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a ` ~ Mark Loye -2- September 8, 1983 A recent rainfall event had coincidentally breached the road and drainage systems at the site, resulting in general slope wash. The areas affected demonstrated denudation of relatively thin (6 to 12 inches) colluvial soils and subsoils, exposing relatively highly slakeable bedrock. The event did not appear to result in deep gully formation. In will take this into consideration when reviewiny their final ditch desiyn submittal, includiny the failure scenarios and consequence discussions which we requested during our preliminary meeting. The operator's representatives suggested that the design was being revised to accommodate a lower frequency flood event's capacity. North Pit Landslide The ]andslide now appears to affect approximately 2/3 of the northeastern wa]] of the northern pit. The basal plain of movement of the landslide daylights near the bottom of the existing pit. The landslide's headscarp, now an approximately 25-foot high exposure, exists approximately 1,000 feet upslope from the pit wall. The operator reports that the headscarp feature predated mining activity representing the existence of a large paleolandslide. This fact was not presented oy the operator during their application phase. Excavation of the north pit has reactivated and yrossly accelerated landslidinq activity. The operator's attempted dewatering activities have failed to stabilize the landslide mass. It is impossible for the operator to mainiain a normal terraced pit wall configuration as a result of the active landslide. Movement of the northeastern wall is monitored using standard surveying techiques to allow for rapid evacuation of the pit, if necessary. The required excavation of landslide debris to access the ore body has rendered recovery of the entire ore resource uneconomical within the north pit. Excavation will be complete within the near future and the operator plans to cease mining within the northern pit. The operator's representatives estimate that approximately 90% of the oriyinally projected high yrade ore will have been recovered from the northern pit. The terrain between the crest of the northeastern pit wall of the northern pit and the headscarp of the slide now consists of horst and graben blocks of landslide-affected ground. Upen fissures and scarps are common, as is disorientation of trees and plant life within the affected area. The magnitude of the slide suggests that it would be impractical to attempt standard landslide control techniques. The only conceivable control activity possible at this time appears to be the backfilling of the north pit, which would reestablish some confinement of the toe of the landslide mass. This could serve to resist continued movement of the landslide mass. However, the greatly extended landslide mass will probably continue to internally readjust itself under the forces of gravity, until it achieves both external and internal equilibrium, at some unknown time in the future.