1997-11-13_REVISION - M1977493 (50)

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Woodward4lyde Mr. Bryce Romig Climax Molybdenum Company October 14, 1997 Page 3 probably less than 50 cubic yards in any given event. This type of sliding (soil slips) can be expected on about a 200 ft length of the slope near where the recent slides occurred. Elsewhere, the slope angle is less steep and the colluvium more stable. We do not believe that the potential soil slips threaten the performance of the dam or reservoir. Deep Seated Landslides We observed no indications of deep seated landsliding in the area east of the recent soil slips and believe that this slope will remain stable during the raising of the reservoir water level. However, the shear surface west of the recent soil slips may be the slip surface of an ancient landslide, or a fault trace. The shear surface orientation is constant with major joint set, as could be expected for a landslide, and with regional faulting. The striations on the surface and the local topography are consistent with what could have been a large ancient landslide. Whether it was generated by landsliding or faulting, the surface is a weakness in the reservoir • slope. The recent excavation of oxide tailing and near surface soils has changed the slope configuration and may have reduced the passive resistance at the slide toe. Thus, there is a small risk that sliding could occur on this surface as the reservoir is raised with fresh water to unprecedented levels. We do not believe that movement is likely, because there currently appears to be relatively little driving force that would cause movement. However, it would be prudent to install several observation points to monitor for slope movement, during the first reservoir filling. Recommendations for Monitoring We recommend surface monuments be installed to monitor movements during next seasons' reservoir filling. Three surface monuments should be installed near the locations shown on Figure 3. We recommend that the monuments consist of a vertical hole 4 to 6 inches in diameter drilled to a depth of 2 to 3 feet. A No. 4 steel reinforcing bar should be inserted in the hole and the hole backfilled with lean concrete or other suitable backfill material. Two to three baseline readings should be taken at each monument prior to reservoir filling The movement points should then be monitored every 2 to 3 weeks during the spring reservoir filling. If only nominal movement has been observed in the monuments after the first year of observations,the measurements should continue twice a year for the next year to . two years. If subsequent movements are not observed, observations can be suspended. - 2443539361LE2.DOC 723/97(1:50 PM)JRPT