Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Pablo DamLocation: MontanaSummary: Removal and replacement of an existing outlet worksPablo Dam is located near Polson, Montana on the Flathead Indian Reservation.The embankment dam is an earthfill structure consisting of a main dam and dikes,which flank both sides of the dam, south and north. The crest elevation of the maindam is at 3220, and the dikes are at 3217. The main dam has a structural height of43 feet, a crest length of 10,550 feet, a crest width of 20 feet, a 3:1 upstream slope,and a 2:1 downstream slope. The north dike has a crest length of 5850 feet, and thesouth dike has a crest length of 10,250 feet. The crest width of both dikes is 12 feet.Pablo Dam was constructed in three phases over 24 years. In 1911, theembankment was constructed to elevation 3202. The second construction in 1918raised the embankment dam to elevation 3209, and the final construction in 1934raised the dam to the present elevation 3220. Pablo Dam is an offstream structurethat is fed by the Pablo Feeder Canal. The purpose of the embankment dam is toimpound water for irrigation. The reservoir has a capacity of 28,400 acre-feet atelevation 3211.0.The original outlet works was situated at the maximum section of the dam andconsisted of a 42-foot high concrete intake structure with two 3- by 5-foot slidegates. The original outlet works consisted of three box shaped conduits. The middleand south conduits were 172 feet long and 4.5 feet wide by 5 feet high. The northconduit was about 136 feet long and 3.0 feet square. This north conduit wasabandoned prior to the third phase of original construction.Differential settlement between the intake tower and the outlet works conduitscaused some offset in “sliding joints.” This settlement was expected, as “slidingjoints” (no reinforcement crossing the joint) were included in the original design.However, continued settlement of the intake structure and the first 13 feet of theconduits required grouting of the foundation shortly after construction. No furthersettlement has been detected in the last 50 plus years. The first sliding joint isdisplaced vertically about 2 inches and sprays water at high reservoir head. Mortarfilling in all sliding joints was disbonded, cracked, and deteriorating. Tensile crackswere also discovered along the length of the conduit. Water was commonly leakingfrom both the cracks and the sliding joints, and there are signs of possible internalerosion of embankment material occurring in a few areas. Spalling concrete hadbeen discovered in the walls of the conduits. The concrete in the center wall at theB-72