1998-10-28_GENERAL DOCUMENTS - C1981039

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i i • Interoffice Memorandum Walker to Binns July 1, 1998 Page 2 of 2 older slide. At near the same elevation, the crest escarpments of the newer slide are shown in Pictures 9 and 10. There is rill and gully erosion of reaches above the escarpments (directly below the high-flow discharge from the pond primary spillway. Although this rill and gully erosion is most pronounced above the southern slide, it extends for some distance to the north, and for a short distance to the south across the slope. The more extensive erosion features are directly below the discharge of the primary spillway barrel of the pond. There is a baffle plate, half way up the barrel outlet, with a 6" diameter pipe and plastic downspout sleeve from the barrel down the slope. The sleeve discharges at the crest of the slide (you are standing at the discharge outlet of the sleeve in Picture 10). The soils and talus debris appear to be permeable. There was a sink hole approximately 6 feet in front of you in Picture 9, near the center of the escarpment in Picture 10. There is a depression below the base of the primary spillway riser in the pond (Picture 8). The riser is tilted slightly out of plumb, away from the embankment. A concrete "mushroom"at the base indicated a spreading of concrete on an original surface during construction. This depression could be the result of excavation after construction, subsidence, or loss of fines (pumping) by seepage through the embankment. Of these three,the last two are most likely. There was no evidence of spoil material indicating excavation. During construction,the base of the embankment was likely compacted. However, areas under the barrel may not have been properly compacted. There was no flow into or out of the depression below the riser at the time of inspection (water level in the pond was at or just below this elevation). I saw no indication of seepage or loss of fines contiguous to the barrel, but my inspection was limited to the access provided on the southern portion of the inlet side and heavily vegetated areas of the outslope. I saw no indication of embankment failure. The geology, topography, and mining history of the site suggest the area is predisposed to sliding. Seepage through the pond embankment or bottom, and the primary riser discharges, appear to have abetted the current slide. The potential for additional sliding is high, higher if the pond is left in place. Additional sliding, or failure of the pond, do not appear to present a threat to life or property. Continual sliding does impose a potential for temporary degradation of water quality or mud- flow damming of the creek. Mud flows have, and would likely continue to, reach the creek (see Picture 5). Such damming is not likely to endanger the road. The slide has scoured the slope to bedrock (Picture 6). There just isn't a lot of material to form a substantial dam across the valley. Should the dam be retained, I suggest: grouting of the base of the primary riser to arrest loss of materials under the riser and through the embankment, and to prevent failure of structural elements of the riser; lining the pond with bentonite; and replacement of the exit low-flow discharge plastic tube with metal pipe, and the extension of the exit pipe to the base of the slope.