2008-04-15_REVISION - M1980244 (256)

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~_ SMITH WILLIAMS CONSULTANTS, INC. hardening parameters were based on settlement data from the SGOSA, which contains similar materials as the AGOSA. The Carlton Mill tailings were modeled as a stiff sandy soil, with an effective friction angle of 29 degrees and no cohesion. The modulus was set at 4,900 psi, which corresponds to a consolidated tailings based on data presented in Qiu and Sego (2001). These properties are considered appropriate for the tailings, which have been in-place for over 12 years and are covered with 100 to 300 feet of overburden materials. The results of the settlement calculations are presented in Appendix C. The results of the analysis indicate that the anticipated vertical settlements along the regraded AGOSA slopes will likely be less than 12 inches under the applied ore loading. The anticipated strain in the geomembrane liner is estimated to be less than 2 %, therefore the composite liner system will not be impacted by settlement of the AGOSA under the design loads. 5.3 Site Surface Hydrology Surface flows resulting from storm events will be controlled and routed around the Phase 5 VLF • using surface-water diversion structures. The size and geometry of the surface-water control structures- are based on storm-water flows from the 100-year/24-hour storm event for both the operational and post closure VLF configuration. There are two primary surface water diversion structures for the Phase 5 VL- F design. One structure is located north of the Phase 5 VLF and consists of a sedimentation pond (see Drawing A510). This pond will collect water diverted from existing surface water channels. The other structure is a diversion channel located along the southeastern edge of the VLF, and will route surface water along Squaw Mountain. The estimated flow volumes and peak flows throughout the site were calculated using the Natural Resources Conservation Service TR-55 method for small watersheds. The total - catchment area during operations along the north end of the VLF is 74 acres, while the catchment for the south end is 9.1 acres. Operational and post closure sub-watershed areas and diversion channel flow paths for the models are shown on Drawings A500 and A610. Mining is a dynamic activity, and the topography within the contributing basins may change daily. From time to time, it may be necessary for CC&V to re-grade areas, super-elevate haul roads, and redirect runoff to other areas. These activities will cause the operational surface-water drainage patterns to be different from those shown on the drawings. The operational sediment S:\PROJECTS\1725 CCBV PHASE 5 VLF\H2 - DESIGNWREQUA VLF\VLF PHASE 5 FINAL REPORT V2.DOC 37