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TETRA TECH RMC Mr. Jeff Gregg January 28, 2003 Page 5 resultant factor of safety was 1.6 without the surcharge and 1.9 with the surcharge (identical to that found at Phase 5 above). Phase 8 -Phase 8 will be surrounded by a slurry wall on all sides, except for the east near the Cole Ditch. The slurry wall scenario modeled was based on drilling information in the southwest comer of Phase 8. The profile modeled consisted of eight feet of overburden clay overlying 34 feet of sand and gravel on top of claystone. The slurry wall was setback 50 feet from the mine highwall and the Godding Hollow Ditch was 25 feet from the slurry wall. The factor of safety was 1.8 without the surcharge and 2.0 with the surcharge. For the east boundary of Phase 8, we modeled eight feet of overburden clay over 30 feet of sand and gravel on top of claystone. The concrete lined, Cole Ditch was setback 65 feet from the highwall. The resulting factor of safety was 1.5 with the surcharge and 1.7 without the surcharge. Phase 9 -Phase 9 will be enclosed on all sides by a slurry wall, with the exception of the west side near the Cole Ditch. The Cole Ditch also applies to the east side of Phase 8 (factors of safety of 1.5 and 1.7 above). The most critical slurry wall section in Phase 9 is in the southwest corner. For this analysis, we modeled 12 feet of overburden clay overlying 30 feet of sand and gravel on top of claystone. The slurry wall setback 50 feet from the highwall, the relocated McCormick Ditch was setback 25 feet from the slurry wall, and the ground surface was sloped upward to the south. The factor of safety at the ditch was 1.5 without the surcharge and 1.9 with the surchazge. Stability analyses were mn on 100 randomly generated circles generated at the mine excavation and highwall, as well as the nearby permanent structure. The Simplified Bishop Method was used to estimate the factor of safety on the most critical surface. Input files and graphic profiles of each analysis are attached. The mine plan incorporates minimum setbacks which are accounted for in our stability analysis. Based on our analysis, mining should not pose a hazard to nearby permanent structures. LIMIATIONS Our review is based on regional geologic mapping, present mining plans, bore hole data, and stability analyses using typical soil strength pazameters. Should the mining plans change or subsurface conditions vary from those portrayed in this letter, we should be contacted in order to re-evaluate the potential affects on man-made structures.