GENERAL38899 - Laserfiche WebLink

Home Browse Search

!V[emo to Dan Hernandez HawksNest East Landslide page 2 the subdued surface of this wetland, I conclude that the ca[-o-nine tail wetland seep is a decade or more in age. Immediately down slope and to the west of this wetland and drainage Swale, the general landslide mass is now largely saturated. Walking across the mass results in water-filled impressions and puddled water stands in natural depressions in the slope. However, the vegetation has not yet responded to reflect this high level of saturation. In addition, this winter and spring (to date) have been abnormally dry in the North Fork Valley. I conclude from the observations afforded me by a one hour field examination and reconnaissance that the saturation of the landslide mass has recently (within the past several years) significantly increased. Further, it appears the increased flow is issuing as ground water in the general vicinity of the previously existing cat-o-nine tail wetland above and [o the east of the landslide mass. However, it appears the flow has increased, even though the immediate vicinity is experiencing a drier than normal winter and spring. Fortunately, the landslide mass has not experienced a recent acceleration of failure. However, based upon its current degree of saturation, it appears to be at or near the threshold of accelerated failure (meta-stable). In no case should the landslide mass be excavated or loaded. The vibration alone of heavy equipment might be sufficient to trigger an acceleration in its normal creep rate. The cause of the increased likelihood of failure of this.historically active pre-historic landslide mass is an increase in it's degree of saturation. The most direct means of decreasing its current failure risk is to decrease its degree of saturation. Without concern for the cause of the increased saturation, the most direct intervention would be to intercept the effluent ground water and divert it from the landslide mass. This could be accomplished by excavating and constructing french drains above and flanking the lands]ide mass. With the passage of time, this should reduce the saturation of the landslide mass, which will increase it's slope stability. This excavation would best be accomplished during the normally drier season of ]ate summer and eazly fall. At your suggestion I discussed this situation with Jim McArdle, who also visited the site recently. At Jim's suggestion we examined the historic (pre-program) mine maps of this azea stored in the CGS. We found no evidence to suggest that an historic mine working portal existed in the immediate area of the distinct "portal-like pocket" I described here above. References and maps were found for [he Axelton Mine, as well as [he Oliver #2 and #3 mines. Examination of the mine workings maps for the permitted HawksNest East and West Mines also discerned no portal in the immediate vicinity of this suspicious landform and the cat-o-nine tail wetland seep area. I conclude that this effluent wetland may be the natural result of the surface interception of a penetrating bedrock shear zone. These fractured zones accommodate infiltration up slope and express effluent flow of ground waters as springs' and seep areas at lower elevations on [he affected slopes.