Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsDuring an inspection of the interior of the conduits in April 2001, it was discoveredthat material had been deposited inside the middle conduit near an opening in aconstruction joint. This was occurring through a hole in the floor of the middleconduit at a construction joint near station 1+30. Approximately 1 ft 3 of silt and finesand were deposited on the floor. However, this deposit was observed during thewinter when no irrigation releases are made. More deposition may have occurredduring irrigation season that was washed downstream and not observed.Consequently, the total volume of material could have been much greater than the1 ft 3 observed in 2001. Reclamation theorized that plugging this opening couldresult in redirecting the erosion through a different hole or crack in the conduit.Also, redirecting the erosion might cause a more dangerous path to develop alongthe foundation contact of the conduits, and a piping exit might develop downstreamof the embankment dam. If the exit point were located within the outlet channel,early detection would be very difficult.Another area of concern was the condition of the north conduit that was reportedlyplugged at each end prior to the final raise of Pablo Dam in 1932, but was neverconfirmed. Therefore, it could be possible that a nearly full reservoir head couldexist at the end of the north conduit, which was less than 100 feet from thedownstream toe of the dam. After much discussion between all involved parties, itwas decided to completely remove and replace the original outlet works.As an interim measure, a temporary patch was installed over the opening to preventadditional material from being eroded into the conduit while allowing for relief ofwater pressures. The patch consisted of filter fabric under a metal screen. DuringMarch 2002, the geotextile portion of the patch ruptured and approximately 0.5 ft 3of silt and fine sand were deposited into the conduit. The patch was repaired soonafter the rupture was discovered. Reservoir level restrictions were implemented inApril 2003 and were to be kept in place until the removal and replacementmodifications could be completed.The construction of a new outlet works began in November 2004 and wascompleted in the by the spring of 2005. The major aspects of the work included:• Construction of a cofferdam to maintain an area free of water duringconstruction.• Clearing, grubbing, and stripping prior to excavation.• Removing existing embankment dam slope protection.• Excavating embankment materials to accommodate construction of the newoutlet works (Slopes transverse to the dam centerline were excavated at 4H:1V).B-74