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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />10 <br />to make up for unavoidable seepage losses through or <br />under the dike. <br />Case Histories <br />AV Watkins Dam [5] <br />The event: <br /> Concentrated seep discharging 1-2 cfs (500-1000 <br />gpm) <br /> Sand boils w/cloudy discharge – active piping <br />(internal erosion) <br /> Significant sediment deposition in downstream toe <br />ditch <br /> Sloughing/cracking of downstream slope <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />The intervention: <br /> Initial attempt to place sand filter material fails – <br />sand washes away <br /> Thick filter and stability berm placed over seeps – <br />still 100-200 gpm cloudy seepage in South Drain <br /> Large berm constructed at upstream sinkholes <br />(seepage entrances) to reduce downstream <br />seepage discharge <br /> <br />Lake Needwood Dam [4] <br />The event: <br /> Concentrated seep in left downstream groin about <br />4 to 6 inches in diameter; later measured at 80 <br />gpm as reservoir was lowering) <br /> Entrained sediment noted in ‘styrofoam coffee cup <br />test’ – active piping (internal erosion) <br /> Piezometers show significant rise of phreatic <br />surface in dam – concern for downstream slope <br />stability <br /> Toe area of dam very spongy – high uplift pressure <br /> <br />The intervention: <br /> Maximum outlet works discharge implemented <br /> Use of inverted filter approach <br /> Initial attempt to place gravel as coarse filter <br />material to reduce discharge velocity fails – gravel <br />washing away <br /> Then used geotextile as filter, ballasted by <br />sandbags and gravel <br /> Gravel ballast 5-6 feet thick required to control <br />discharge while reservoir pool was lowered <br />Washakie Dam [4] <br />The event: <br /> Unexplained, persisting wet area on new stability <br />berm upon refilling reservoir; high piezometer <br />readings within new chimney filter <br /> Upon refilling the reservoir after the new berm <br />construction, a wet area was observed and test pit <br />excavated to investigate <br /> Concentrated seepage flow > 80 gpm and <br />appearing to increase entering bottom of shallow <br />test pit <br /> Entrained sediment observed in seepage flow into <br />the test pit – active piping (internal erosion) <br /> <br />Downstream <br />filter/gravel berm <br />Upstream sinkholes choke <br />filter/berm <br />Slough <br />Sand <br />deposition <br />Actively piping <br />sand boil