borrow areas or other outcrops locations. However, the riprap borrow areas were not fully developed in the Report and blasting may be required to produce dam face riprap. Other Issues Not Identified In The Design Report: The following issues not addressed in the Report and are listed here for your consideration. The six percent of bentonite amendment needed for Zone 1 presented in Section 4.4 was estimated based on established references which correlate material grading and reagent additions to permeability. This percent addition was confirmed from the evaluations performed by WyoBen, Inc. of Billings, Montana. Foundation design assumed that all material below the Dam is unsuitable for the Dam foundation in its natural state and would be excavated and replaced with compacted suitable soils. As such, consolidation calculations were not required. If in-place material is determined to be suitable and not to requiring excavation, than consolidation of the foundation needs to be evaluated. The Dam will have some operation and maintenance (O&M) costs. The manually-operated valves will require some labor costs to operate, though the extent of 0&M costs are dependent on CMC's future operational plans. Typical O&M issues include monitoring the Dam for foundation subsidence, face sloughing and crest erosion. Siltation and the harsh environment created by the low pH seepwater and extreme weather conditions will require regular maintenance for: the conveyance pipeline valves, headwall concrete, pipe and discharge apron riprap; the spillway concrete and riprap; the Chalk Mountain discharge pipeline outlet; pipe and discharge apron riprap; the dam crest surfacing; and, the dam face riprap. CMC has indicated that they may consider a smaller scale collection/sump system to convey seepwater to the No. 1 Dam Pump Station as a more cost-effective alterative to Dam construction. Any alternate design should consider the Robinson Lake high water level of approximately 10,892 ft MSL. This elevation was the critical design parameter for the Dam crest elevation. Any structure with a lower crest elevation would not prevent the commingling of Robinson Lake water with seepwater if Robinson Lake was allowed to reach it's high water elevation. U:ISPELLMAIACLIMAX.MOLVM IMAX\DFSSUMLT.D I 11/22/97 - - - - -- V Environmental